vlf-disciplined OCXO circuit
Does anyone know of designs for disciplined OCXOs that are referenced to
off-air, especially VLF, signals other than GPS/GNSS please? With modules
for the latter being so cheap this might seem pointless but there are some
potential advantages.
- GPS reception at indoor locations where mechanical clocks need to be
monitored is often (usually?) unavailable because of shadowing and building
absorbtion and it's usually inconvenient to run a cable. - GPS is increasingly likely to be jammed either by criminal elements
or "state actors". - VLF (the likes of WWVB / MSF / DCF77 / e-Loran) is more likely to be
received indoors. - E-Loran is being tipped as an off-air time source to back up GPS and
will become increasingly available. - There's the possibility of a multi-standard receiver that might find
and lock to any available source, and potentially to several.
Obviously there are a lot of very cheap modules around to receive the
signals but these discard the carrier and just output the time-code logic
signal. I have seen a design for an MSF-locked standard in discrete
components and more recently an MSF receiver implemented as
direct-conversion SDR on a Raspberry Pi Pico which phase locks its internal
digital LO to the carrier but I suspect that its phase noise would be pretty
ropey - really intended as a time, not frequency, source.
- John Haine.
Hi
The most basic question here is “where do you want to do this?”
VLF (even Loran) is a limited range sort of thing. If you live in the US, your choices are different than
the UK. If you live in NZ, your choices are a bit limited … sorry about that …
Next layer once you have a site “in range”, just how far is that?
If you happen to be 10’s of miles from the site, you will not have much propagation to deal with.
It’s a lot more likely that you are hundreds (or maybe even thousands) of miles away. Miami to
WWVB is a bit over 2,000 miles.
Over a long path, you will indeed get into propagation effects. These can be pretty substantial.
Most of it is predictable, there will always be grubby side effects that come in at some level. If
you are after a “modern” sort of GPSDO performance, you will need to come up with a method
of dealing with those. Does this apply to you? Let’s assume it does and move on.
One traditional approach to dealing with this is to go to a Rb rather than an OCXO as the standard.
You then decide on a “safe” time range during the day. It might be the middle of the night. You
take your data then and shove it into the control function. Your correction process is a bit slow
as a result.
No this is not even close to a full list of what to dig into. There are already enough choices and
turns in the road that covering all the cases is heading into a lot of things that simply will not
matter.
Yes, we could dive into your “why” section. Needless to say, there is a lot of debate there and
a wide number of fixes for this or fixes for that. None of these systems by themselves are immune
to “issues”. There are alternatives past those on your list.
Fun !!!
Bob
On Sep 22, 2025, at 5:19 AM, john.haine--- via time-nuts time-nuts@lists.febo.com wrote:
Does anyone know of designs for disciplined OCXOs that are referenced to
off-air, especially VLF, signals other than GPS/GNSS please? With modules
for the latter being so cheap this might seem pointless but there are some
potential advantages.
- GPS reception at indoor locations where mechanical clocks need to be
monitored is often (usually?) unavailable because of shadowing and building
absorbtion and it's usually inconvenient to run a cable. - GPS is increasingly likely to be jammed either by criminal elements
or "state actors". - VLF (the likes of WWVB / MSF / DCF77 / e-Loran) is more likely to be
received indoors. - E-Loran is being tipped as an off-air time source to back up GPS and
will become increasingly available. - There's the possibility of a multi-standard receiver that might find
and lock to any available source, and potentially to several.
Obviously there are a lot of very cheap modules around to receive the
signals but these discard the carrier and just output the time-code logic
signal. I have seen a design for an MSF-locked standard in discrete
components and more recently an MSF receiver implemented as
direct-conversion SDR on a Raspberry Pi Pico which phase locks its internal
digital LO to the carrier but I suspect that its phase noise would be pretty
ropey - really intended as a time, not frequency, source.
- John Haine.
time-nuts mailing list -- time-nuts@lists.febo.com
To unsubscribe send an email to time-nuts-leave@lists.febo.com
john.haine--- via time-nuts writes:
Does anyone know of designs for disciplined OCXOs that are referenced to
off-air, especially VLF, signals other than GPS/GNSS please?
I played a lot with that many years ago.
You can do a lot with a small microcontroller with an ADC which does
something like a million samples per second.
My favourite was this:
Run the ADC at 1 MHz from your reference clock, sum the samples into
a 1000 long circular buffer:
alpha = 1e-5; // play with this
n = 0
while (1) {
x = get_sample()
buffer[x] += (x - buffer[x]) * alpha
x +=1
if x == 1000:
x = 0;
}
This buffer now holds the output of a 1kHz combfilter over your input
signal, which means you can extract the phase and amplitude of any
signal on a multiple of 1kHz from that single buffer.
Simply multiply the 1000 samples in the buffer with sin() and cos()
of your desired frequency and calculate the magnitude and angle of
the resulting vector:
fx = 60 // 60 khz
ssin = 0.0
scos = 0.0
for i in range(1000):
ssin += sin(2 * PI * i / 1000 * buffer[i]
scos += sin(2 * PI * i / 1000 * buffer[i]
ampl = hypot(ssin, scos)
angle = atan2(ssin, scos)
Now do that for as many signals as you want, and steer your
reference clock accordingly.
If you want to also track signals on half kHz (like DCF77), make
the buffer 2000 samples.
One paticularly interesting case is using a buffer exactly one
second long (=1 million in this example), that would allow you
to extract both the phase and the modulation from signals
like DCF77 and WWVB
The Pi2 has a 500kS ADC and two CPU cores, so it is nearly perfect,
provided you can lock the frequency to your house standard - or
just use an external ADC, plenty of eval boards out there.
I didn't document very much of the fun I had, but there is
some stuff here:
https://phk.freebsd.dk/loran-c/CW/
--
Poul-Henning Kamp | UNIX since Zilog Zeus 3.20
phk@FreeBSD.ORG | TCP/IP since RFC 956
FreeBSD committer | BSD since 4.3-tahoe
Never attribute to malice what can adequately be explained by incompetence.
I have a design for a WWVB Receiver, which is an SDR running on a 600 MHz
ARM M7. It phase locks the receiver to the incoming 60 kHz signal, by
steering an on-board TCVCXO. It extracts the AM modulation, phase
modulation, and only extracts carrier frequency during a known phase state
window. This constitutes a "sensor", which could then be used to steer a
separate secondary frequency/time standard.
After running it for a year, and taking some data, I would summarize that,
at any distance from the transmitter, the WWVB "sensor" is two or three
orders of magnitude less accurate than even the simplest GPS receiver
system.
The WWVB signal as transmitted is derived from time/frequency standards
that are derived from the same source as the GPS system. But the
propagation mechanism is much less stable.
So, many day averaging and some time windowing and gating, and ionosphere
disturbance intelligence are going to be required to extract something more
accurate from WWVB.
So, whether you are looking for time or frequency information, you will
need to decide just how much accuracy is important to you. If you are
monitoring some mechanical clocks, or looking for fractional milli-second
accuracy, it might be OK. If you are trying to provide a back-up for a GPS
system, beware.
I live in Austin Texas, which is about 900 miles from the WWVB transmitter.
The main night time propagation mode is that the signal is refracted by the
lower D layer, which appears to move up and down day to night, with
multipath, as well as appears to move up and down with solar flare activity
which can also charge the ionosphere.
Daytime propagation mode is mostly ground wave. (More loss, but more
stable.) And here in Austin we can get total cancellation (and half cycle
slips) of the received signal as the propagation mode changes between day
and night propagation modes
I typically find that the day to night path difference is on the order of
22 microseconds and a typical minor solar flare will create a few hours
long 5 or 6 microsecond time change. Bigger flares, bigger impact.
If you look at the VLF systems that Hewlett-Packard sold prior to GPS
availability, the OCXO secondary standard was not managed directly by the
VLF receiver. The receiver drove a paper chart recorder, and the lab
personnel were expected to examine the paper tape, at the same time every
afternoon (when the path was most stable and repeatable) and then use their
judgement to manually adjust the OCXO secondary standard to the 'correct'
frequency.
This is not a simple algorithm to convert to software.
The VLF signals do penetrate wooden buildings (homes) reasonably well. But
they are also subject to interference.
Any lightning activity within about 150 miles causes signal degradation and
signals become unusable when the lightning activity is within about 50
miles.
Here in the US, with power systems at 60 Hz, the harmonics from any
computer monitor screen refreshed at 60 Hz (or a multiple) can cause
interference to the WWVB receiver. So I find that the WWVB antenna should
not be within 15 to 20 feet of any computer monitor screen, which limits
antenna placement in the modern home or lab.
I have a friend who has one of my receivers, and is attempting to do
multiple day averaging to perhaps do very long term steering of a rubidium
secondary standard. Perhaps more to come on that topic, someday.
As Bob says, fun.
--- Graham
==
On Mon, Sep 22, 2025 at 1:02 PM Bob Camp via time-nuts <
time-nuts@lists.febo.com> wrote:
Hi
The most basic question here is “where do you want to do this?”
VLF (even Loran) is a limited range sort of thing. If you live in the US,
your choices are different than
the UK. If you live in NZ, your choices are a bit limited … sorry about
that …
Next layer once you have a site “in range”, just how far is that?
If you happen to be 10’s of miles from the site, you will not have much
propagation to deal with.
It’s a lot more likely that you are hundreds (or maybe even thousands) of
miles away. Miami to
WWVB is a bit over 2,000 miles.
Over a long path, you will indeed get into propagation effects. These can
be pretty substantial.
Most of it is predictable, there will always be grubby side effects that
come in at some level. If
you are after a “modern” sort of GPSDO performance, you will need to come
up with a method
of dealing with those. Does this apply to you? Let’s assume it does and
move on.
One traditional approach to dealing with this is to go to a Rb rather than
an OCXO as the standard.
You then decide on a “safe” time range during the day. It might be the
middle of the night. You
take your data then and shove it into the control function. Your
correction process is a bit slow
as a result.
No this is not even close to a full list of what to dig into. There are
already enough choices and
turns in the road that covering all the cases is heading into a lot of
things that simply will not
matter.
Yes, we could dive into your “why” section. Needless to say, there is a
lot of debate there and
a wide number of fixes for this or fixes for that. None of these systems
by themselves are immune
to “issues”. There are alternatives past those on your list.
Fun !!!
Bob
On Sep 22, 2025, at 5:19 AM, john.haine--- via time-nuts <
time-nuts@lists.febo.com> wrote:
Does anyone know of designs for disciplined OCXOs that are referenced to
off-air, especially VLF, signals other than GPS/GNSS please? With
modules
for the latter being so cheap this might seem pointless but there are
some
potential advantages.
-
GPS reception at indoor locations where mechanical clocks need to
be
monitored is often (usually?) unavailable because of shadowing and
building
absorbtion and it's usually inconvenient to run a cable.
-
GPS is increasingly likely to be jammed either by criminal elements
or "state actors".
-
VLF (the likes of WWVB / MSF / DCF77 / e-Loran) is more likely to
be
received indoors.
-
E-Loran is being tipped as an off-air time source to back up GPS
and
will become increasingly available.
-
There's the possibility of a multi-standard receiver that might
find
and lock to any available source, and potentially to several.
Obviously there are a lot of very cheap modules around to receive the
signals but these discard the carrier and just output the time-code logic
signal. I have seen a design for an MSF-locked standard in discrete
components and more recently an MSF receiver implemented as
direct-conversion SDR on a Raspberry Pi Pico which phase locks its
internal
digital LO to the carrier but I suspect that its phase noise would be
pretty
ropey - really intended as a time, not frequency, source.
-
John Haine.
time-nuts mailing list -- time-nuts@lists.febo.com
To unsubscribe send an email to time-nuts-leave@lists.febo.com
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John,
Not a guru...but I would worry about the phase shift and potential reflection/refraction/noise effects of disciplining an oscillator to a far-away VLF carrier signal. Not sure what the relative accuracy might be...but it's probably quite a bit less than disciplining to GPS...and maybe even WWV. At the "right" times of day, WWV can get you to about 1-2 Hz @ 10 MHz. Wonder what VLF might be?
Interesting!
Regards,
Kirk, NT0ZRochester, MN
My book, "Stealth Amateur Radio," is now available from www.stealthamateur.com and on the Amazon Kindle (soon)
On Monday, September 22, 2025 at 10:01:45 AM CDT, john.haine--- via time-nuts <time-nuts@lists.febo.com> wrote:
Does anyone know of designs for disciplined OCXOs that are referenced to
off-air, especially VLF, signals other than GPS/GNSS please? With modules
for the latter being so cheap this might seem pointless but there are some
potential advantages.
* GPS reception at indoor locations where mechanical clocks need to be
monitored is often (usually?) unavailable because of shadowing and building
absorbtion and it's usually inconvenient to run a cable.
* GPS is increasingly likely to be jammed either by criminal elements
or "state actors".
* VLF (the likes of WWVB / MSF / DCF77 / e-Loran) is more likely to be
received indoors.
* E-Loran is being tipped as an off-air time source to back up GPS and
will become increasingly available.
* There's the possibility of a multi-standard receiver that might find
and lock to any available source, and potentially to several.
Obviously there are a lot of very cheap modules around to receive the
signals but these discard the carrier and just output the time-code logic
signal. I have seen a design for an MSF-locked standard in discrete
components and more recently an MSF receiver implemented as
direct-conversion SDR on a Raspberry Pi Pico which phase locks its internal
digital LO to the carrier but I suspect that its phase noise would be pretty
ropey - really intended as a time, not frequency, source.
* John Haine.
time-nuts mailing list -- time-nuts@lists.febo.com
To unsubscribe send an email to time-nuts-leave@lists.febo.com
Hi Graham:
I'd add that some WWVB clocks go crazy if they are near Wi-Fi, which includes routers and cell phones.
https://prc68.com/I/Loop.shtml#RFI
--
Have Fun,
Brooke Clarke
https://www.PRC68.com
axioms:
- The extent to which you can fix or improve something will be limited by how well you understand how it works.
- Everybody, with no exceptions, holds false beliefs.
-------- Original Message --------
I have a design for a WWVB Receiver, which is an SDR running on a 600 MHz
ARM M7. It phase locks the receiver to the incoming 60 kHz signal, by
steering an on-board TCVCXO. It extracts the AM modulation, phase
modulation, and only extracts carrier frequency during a known phase state
window. This constitutes a "sensor", which could then be used to steer a
separate secondary frequency/time standard.
After running it for a year, and taking some data, I would summarize that,
at any distance from the transmitter, the WWVB "sensor" is two or three
orders of magnitude less accurate than even the simplest GPS receiver
system.
The WWVB signal as transmitted is derived from time/frequency standards
that are derived from the same source as the GPS system. But the
propagation mechanism is much less stable.
So, many day averaging and some time windowing and gating, and ionosphere
disturbance intelligence are going to be required to extract something more
accurate from WWVB.
So, whether you are looking for time or frequency information, you will
need to decide just how much accuracy is important to you. If you are
monitoring some mechanical clocks, or looking for fractional milli-second
accuracy, it might be OK. If you are trying to provide a back-up for a GPS
system, beware.
I live in Austin Texas, which is about 900 miles from the WWVB transmitter.
The main night time propagation mode is that the signal is refracted by the
lower D layer, which appears to move up and down day to night, with
multipath, as well as appears to move up and down with solar flare activity
which can also charge the ionosphere.
Daytime propagation mode is mostly ground wave. (More loss, but more
stable.) And here in Austin we can get total cancellation (and half cycle
slips) of the received signal as the propagation mode changes between day
and night propagation modes
I typically find that the day to night path difference is on the order of
22 microseconds and a typical minor solar flare will create a few hours
long 5 or 6 microsecond time change. Bigger flares, bigger impact.
If you look at the VLF systems that Hewlett-Packard sold prior to GPS
availability, the OCXO secondary standard was not managed directly by the
VLF receiver. The receiver drove a paper chart recorder, and the lab
personnel were expected to examine the paper tape, at the same time every
afternoon (when the path was most stable and repeatable) and then use their
judgement to manually adjust the OCXO secondary standard to the 'correct'
frequency.
This is not a simple algorithm to convert to software.
The VLF signals do penetrate wooden buildings (homes) reasonably well. But
they are also subject to interference.
Any lightning activity within about 150 miles causes signal degradation and
signals become unusable when the lightning activity is within about 50
miles.
Here in the US, with power systems at 60 Hz, the harmonics from any
computer monitor screen refreshed at 60 Hz (or a multiple) can cause
interference to the WWVB receiver. So I find that the WWVB antenna should
not be within 15 to 20 feet of any computer monitor screen, which limits
antenna placement in the modern home or lab.
I have a friend who has one of my receivers, and is attempting to do
multiple day averaging to perhaps do very long term steering of a rubidium
secondary standard. Perhaps more to come on that topic, someday.
As Bob says, fun.
--- Graham
==
On Mon, Sep 22, 2025 at 1:02 PM Bob Camp via time-nuts <
time-nuts@lists.febo.com> wrote:
Hi
The most basic question here is “where do you want to do this?”
VLF (even Loran) is a limited range sort of thing. If you live in the US,
your choices are different than
the UK. If you live in NZ, your choices are a bit limited … sorry about
that …
Next layer once you have a site “in range”, just how far is that?
If you happen to be 10’s of miles from the site, you will not have much
propagation to deal with.
It’s a lot more likely that you are hundreds (or maybe even thousands) of
miles away. Miami to
WWVB is a bit over 2,000 miles.
Over a long path, you will indeed get into propagation effects. These can
be pretty substantial.
Most of it is predictable, there will always be grubby side effects that
come in at some level. If
you are after a “modern” sort of GPSDO performance, you will need to come
up with a method
of dealing with those. Does this apply to you? Let’s assume it does and
move on.
One traditional approach to dealing with this is to go to a Rb rather than
an OCXO as the standard.
You then decide on a “safe” time range during the day. It might be the
middle of the night. You
take your data then and shove it into the control function. Your
correction process is a bit slow
as a result.
No this is not even close to a full list of what to dig into. There are
already enough choices and
turns in the road that covering all the cases is heading into a lot of
things that simply will not
matter.
Yes, we could dive into your “why” section. Needless to say, there is a
lot of debate there and
a wide number of fixes for this or fixes for that. None of these systems
by themselves are immune
to “issues”. There are alternatives past those on your list.
Fun !!!
Bob
On Sep 22, 2025, at 5:19 AM, john.haine--- via time-nuts <
time-nuts@lists.febo.com> wrote:
Does anyone know of designs for disciplined OCXOs that are referenced to
off-air, especially VLF, signals other than GPS/GNSS please? With
modules
for the latter being so cheap this might seem pointless but there are
some
potential advantages.
-
GPS reception at indoor locations where mechanical clocks need to
be
monitored is often (usually?) unavailable because of shadowing and
building
absorbtion and it's usually inconvenient to run a cable.
-
GPS is increasingly likely to be jammed either by criminal elements
or "state actors".
-
VLF (the likes of WWVB / MSF / DCF77 / e-Loran) is more likely to
be
received indoors.
-
E-Loran is being tipped as an off-air time source to back up GPS
and
will become increasingly available.
-
There's the possibility of a multi-standard receiver that might
find
and lock to any available source, and potentially to several.
Obviously there are a lot of very cheap modules around to receive the
signals but these discard the carrier and just output the time-code logic
signal. I have seen a design for an MSF-locked standard in discrete
components and more recently an MSF receiver implemented as
direct-conversion SDR on a Raspberry Pi Pico which phase locks its
internal
digital LO to the carrier but I suspect that its phase noise would be
pretty
ropey - really intended as a time, not frequency, source.
-
John Haine.
time-nuts mailing list -- time-nuts@lists.febo.com
To unsubscribe send an email to time-nuts-leave@lists.febo.com
time-nuts mailing list -- time-nuts@lists.febo.com
To unsubscribe send an email to time-nuts-leave@lists.febo.com
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I built a 162kHz reference based on the timing signal broadcasted by the Allouis station in France.
It is reliable, except every Tuesday mornings when it shuts down for maintenance !
Compared to GPS PPS I can reach about 10-11 ADEV in day time and 10-12 in a week.
Naturally impacted by perturbations on the path, day / night ionospheric disturbances and others.
But the other powerful commercial LF transmitters have now been shutdown (at least here in Europe)
Gilles.
Le 22 sept. 2025 à 21:13, Graham / KE9H via time-nuts time-nuts@lists.febo.com a écrit :
I have a design for a WWVB Receiver, which is an SDR running on a 600 MHz
ARM M7. It phase locks the receiver to the incoming 60 kHz signal, by
steering an on-board TCVCXO. It extracts the AM modulation, phase
modulation, and only extracts carrier frequency during a known phase state
window. This constitutes a "sensor", which could then be used to steer a
separate secondary frequency/time standard.
After running it for a year, and taking some data, I would summarize that,
at any distance from the transmitter, the WWVB "sensor" is two or three
orders of magnitude less accurate than even the simplest GPS receiver
system.
The WWVB signal as transmitted is derived from time/frequency standards
that are derived from the same source as the GPS system. But the
propagation mechanism is much less stable.
So, many day averaging and some time windowing and gating, and ionosphere
disturbance intelligence are going to be required to extract something more
accurate from WWVB.
So, whether you are looking for time or frequency information, you will
need to decide just how much accuracy is important to you. If you are
monitoring some mechanical clocks, or looking for fractional milli-second
accuracy, it might be OK. If you are trying to provide a back-up for a GPS
system, beware.
I live in Austin Texas, which is about 900 miles from the WWVB transmitter.
The main night time propagation mode is that the signal is refracted by the
lower D layer, which appears to move up and down day to night, with
multipath, as well as appears to move up and down with solar flare activity
which can also charge the ionosphere.
Daytime propagation mode is mostly ground wave. (More loss, but more
stable.) And here in Austin we can get total cancellation (and half cycle
slips) of the received signal as the propagation mode changes between day
and night propagation modes
I typically find that the day to night path difference is on the order of
22 microseconds and a typical minor solar flare will create a few hours
long 5 or 6 microsecond time change. Bigger flares, bigger impact.
If you look at the VLF systems that Hewlett-Packard sold prior to GPS
availability, the OCXO secondary standard was not managed directly by the
VLF receiver. The receiver drove a paper chart recorder, and the lab
personnel were expected to examine the paper tape, at the same time every
afternoon (when the path was most stable and repeatable) and then use their
judgement to manually adjust the OCXO secondary standard to the 'correct'
frequency.
This is not a simple algorithm to convert to software.
The VLF signals do penetrate wooden buildings (homes) reasonably well. But
they are also subject to interference.
Any lightning activity within about 150 miles causes signal degradation and
signals become unusable when the lightning activity is within about 50
miles.
Here in the US, with power systems at 60 Hz, the harmonics from any
computer monitor screen refreshed at 60 Hz (or a multiple) can cause
interference to the WWVB receiver. So I find that the WWVB antenna should
not be within 15 to 20 feet of any computer monitor screen, which limits
antenna placement in the modern home or lab.
I have a friend who has one of my receivers, and is attempting to do
multiple day averaging to perhaps do very long term steering of a rubidium
secondary standard. Perhaps more to come on that topic, someday.
As Bob says, fun.
--- Graham
==
On Mon, Sep 22, 2025 at 1:02 PM Bob Camp via time-nuts <
time-nuts@lists.febo.com> wrote:
Hi
The most basic question here is “where do you want to do this?”
VLF (even Loran) is a limited range sort of thing. If you live in the US,
your choices are different than
the UK. If you live in NZ, your choices are a bit limited … sorry about
that …
Next layer once you have a site “in range”, just how far is that?
If you happen to be 10’s of miles from the site, you will not have much
propagation to deal with.
It’s a lot more likely that you are hundreds (or maybe even thousands) of
miles away. Miami to
WWVB is a bit over 2,000 miles.
Over a long path, you will indeed get into propagation effects. These can
be pretty substantial.
Most of it is predictable, there will always be grubby side effects that
come in at some level. If
you are after a “modern” sort of GPSDO performance, you will need to come
up with a method
of dealing with those. Does this apply to you? Let’s assume it does and
move on.
One traditional approach to dealing with this is to go to a Rb rather than
an OCXO as the standard.
You then decide on a “safe” time range during the day. It might be the
middle of the night. You
take your data then and shove it into the control function. Your
correction process is a bit slow
as a result.
No this is not even close to a full list of what to dig into. There are
already enough choices and
turns in the road that covering all the cases is heading into a lot of
things that simply will not
matter.
Yes, we could dive into your “why” section. Needless to say, there is a
lot of debate there and
a wide number of fixes for this or fixes for that. None of these systems
by themselves are immune
to “issues”. There are alternatives past those on your list.
Fun !!!
Bob
On Sep 22, 2025, at 5:19 AM, john.haine--- via time-nuts <
time-nuts@lists.febo.com> wrote:
Does anyone know of designs for disciplined OCXOs that are referenced to
off-air, especially VLF, signals other than GPS/GNSS please? With
modules
for the latter being so cheap this might seem pointless but there are
some
potential advantages.
-
GPS reception at indoor locations where mechanical clocks need to
be
monitored is often (usually?) unavailable because of shadowing and
building
absorbtion and it's usually inconvenient to run a cable.
-
GPS is increasingly likely to be jammed either by criminal elements
or "state actors".
-
VLF (the likes of WWVB / MSF / DCF77 / e-Loran) is more likely to
be
received indoors.
-
E-Loran is being tipped as an off-air time source to back up GPS
and
will become increasingly available.
-
There's the possibility of a multi-standard receiver that might
find
and lock to any available source, and potentially to several.
Obviously there are a lot of very cheap modules around to receive the
signals but these discard the carrier and just output the time-code logic
signal. I have seen a design for an MSF-locked standard in discrete
components and more recently an MSF receiver implemented as
direct-conversion SDR on a Raspberry Pi Pico which phase locks its
internal
digital LO to the carrier but I suspect that its phase noise would be
pretty
ropey - really intended as a time, not frequency, source.
-
John Haine.
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Hi John,
These older for the 80s / 90s Spectracom Units did just what you are
looking for.
Unfortunately, they do not work anymore with WWVB as of the phase shift
modulation that was introduced in 2012
Working on a mod for to make them work now a days
here is a link to instruction manual for one of the 8164
https://manuals.repeater-builder.com/te-files/MISCELLANEOUS/Spectracom%208164%20Instruction.pdf
Best,
Mike Fowler
On Mon, Sep 22, 2025 at 10:01 AM john.haine--- via time-nuts <
time-nuts@lists.febo.com> wrote:
Does anyone know of designs for disciplined OCXOs that are referenced to
off-air, especially VLF, signals other than GPS/GNSS please? With modules
for the latter being so cheap this might seem pointless but there are some
potential advantages.
-
GPS reception at indoor locations where mechanical clocks need to
be
monitored is often (usually?) unavailable because of shadowing and building
absorbtion and it's usually inconvenient to run a cable.
-
GPS is increasingly likely to be jammed either by criminal elements
or "state actors".
-
VLF (the likes of WWVB / MSF / DCF77 / e-Loran) is more likely to
be
received indoors.
-
E-Loran is being tipped as an off-air time source to back up GPS
and
will become increasingly available.
-
There's the possibility of a multi-standard receiver that might
find
and lock to any available source, and potentially to several.
Obviously there are a lot of very cheap modules around to receive the
signals but these discard the carrier and just output the time-code logic
signal. I have seen a design for an MSF-locked standard in discrete
components and more recently an MSF receiver implemented as
direct-conversion SDR on a Raspberry Pi Pico which phase locks its internal
digital LO to the carrier but I suspect that its phase noise would be
pretty
ropey - really intended as a time, not frequency, source.
-
John Haine.
time-nuts mailing list -- time-nuts@lists.febo.com
To unsubscribe send an email to time-nuts-leave@lists.febo.com
Hi,
I calculate the amplitude and phase of a sampled signal with the
goertzel algo https://en.wikipedia.org/wiki/Goertzel_algorithm.
It calculates the discrete fourier transform, i.e. amplitude and phase
for a single frequency at the cost of one multiply per sample.
No need to buffer samples.
const float fcc=2.0f*cosf(M_PI/16.0f); // signal freq / sample freq
= 1/32
fq2=0.0f;fq1=0.0f;
for(nn=0;nn<RECLEN;nn++){
while(1){ // wait for sample
if((ADC_ISR&(1<<2))>0){ // got it
dd=ADC_DR; // get sample
fq0=fq1*fcc-fq2+dd; // undamped second order recursive
filter
fq2=fq1;fq1=fq0;
break;
}
}
}
// unwrap result
double complex z =(float)(fq1-fq2cosf(M_PI/16))+ // Real part
I(float)(fq2*sinf(M_PI/16)); // imaginary part
Cheers
Detlef
Am 22.09.2025 um 19:12 schrieb Poul-Henning Kamp via time-nuts:
john.haine--- via time-nuts writes:
Does anyone know of designs for disciplined OCXOs that are referenced to
off-air, especially VLF, signals other than GPS/GNSS please?
I played a lot with that many years ago.
You can do a lot with a small microcontroller with an ADC which does
something like a million samples per second.
My favourite was this:
Run the ADC at 1 MHz from your reference clock, sum the samples into
a 1000 long circular buffer:
alpha = 1e-5; // play with this
n = 0
while (1) {
x = get_sample()
buffer[x] += (x - buffer[x]) * alpha
x +=1
if x == 1000:
x = 0;
}
This buffer now holds the output of a 1kHz combfilter over your input
signal, which means you can extract the phase and amplitude of any
signal on a multiple of 1kHz from that single buffer.
Simply multiply the 1000 samples in the buffer with sin() and cos()
of your desired frequency and calculate the magnitude and angle of
the resulting vector:
fx = 60 // 60 khz
ssin = 0.0
scos = 0.0
for i in range(1000):
ssin += sin(2 * PI * i / 1000 * buffer[i]
scos += sin(2 * PI * i / 1000 * buffer[i]
ampl = hypot(ssin, scos)
angle = atan2(ssin, scos)
Now do that for as many signals as you want, and steer your
reference clock accordingly.
If you want to also track signals on half kHz (like DCF77), make
the buffer 2000 samples.
One paticularly interesting case is using a buffer exactly one
second long (=1 million in this example), that would allow you
to extract both the phase and the modulation from signals
like DCF77 and WWVB
The Pi2 has a 500kS ADC and two CPU cores, so it is nearly perfect,
provided you can lock the frequency to your house standard - or
just use an external ADC, plenty of eval boards out there.
I didn't document very much of the fun I had, but there is
some stuff here:
https://phk.freebsd.dk/loran-c/CW/
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Anyone thought of, or implemented, a 50/60Hz VLF receiver?
Yes it would have horrible characteristics compared to a proper VLF
station, especially during power cuts. Personally I’m thinking more of
measuring the 50/60Hz “signal” than using it as a time source. And yes I’m
aware that it’s possible to do such measurements by plugging directly into
the grid, but I’d prefer not to do that for reasons.
I calculate the amplitude and phase of a sampled signal with the
goertzel algo https://en.wikipedia.org/wiki/Goertzel_algorithm.
The point about my suggestion, is that the per-sample math is very
trivial, and you can do the "heavy" math in the background at a
much lower rate and on multiple different carriers at the same time.
--
Poul-Henning Kamp | UNIX since Zilog Zeus 3.20
phk@FreeBSD.ORG | TCP/IP since RFC 956
FreeBSD committer | BSD since 4.3-tahoe
Never attribute to malice what can adequately be explained by incompetence.
Hi
Back in the day a lot of us ran setups that locked to Loran or WWVB. Of the two, Loran seemed
to be the more common / more stable choice. We ran an Austron Loran device that did pretty well.
With WWVB, the answer was a strip chart recorder and working out what a wiggly line actually
was telling us.
Part of the “why” was our location. A master station for Loran was way closer than WWVB.
Since the master in a Loran chain transmits at much higher power than the other stations, that’s
the one you want to be near.
If we had been up in the northeastern US, WWVB happened to share 60KHz with other stations.
That tended to mess things up a bit. I’m glad we didn’t have to deal with that.
All of this was targeted at manually steering the house standard Rb to keep it on frequency. I was
amazed at just how little it drifted each month.
Bob
On Sep 23, 2025, at 11:51 AM, Michael Fowler via time-nuts time-nuts@lists.febo.com wrote:
Hi John,
These older for the 80s / 90s Spectracom Units did just what you are
looking for.
Unfortunately, they do not work anymore with WWVB as of the phase shift
modulation that was introduced in 2012
Working on a mod for to make them work now a days
here is a link to instruction manual for one of the 8164
https://manuals.repeater-builder.com/te-files/MISCELLANEOUS/Spectracom%208164%20Instruction.pdf
Best,
Mike Fowler
On Mon, Sep 22, 2025 at 10:01 AM john.haine--- via time-nuts <
time-nuts@lists.febo.com> wrote:
Does anyone know of designs for disciplined OCXOs that are referenced to
off-air, especially VLF, signals other than GPS/GNSS please? With modules
for the latter being so cheap this might seem pointless but there are some
potential advantages.
-
GPS reception at indoor locations where mechanical clocks need to
be
monitored is often (usually?) unavailable because of shadowing and building
absorbtion and it's usually inconvenient to run a cable.
-
GPS is increasingly likely to be jammed either by criminal elements
or "state actors".
-
VLF (the likes of WWVB / MSF / DCF77 / e-Loran) is more likely to
be
received indoors.
-
E-Loran is being tipped as an off-air time source to back up GPS
and
will become increasingly available.
-
There's the possibility of a multi-standard receiver that might
find
and lock to any available source, and potentially to several.
Obviously there are a lot of very cheap modules around to receive the
signals but these discard the carrier and just output the time-code logic
signal. I have seen a design for an MSF-locked standard in discrete
components and more recently an MSF receiver implemented as
direct-conversion SDR on a Raspberry Pi Pico which phase locks its internal
digital LO to the carrier but I suspect that its phase noise would be
pretty
ropey - really intended as a time, not frequency, source.
-
John Haine.
time-nuts mailing list -- time-nuts@lists.febo.com
To unsubscribe send an email to time-nuts-leave@lists.febo.com
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To unsubscribe send an email to time-nuts-leave@lists.febo.com
Once upon a time long ago, in a far distant place, I used to watch a signal transmitted by the USN in US, between 50 and 60Hz. This was received with a small, many turned, loop antenna, a preamp, and the 440A Mini. Ubiquitous® FFT analyzer. (That was all there was in those days). A few hundred watts of US located transmitter was unmistakably identifiable on the FFT display, between the 50 and 60 Hz peaks, with a minute or so of integration time. The "bloody" project was abandoned in favor of the current USN VLF transmissions.
Lester B Veenstra K1YCM MØYCM W8YCM 6Y6Y W8YCM/6Y 6Y8LV (Reformed USNSG CTM1)
lester@veenstras.com
452 Stable Ln
Keyser WV 26726 USA
GPS: 39.336826 N 78.982287 W (Google)
GPS: 39.33682 N 78.9823741 W (GPSDO)
Telephones:
Home: +1-304-289-6057
US cell +1-304-790-9192
Jamaica cell: +1-876-456-8898
-----Original Message-----
From: Deirdre O'Byrne via time-nuts [mailto:time-nuts@lists.febo.com]
Sent: Wednesday, September 24, 2025 6:00 AM
To: Discussion of precise time and frequency measurement
Cc: Deirdre O'Byrne
Subject: [time-nuts] Re: vlf-disciplined OCXO circuit
Anyone thought of, or implemented, a 50/60Hz VLF receiver?
Yes it would have horrible characteristics compared to a proper VLF
station, especially during power cuts. Personally I’m thinking more of
measuring the 50/60Hz “signal” than using it as a time source. And yes I’m
aware that it’s possible to do such measurements by plugging directly into
the grid, but I’d prefer not to do that for reasons.
time-nuts mailing list -- time-nuts@lists.febo.com
To unsubscribe send an email to time-nuts-leave@lists.febo.com
Bob Camp via time-nuts writes:
Part of the “why” was our location. A master station for Loran was way closer than WWVB.
Since the master in a Loran chain transmits at much higher power than the other stations, that’s
the one you want to be near.
Actually, that's not quite correct...
You wanted to track the master because it's Cs clocks were /only/
adjusted to track UTC, whereas the slaves adjusted their Cs's to track
the master as specified under the current circumstances.
Current circumstances included the ionosphere, amount of atmospheric
moisture, amount of ground moisture, snow/ice cover and in one rare
case "somebody built a steel bridge." [source: Prof. Dave Mills.]
It is true that the master transmits more power, nine pulses rather
than eight, but the slave code is balanced, the master code is not.
The imbalance makes the master signal sensitive to "almost perfectly
on frequency" CW signals, and the extra power is nowhere near
enough to compensate for that handicap.
(Even Dave had never been able to find out why the master code was
unbalanced and leaned towards it just being a typo.)
Both the slave adjustments and the CW RFI had taus in the
hour-day range, which was fine if you had a local Cs and not
so fine otherwise.
Poul-Henning
See also: https://phk.freebsd.dk/loran-c/theoretical_spectrum/
--
Poul-Henning Kamp | UNIX since Zilog Zeus 3.20
phk@FreeBSD.ORG | TCP/IP since RFC 956
FreeBSD committer | BSD since 4.3-tahoe
Never attribute to malice what can adequately be explained by incompetence.
Do I have this right? If the 60 KHz VLF time transmission is phase encoded, can that carrier be at all suitable for maintaining a gps-style standard at 60 KHz?
Don
----- Original Message -----
From: "Poul-Henning Kamp via time-nuts" time-nuts@lists.febo.com
To: "Discussion of precise time and frequency measurement" time-nuts@lists.febo.com
Cc: "Poul-Henning Kamp" phk@phk.freebsd.dk
Sent: Wednesday, September 24, 2025 11:57:43 AM
Subject: [time-nuts] Re: vlf-disciplined OCXO circuit
Bob Camp via time-nuts writes:
Part of the “why” was our location. A master station for Loran was way closer than WWVB.
Since the master in a Loran chain transmits at much higher power than the other stations, that’s
the one you want to be near.
Actually, that's not quite correct...
You wanted to track the master because it's Cs clocks were /only/
adjusted to track UTC, whereas the slaves adjusted their Cs's to track
the master as specified under the current circumstances.
Current circumstances included the ionosphere, amount of atmospheric
moisture, amount of ground moisture, snow/ice cover and in one rare
case "somebody built a steel bridge." [source: Prof. Dave Mills.]
It is true that the master transmits more power, nine pulses rather
than eight, but the slave code is balanced, the master code is not.
The imbalance makes the master signal sensitive to "almost perfectly
on frequency" CW signals, and the extra power is nowhere near
enough to compensate for that handicap.
(Even Dave had never been able to find out why the master code was
unbalanced and leaned towards it just being a typo.)
Both the slave adjustments and the CW RFI had taus in the
hour-day range, which was fine if you had a local Cs and not
so fine otherwise.
Poul-Henning
See also: https://phk.freebsd.dk/loran-c/theoretical_spectrum/
--
Poul-Henning Kamp | UNIX since Zilog Zeus 3.20
phk@FreeBSD.ORG | TCP/IP since RFC 956
FreeBSD committer | BSD since 4.3-tahoe
Never attribute to malice what can adequately be explained by incompetence.
time-nuts mailing list -- time-nuts@lists.febo.com
To unsubscribe send an email to time-nuts-leave@lists.febo.com
Don Latham
PO Box 404,
Frenchtown, MT, 59846
406-626-4304
Am 2025-09-24 12:00, schrieb Deirdre O'Byrne via time-nuts:
Anyone thought of, or implemented, a 50/60Hz VLF receiver?
Yes it would have horrible characteristics compared to a proper VLF
station, especially during power cuts. Personally I’m thinking more of
measuring the 50/60Hz “signal” than using it as a time source. And yes
I’m
aware that it’s possible to do such measurements by plugging directly
into
the grid, but I’d prefer not to do that for reasons.
AC power does not work as a frequency reference because the frequency
is used to synchronize the many generators, so it is a moving target.
It is not always as bad as that:
<
https://www.flickr.com/photos/137684711@N07/38870750440/in/datetaken/
because they could not work out in 2018 who was responsible for blind
power
compensation for some en/exclaves on the Balkans. That made wall clocks
lag some minutes.
Signal preparation was done with a small 5V mains transformer +
attenuator.
The SR-620 did survive. :-)
cheers, Gerhard
In the 1980's, on SSBN submarines, we got a new sensor to track LORAN-C.
This replaced the WPN-3.
We could track traditional multi station LORAN-C, but, we could also
track Phase shift LORAN-C.
While in port we had to track any LORAN-C station for a period of time.
From this we computed the frequency drift of our on board Cesium standard.
This drift rate was entered into the computer.
I do not remember where we got the relative drift rates of the different
LORAN-C stations from, but, this data was also entered into the computer.
From this we could track any two Loran-C stations with our onboard
standard representing the third station.
We did not have to track any master station.
This was one of the fix sources that we use to determine the position of
the submarine to ensure we were within "firing specs" if the order came
to launch.
Glenn Little
ETCS(SS) USN Ret
On 9/24/2025 1:57 PM, Poul-Henning Kamp via time-nuts wrote:
Bob Camp via time-nuts writes:
Part of the “why” was our location. A master station for Loran was way closer than WWVB.
Since the master in a Loran chain transmits at much higher power than the other stations, that’s
the one you want to be near.
Actually, that's not quite correct...
You wanted to track the master because it's Cs clocks were /only/
adjusted to track UTC, whereas the slaves adjusted their Cs's to track
the master as specified under the current circumstances.
Current circumstances included the ionosphere, amount of atmospheric
moisture, amount of ground moisture, snow/ice cover and in one rare
case "somebody built a steel bridge." [source: Prof. Dave Mills.]
It is true that the master transmits more power, nine pulses rather
than eight, but the slave code is balanced, the master code is not.
The imbalance makes the master signal sensitive to "almost perfectly
on frequency" CW signals, and the extra power is nowhere near
enough to compensate for that handicap.
(Even Dave had never been able to find out why the master code was
unbalanced and leaned towards it just being a typo.)
Both the slave adjustments and the CW RFI had taus in the
hour-day range, which was fine if you had a local Cs and not
so fine otherwise.
Poul-Henning
See also: https://phk.freebsd.dk/loran-c/theoretical_spectrum/
--
Glenn Little ARRL Technical Specialist QCWA LM 28417
Amateur Callsign: WB4UIV wb4uiv@arrl.net AMSAT LM 2178
QTH: Goose Creek, SC USA (EM92xx) USSVI, FRA, NRA-LM ARRL TAPR
"It is not the class of license that the Amateur holds but the class
of the Amateur that holds the license"
sure. The PLL tracking loop in the receiver tracks the carrier phase. A typical Costas loop will work nicely for BPSK.
On Wed, 24 Sep 2025 15:23:57 -0400 (EDT), Don Latham via time-nuts time-nuts@lists.febo.com wrote:
Do I have this right? If the 60 KHz VLF time transmission is phase encoded, can that carrier be at all suitable for maintaining a gps-style standard at 60 KHz?
Don
----- Original Message -----
From: "Poul-Henning Kamp via time-nuts"
To: "Discussion of precise time and frequency measurement"
Cc: "Poul-Henning Kamp"
Sent: Wednesday, September 24, 2025 11:57:43 AM
Subject: [time-nuts] Re: vlf-disciplined OCXO circuit
Bob Camp via time-nuts writes:
Part of the “why” was our location. A master station for Loran was way closer than WWVB.
Since the master in a Loran chain transmits at much higher power than the other stations, that’s
the one you want to be near.
Actually, that's not quite correct...
You wanted to track the master because it's Cs clocks were /only/
adjusted to track UTC, whereas the slaves adjusted their Cs's to track
the master as specified under the current circumstances.
Current circumstances included the ionosphere, amount of atmospheric
moisture, amount of ground moisture, snow/ice cover and in one rare
case "somebody built a steel bridge." [source: Prof. Dave Mills.]
It is true that the master transmits more power, nine pulses rather
than eight, but the slave code is balanced, the master code is not.
The imbalance makes the master signal sensitive to "almost perfectly
on frequency" CW signals, and the extra power is nowhere near
enough to compensate for that handicap.
(Even Dave had never been able to find out why the master code was
unbalanced and leaned towards it just being a typo.)
Both the slave adjustments and the CW RFI had taus in the
hour-day range, which was fine if you had a local Cs and not
so fine otherwise.
Poul-Henning
See also: https://phk.freebsd.dk/loran-c/theoretical_spectrum/
--
Poul-Henning Kamp | UNIX since Zilog Zeus 3.20
phk@FreeBSD.ORG | TCP/IP since RFC 956
FreeBSD committer | BSD since 4.3-tahoe
Never attribute to malice what can adequately be explained by incompetence.
time-nuts mailing list -- time-nuts@lists.febo.com
To unsubscribe send an email to time-nuts-leave@lists.febo.com
Don Latham
PO Box 404,
Frenchtown, MT, 59846
406-626-4304
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To unsubscribe send an email to time-nuts-leave@lists.febo.com
There kind of is a difference, isn’t there, from a FFT or digitally mixing with a sinusoid (generated however). If it’s a pure sinusoid as input, not much difference, but if there’s other signals, all these techniques have different frequency domain passbands (i.e. compare rectangular window vs Hamming vs Blackman, etc.) or, the equivalent, different relative weighting of the time domain samples in the current estimate of the amplitude and phase.
”other signals” here could be things like noise from quantization, from whatever your input receiver looks like, numerical issues (e.g. if you increment an integer counter, then multiply by frequency, then call sin/cos, you’ll get different answers than, say, an incremental CORDIC generator).
For a lot of applications “it doesn’t make any difference” - but here on time-nuts, we do worry about 1E-12 kinds of uncertainties (sometimes… when I set my alarm clock, I’m not worried about 60 second uncertainty, which is 1e-2 kinds of uncertainty)
On Tue, 23 Sep 2025 20:17:34 +0200, dschuecker via time-nuts time-nuts@lists.febo.com wrote:
Hi,
I calculate the amplitude and phase of a sampled signal with the
goertzel algo https://en.wikipedia.org/wiki/Goertzel_algorithm.
It calculates the discrete fourier transform, i.e. amplitude and phase
for a single frequency at the cost of one multiply per sample.
No need to buffer samples.
const float fcc=2.0f*cosf(M_PI/16.0f); // signal freq / sample freq
= 1/32
fq2=0.0f;fq1=0.0f;
for(nn=0;nn0){ // got it
dd=ADC_DR; // get sample
fq0=fq1*fcc-fq2+dd; // undamped second order recursive
filter
fq2=fq1;fq1=fq0;
break;
}
}
}
// unwrap result
double complex z =(float)(fq1-fq2cosf(M_PI/16))+ // Real part
I(float)(fq2*sinf(M_PI/16)); // imaginary part
Cheers
Detlef
Am 22.09.2025 um 19:12 schrieb Poul-Henning Kamp via time-nuts:
john.haine--- via time-nuts writes:
Does anyone know of designs for disciplined OCXOs that are referenced to
off-air, especially VLF, signals other than GPS/GNSS please?
I played a lot with that many years ago.
You can do a lot with a small microcontroller with an ADC which does
something like a million samples per second.
My favourite was this:
Run the ADC at 1 MHz from your reference clock, sum the samples into
a 1000 long circular buffer:
alpha = 1e-5; // play with this
n = 0
while (1) {
x = get_sample()
buffer[x] += (x - buffer[x]) * alpha
x +=1
if x == 1000:
x = 0;
}
This buffer now holds the output of a 1kHz combfilter over your input
signal, which means you can extract the phase and amplitude of any
signal on a multiple of 1kHz from that single buffer.
Simply multiply the 1000 samples in the buffer with sin() and cos()
of your desired frequency and calculate the magnitude and angle of
the resulting vector:
fx = 60 // 60 khz
ssin = 0.0
scos = 0.0
for i in range(1000):
ssin += sin(2 * PI * i / 1000 * buffer[i]
scos += sin(2 * PI * i / 1000 * buffer[i]
ampl = hypot(ssin, scos)
angle = atan2(ssin, scos)
Now do that for as many signals as you want, and steer your
reference clock accordingly.
If you want to also track signals on half kHz (like DCF77), make
the buffer 2000 samples.
One paticularly interesting case is using a buffer exactly one
second long (=1 million in this example), that would allow you
to extract both the phase and the modulation from signals
like DCF77 and WWVB
The Pi2 has a 500kS ADC and two CPU cores, so it is nearly perfect,
provided you can lock the frequency to your house standard - or
just use an external ADC, plenty of eval boards out there.
I didn't document very much of the fun I had, but there is
some stuff here:
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Many thanks for all the responses! A few random thoughts in response.
- I'm based in the eastern UK near Cambridge. Anthorn where the only
"nearby" eLoran station, and also MSF, is 370 km / 230 miles. Mainflingen
(DCF77) is 666 km/ 535 miles. DCF77 signals are largely available in the UK
and most of the cheap RC clocks use them. However in some locations they
are a bit dodgy - for example Bristol University where I sometimes work put
RC clocks in most public areas and they are often wrong or spend all their
time trying to re-sync. MSF and eLoran would work better I suspect though
consumer MSF receivers are less available as the market is more UK-based
while DCF 77 works over most of Europe. - Using an online SDR based in Bedford which is 40 miles of so west of
here eLoran from Anthorn is quite strong but DCF and MSF not observable -
that may be the antennas used as much as anything. - Unlike WWVB MSF does not have a phase modulation component, and its
carrier is switched off completely by the keying. Phase locking to its
carrier therefore has to gate the PLL but there is one design at least that
does this. - DCF77 has a spread-spectrum phase mod component and its carrier is
not switched off completely. This makes it much more robust if a properly
designed coherent receiver is used and I've seen one design that does this
in software. Unfortunately - I wasn't aware that eLoran has a phase mod component, I need to read
up on its waveform design more. But is being toted as a GPS/GNSS backup for
timing in Europe where we have a large and unfriendly neighbour.
I'll study the responses in more detail and follow up if this project makes
it to the starting blocks.
Thanks everyone for your input!
- John
-----Original Message-----
From: john.haine--- via time-nuts time-nuts@lists.febo.com
Sent: 22 September 2025 10:20
To: 'Discussion of precise time and frequency measurement'
time-nuts@lists.febo.com
Cc: john.haine@haine-online.net
Subject: [time-nuts] vlf-disciplined OCXO circuit
Does anyone know of designs for disciplined OCXOs that are referenced to
off-air, especially VLF, signals other than GPS/GNSS please? With modules
for the latter being so cheap this might seem pointless but there are some
potential advantages.
- GPS reception at indoor locations where mechanical clocks need to be
monitored is often (usually?) unavailable because of shadowing and building
absorbtion and it's usually inconvenient to run a cable. - GPS is increasingly likely to be jammed either by criminal elements
or "state actors". - VLF (the likes of WWVB / MSF / DCF77 / e-Loran) is more likely to be
received indoors. - E-Loran is being tipped as an off-air time source to back up GPS and
will become increasingly available. - There's the possibility of a multi-standard receiver that might find
and lock to any available source, and potentially to several.
Obviously there are a lot of very cheap modules around to receive the
signals but these discard the carrier and just output the time-code logic
signal. I have seen a design for an MSF-locked standard in discrete
components and more recently an MSF receiver implemented as
direct-conversion SDR on a Raspberry Pi Pico which phase locks its internal
digital LO to the carrier but I suspect that its phase noise would be pretty
ropey - really intended as a time, not frequency, source.
- John Haine.
time-nuts mailing list -- time-nuts@lists.febo.com To unsubscribe send an
email to time-nuts-leave@lists.febo.com
Hi
Yes and no. As noted earlier you can double the carrier and lock to that.
One basic issue with 60 KHz is phase slips. You really want to ID the “cycle” you are tracking. Loran gear does this all the time. Doing that with WWVB a bit of a challenge.
How much of an issue are phase slips on WWVB? It depends a lot on where you are in the US. If we’re talking about MSF on 60 KHz, most folks in the UK aren’t quite as far from the transmitter as the major population centers in the US are from WWVB.
I’m guessing we’re talking about WWVB and you are somewhere in the US. If you are in the north east, decoding the modulation has the added benefit of sorting out interference from MSF. It also helps a bit with sorting out all of the RFI from the vast number of things running 60KHz switching power supplies (and no filtering ….).
To have something reasonably reliable you probably will be decoding the modulation. These days some sort of SDR is likely to be involved along with some processing somewhere. Tossing in modulation decoding is “just code” :) :).
Bob
On Sep 24, 2025, at 3:23 PM, Don Latham via time-nuts time-nuts@lists.febo.com wrote:
Do I have this right? If the 60 KHz VLF time transmission is phase encoded, can that carrier be at all suitable for maintaining a gps-style standard at 60 KHz?
Don
----- Original Message -----
From: "Poul-Henning Kamp via time-nuts" time-nuts@lists.febo.com
To: "Discussion of precise time and frequency measurement" time-nuts@lists.febo.com
Cc: "Poul-Henning Kamp" phk@phk.freebsd.dk
Sent: Wednesday, September 24, 2025 11:57:43 AM
Subject: [time-nuts] Re: vlf-disciplined OCXO circuit
Bob Camp via time-nuts writes:
Part of the “why” was our location. A master station for Loran was way closer than WWVB.
Since the master in a Loran chain transmits at much higher power than the other stations, that’s
the one you want to be near.
Actually, that's not quite correct...
You wanted to track the master because it's Cs clocks were /only/
adjusted to track UTC, whereas the slaves adjusted their Cs's to track
the master as specified under the current circumstances.
Current circumstances included the ionosphere, amount of atmospheric
moisture, amount of ground moisture, snow/ice cover and in one rare
case "somebody built a steel bridge." [source: Prof. Dave Mills.]
It is true that the master transmits more power, nine pulses rather
than eight, but the slave code is balanced, the master code is not.
The imbalance makes the master signal sensitive to "almost perfectly
on frequency" CW signals, and the extra power is nowhere near
enough to compensate for that handicap.
(Even Dave had never been able to find out why the master code was
unbalanced and leaned towards it just being a typo.)
Both the slave adjustments and the CW RFI had taus in the
hour-day range, which was fine if you had a local Cs and not
so fine otherwise.
Poul-Henning
See also: https://phk.freebsd.dk/loran-c/theoretical_spectrum/
--
Poul-Henning Kamp | UNIX since Zilog Zeus 3.20
phk@FreeBSD.ORG | TCP/IP since RFC 956
FreeBSD committer | BSD since 4.3-tahoe
Never attribute to malice what can adequately be explained by incompetence.
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PO Box 404,
Frenchtown, MT, 59846
406-626-4304
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Hi,
I tried with a 262144 samples 'pure' ( 8 bytes for a double ) sinewave.
1.310720000000000e+05 - 1.314745628841594e-07i for FFT
1.310730000003061e+05 + 1.651948562937711e-06i for Goertzel
1.310719999999999e+05 - 3.865352482534945e-11i for mixing with a
complex 'carrier'
Now the sine with additional ~ -17dB broadband white noise
1.310543992827178e+05 - 1.285247212381880e+01i FFT
1.310554051787366e+05 - 1.285247050053514e+01i Goertzel
1.310543992827178e+05 + 1.285247199235141e+01i Mixing
I cannot see great difference in the noise impact. But that is only a guess.
I never had numerical issues with FFT or Goertzel or mixing. For 1E-12 a
32-Bit float is not sufficient, a 64-Bit is. For very sharp IIR-filters
you need more than 64 Bit.
Cheers
Detlef
clear
n=2^18;
sig=cos(2pi(n/32)(0:n-1)/n);
noi=0.1randn(1,n);
sigsig.'
noinoi.'
10log10((sigsig.')/(noi*noi.'))
sig=sig+noi;
spsig=fft(sig);
format long
spsig(8193)
M_PI = pi;
fcc=2.0*cos(M_PI/16.0);
fq2=0.0;fq1=0.0;
noi1 =[noi noi(1)];
for(k=1:n+1)
%dd=sig(k);
dd=cos(2pi(n/32)(k-1)/n)+noi1(k);
fq0=fq1fcc-fq2+dd;
fq2=fq1;fq1=fq0;
end
z =(fq1-fq2cos(M_PI/16))+ i(fq2*sin(M_PI/16))
sigexp(i(2pi(n/32)*(0:n-1)/n)).'
return
Am 25.09.2025 um 11:49 schrieb Jim Lux via time-nuts:
There kind of is a difference, isn’t there, from a FFT or digitally mixing with a sinusoid (generated however). If it’s a pure sinusoid as input, not much difference, but if there’s other signals, all these techniques have different frequency domain passbands (i.e. compare rectangular window vs Hamming vs Blackman, etc.) or, the equivalent, different relative weighting of the time domain samples in the current estimate of the amplitude and phase.
”other signals” here could be things like noise from quantization, from whatever your input receiver looks like, numerical issues (e.g. if you increment an integer counter, then multiply by frequency, then call sin/cos, you’ll get different answers than, say, an incremental CORDIC generator).
For a lot of applications “it doesn’t make any difference” - but here on time-nuts, we do worry about 1E-12 kinds of uncertainties (sometimes… when I set my alarm clock, I’m not worried about 60 second uncertainty, which is 1e-2 kinds of uncertainty)
On Tue, 23 Sep 2025 20:17:34 +0200, dschuecker via time-nuts time-nuts@lists.febo.com wrote:
Hi,
I calculate the amplitude and phase of a sampled signal with the
goertzel algo https://en.wikipedia.org/wiki/Goertzel_algorithm.
It calculates the discrete fourier transform, i.e. amplitude and phase
for a single frequency at the cost of one multiply per sample.
No need to buffer samples.
const float fcc=2.0f*cosf(M_PI/16.0f); // signal freq / sample freq
= 1/32
fq2=0.0f;fq1=0.0f;
for(nn=0;nn0){ // got it
dd=ADC_DR; // get sample
fq0=fq1*fcc-fq2+dd; // undamped second order recursive
filter
fq2=fq1;fq1=fq0;
break;
}
}
}
// unwrap result
double complex z =(float)(fq1-fq2cosf(M_PI/16))+ // Real part
I(float)(fq2*sinf(M_PI/16)); // imaginary part
Cheers
Detlef
Am 22.09.2025 um 19:12 schrieb Poul-Henning Kamp via time-nuts:
john.haine--- via time-nuts writes:
Does anyone know of designs for disciplined OCXOs that are referenced to
off-air, especially VLF, signals other than GPS/GNSS please?
I played a lot with that many years ago.You can do a lot with a small microcontroller with an ADC which does
something like a million samples per second.My favourite was this:
Run the ADC at 1 MHz from your reference clock, sum the samples into
a 1000 long circular buffer:alpha = 1e-5; // play with this
n = 0
while (1) {
x = get_sample()
buffer[x] += (x - buffer[x]) * alpha
x +=1
if x == 1000:
x = 0;
}This buffer now holds the output of a 1kHz combfilter over your input
signal, which means you can extract the phase and amplitude of any
signal on a multiple of 1kHz from that single buffer.Simply multiply the 1000 samples in the buffer with sin() and cos()
of your desired frequency and calculate the magnitude and angle of
the resulting vector:fx = 60 // 60 khz
ssin = 0.0
scos = 0.0
for i in range(1000):
ssin += sin(2 * PI * i / 1000 * buffer[i]
scos += sin(2 * PI * i / 1000 * buffer[i]
ampl = hypot(ssin, scos)
angle = atan2(ssin, scos)Now do that for as many signals as you want, and steer your
reference clock accordingly.If you want to also track signals on half kHz (like DCF77), make
the buffer 2000 samples.One paticularly interesting case is using a buffer exactly one
second long (=1 million in this example), that would allow you
to extract both the phase and the modulation from signals
like DCF77 and WWVBThe Pi2 has a 500kS ADC and two CPU cores, so it is nearly perfect,
provided you can lock the frequency to your house standard - or
just use an external ADC, plenty of eval boards out there.I didn't document very much of the fun I had, but there is
some stuff here:
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In case it's relevant ..
I'm in Bedford (I'm not the operator of that SDR though).
I've got a little heap of receivers in case they're interesting for
comparison, loan etc. A radio-4-LW-disciplined oscillator, a Thunderbolt, a
heap of KS-24361 ref 0 GPSDOs, and (not connected) an MSF time receiver, a
Pluto SDR and a Rubidium oscillator.
I also have some random MSF and DCF77 clocks around. The MSF clocks work
pretty well but the DCF77 kitchen clock jumps a couple of hours sometimes
then jumps back a couple of days later. The MSF clock used to get upset by
a VGA monitor when it was close by. Doesn't seem to mind modern LCD ones.
I believe the LW-disciplined oscillator is on borrowed time. The device is
fine but the LW transmitter is in danger of being retired any time soon.
On Thu, Sep 25, 2025 at 1:35 PM john.haine--- via time-nuts <
time-nuts@lists.febo.com> wrote:
Many thanks for all the responses! A few random thoughts in response.
-
I'm based in the eastern UK near Cambridge. Anthorn where the only
"nearby" eLoran station, and also MSF, is 370 km / 230 miles. Mainflingen
(DCF77) is 666 km/ 535 miles. DCF77 signals are largely available in the
UK
and most of the cheap RC clocks use them. However in some locations they
are a bit dodgy - for example Bristol University where I sometimes work put
RC clocks in most public areas and they are often wrong or spend all their
time trying to re-sync. MSF and eLoran would work better I suspect though
consumer MSF receivers are less available as the market is more UK-based
while DCF 77 works over most of Europe.
-
Using an online SDR based in Bedford which is 40 miles of so west
of
here eLoran from Anthorn is quite strong but DCF and MSF not observable -
that may be the antennas used as much as anything.
-
Unlike WWVB MSF does not have a phase modulation component, and its
carrier is switched off completely by the keying. Phase locking to its
carrier therefore has to gate the PLL but there is one design at least that
does this.
-
DCF77 has a spread-spectrum phase mod component and its carrier is
not switched off completely. This makes it much more robust if a properly
designed coherent receiver is used and I've seen one design that does this
in software. Unfortunately
-
I wasn't aware that eLoran has a phase mod component, I need to
read
up on its waveform design more. But is being toted as a GPS/GNSS backup
for
timing in Europe where we have a large and unfriendly neighbour.
I'll study the responses in more detail and follow up if this project makes
it to the starting blocks.
Thanks everyone for your input!
- John
-----Original Message-----
From: john.haine--- via time-nuts time-nuts@lists.febo.com
Sent: 22 September 2025 10:20
To: 'Discussion of precise time and frequency measurement'
time-nuts@lists.febo.com
Cc: john.haine@haine-online.net
Subject: [time-nuts] vlf-disciplined OCXO circuit
Does anyone know of designs for disciplined OCXOs that are referenced to
off-air, especially VLF, signals other than GPS/GNSS please? With modules
for the latter being so cheap this might seem pointless but there are some
potential advantages.
-
GPS reception at indoor locations where mechanical clocks need to
be
monitored is often (usually?) unavailable because of shadowing and building
absorbtion and it's usually inconvenient to run a cable.
-
GPS is increasingly likely to be jammed either by criminal elements
or "state actors".
-
VLF (the likes of WWVB / MSF / DCF77 / e-Loran) is more likely to
be
received indoors.
-
E-Loran is being tipped as an off-air time source to back up GPS
and
will become increasingly available.
-
There's the possibility of a multi-standard receiver that might
find
and lock to any available source, and potentially to several.
Obviously there are a lot of very cheap modules around to receive the
signals but these discard the carrier and just output the time-code logic
signal. I have seen a design for an MSF-locked standard in discrete
components and more recently an MSF receiver implemented as
direct-conversion SDR on a Raspberry Pi Pico which phase locks its internal
digital LO to the carrier but I suspect that its phase noise would be
pretty
ropey - really intended as a time, not frequency, source.
-
John Haine.
time-nuts mailing list -- time-nuts@lists.febo.com To unsubscribe send an
email to time-nuts-leave@lists.febo.com
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In addition to all that has been said above, the WWVB phase modulation
schema always sends the last bit (last second) of the minute as a
known/fixed phase reference, so you do have a way to detect half cycle
phase slips, or to do a gated phase reference for controlling a secondary
oscillator.
--- Graham
==
On Thu, Sep 25, 2025 at 12:10 PM Bob Camp via time-nuts <
time-nuts@lists.febo.com> wrote:
Hi
Yes and no. As noted earlier you can double the carrier and lock to that.
One basic issue with 60 KHz is phase slips. You really want to ID the
“cycle” you are tracking. Loran gear does this all the time. Doing that
with WWVB a bit of a challenge.
How much of an issue are phase slips on WWVB? It depends a lot on where
you are in the US. If we’re talking about MSF on 60 KHz, most folks in the
UK aren’t quite as far from the transmitter as the major population centers
in the US are from WWVB.
I’m guessing we’re talking about WWVB and you are somewhere in the US. If
you are in the north east, decoding the modulation has the added benefit of
sorting out interference from MSF. It also helps a bit with sorting out all
of the RFI from the vast number of things running 60KHz switching power
supplies (and no filtering ….).
To have something reasonably reliable you probably will be decoding the
modulation. These days some sort of SDR is likely to be involved along with
some processing somewhere. Tossing in modulation decoding is “just code” :)
:).
Bob
On Sep 24, 2025, at 3:23 PM, Don Latham via time-nuts <
time-nuts@lists.febo.com> wrote:
Do I have this right? If the 60 KHz VLF time transmission is phase
encoded, can that carrier be at all suitable for maintaining a gps-style
standard at 60 KHz?
Don
----- Original Message -----
From: "Poul-Henning Kamp via time-nuts" time-nuts@lists.febo.com
To: "Discussion of precise time and frequency measurement" <
Cc: "Poul-Henning Kamp" phk@phk.freebsd.dk
Sent: Wednesday, September 24, 2025 11:57:43 AM
Subject: [time-nuts] Re: vlf-disciplined OCXO circuit
Bob Camp via time-nuts writes:
Part of the “why” was our location. A master station for Loran was
way closer than WWVB.
Since the master in a Loran chain transmits at much higher power than
the other stations, that’s
the one you want to be near.
Actually, that's not quite correct...
You wanted to track the master because it's Cs clocks were /only/
adjusted to track UTC, whereas the slaves adjusted their Cs's to track
the master as specified under the current circumstances.
Current circumstances included the ionosphere, amount of atmospheric
moisture, amount of ground moisture, snow/ice cover and in one rare
case "somebody built a steel bridge." [source: Prof. Dave Mills.]
It is true that the master transmits more power, nine pulses rather
than eight, but the slave code is balanced, the master code is not.
The imbalance makes the master signal sensitive to "almost perfectly
on frequency" CW signals, and the extra power is nowhere near
enough to compensate for that handicap.
(Even Dave had never been able to find out why the master code was
unbalanced and leaned towards it just being a typo.)
Both the slave adjustments and the CW RFI had taus in the
hour-day range, which was fine if you had a local Cs and not
so fine otherwise.
Poul-Henning
See also: https://phk.freebsd.dk/loran-c/theoretical_spectrum/
--
Poul-Henning Kamp | UNIX since Zilog Zeus 3.20
phk@FreeBSD.ORG | TCP/IP since RFC 956
FreeBSD committer | BSD since 4.3-tahoe
Never attribute to malice what can adequately be explained by
incompetence.
time-nuts mailing list -- time-nuts@lists.febo.com
To unsubscribe send an email to time-nuts-leave@lists.febo.com
Don Latham
PO Box 404,
Frenchtown, MT, 59846
406-626-4304
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Now put some noise in the system? Or a quantized input (which is in some sense just another noise)? Or an interfering signal with a frequency a bit off from the signal of interest.
That’s where the “window” or “filter shape” starts to make a difference,
On Thu, 25 Sep 2025 16:20:37 +0200, dschuecker via time-nuts time-nuts@lists.febo.com wrote:
Hi,
I tried with a 262144 samples 'pure' ( 8 bytes for a double ) sinewave.
1.310720000000000e+05 - 1.314745628841594e-07i for FFT
1.310730000003061e+05 + 1.651948562937711e-06i for Goertzel
1.310719999999999e+05 - 3.865352482534945e-11i for mixing with a
complex 'carrier'
Now the sine with additional ~ -17dB broadband white noise
1.310543992827178e+05 - 1.285247212381880e+01i FFT
1.310554051787366e+05 - 1.285247050053514e+01i Goertzel
1.310543992827178e+05 + 1.285247199235141e+01i Mixing
I cannot see great difference in the noise impact. But that is only a guess.
I never had numerical issues with FFT or Goertzel or mixing. For 1E-12 a
32-Bit float is not sufficient, a 64-Bit is. For very sharp IIR-filters
you need more than 64 Bit.
Cheers
Detlef
clear
n=2^18;
sig=cos(2pi(n/32)(0:n-1)/n);
noi=0.1randn(1,n);
sigsig.'
noinoi.'
10log10((sigsig.')/(noi*noi.'))
sig=sig+noi;
spsig=fft(sig);
format long
spsig(8193)
M_PI = pi;
fcc=2.0*cos(M_PI/16.0);
fq2=0.0;fq1=0.0;
noi1 =[noi noi(1)];
for(k=1:n+1)
%dd=sig(k);
dd=cos(2pi(n/32)(k-1)/n)+noi1(k);
fq0=fq1fcc-fq2+dd;
fq2=fq1;fq1=fq0;
end
z =(fq1-fq2cos(M_PI/16))+ i(fq2*sin(M_PI/16))
sigexp(i(2pi(n/32)*(0:n-1)/n)).'
return
Am 25.09.2025 um 11:49 schrieb Jim Lux via time-nuts:
There kind of is a difference, isn’t there, from a FFT or digitally mixing with a sinusoid (generated however). If it’s a pure sinusoid as input, not much difference, but if there’s other signals, all these techniques have different frequency domain passbands (i.e. compare rectangular window vs Hamming vs Blackman, etc.) or, the equivalent, different relative weighting of the time domain samples in the current estimate of the amplitude and phase.
”other signals” here could be things like noise from quantization, from whatever your input receiver looks like, numerical issues (e.g. if you increment an integer counter, then multiply by frequency, then call sin/cos, you’ll get different answers than, say, an incremental CORDIC generator).
For a lot of applications “it doesn’t make any difference” - but here on time-nuts, we do worry about 1E-12 kinds of uncertainties (sometimes… when I set my alarm clock, I’m not worried about 60 second uncertainty, which is 1e-2 kinds of uncertainty)
On Tue, 23 Sep 2025 20:17:34 +0200, dschuecker via time-nuts wrote:
Hi,
I calculate the amplitude and phase of a sampled signal with the
goertzel algo https://en.wikipedia.org/wiki/Goertzel_algorithm.
It calculates the discrete fourier transform, i.e. amplitude and phase
for a single frequency at the cost of one multiply per sample.
No need to buffer samples.
const float fcc=2.0f*cosf(M_PI/16.0f); // signal freq / sample freq
= 1/32
fq2=0.0f;fq1=0.0f;
for(nn=0;nn0){ // got it
dd=ADC_DR; // get sample
fq0=fq1*fcc-fq2+dd; // undamped second order recursive
filter
fq2=fq1;fq1=fq0;
break;
}
}
}
// unwrap result
double complex z =(float)(fq1-fq2cosf(M_PI/16))+ // Real part
I(float)(fq2*sinf(M_PI/16)); // imaginary part
Cheers
Detlef
Am 22.09.2025 um 19:12 schrieb Poul-Henning Kamp via time-nuts:
john.haine--- via time-nuts writes:
Does anyone know of designs for disciplined OCXOs that are referenced to
off-air, especially VLF, signals other than GPS/GNSS please?
I played a lot with that many years ago.
You can do a lot with a small microcontroller with an ADC which does
something like a million samples per second.
My favourite was this:
Run the ADC at 1 MHz from your reference clock, sum the samples into
a 1000 long circular buffer:
alpha = 1e-5; // play with this
n = 0
while (1) {
x = get_sample()
buffer[x] += (x - buffer[x]) * alpha
x +=1
if x == 1000:
x = 0;
}
This buffer now holds the output of a 1kHz combfilter over your input
signal, which means you can extract the phase and amplitude of any
signal on a multiple of 1kHz from that single buffer.
Simply multiply the 1000 samples in the buffer with sin() and cos()
of your desired frequency and calculate the magnitude and angle of
the resulting vector:
fx = 60 // 60 khz
ssin = 0.0
scos = 0.0
for i in range(1000):
ssin += sin(2 * PI * i / 1000 * buffer[i]
scos += sin(2 * PI * i / 1000 * buffer[i]
ampl = hypot(ssin, scos)
angle = atan2(ssin, scos)
Now do that for as many signals as you want, and steer your
reference clock accordingly.
If you want to also track signals on half kHz (like DCF77), make
the buffer 2000 samples.
One paticularly interesting case is using a buffer exactly one
second long (=1 million in this example), that would allow you
to extract both the phase and the modulation from signals
like DCF77 and WWVB
The Pi2 has a 500kS ADC and two CPU cores, so it is nearly perfect,
provided you can lock the frequency to your house standard - or
just use an external ADC, plenty of eval boards out there.
I didn't document very much of the fun I had, but there is
some stuff here:
--
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Hi
There are an unfortunate lot of things out there that can take out a 60KHz VLF signal. For cost
reasons, 60kHz is what a lot of low end switchers run at. They are not controlled in any way so
that’s a very approximate number. Since they are typically hooked to the power line, they have a
fine “antenna” to work with. If you count the number of this and that in the typical home, there might
well be a hundred devices with switchers in them. They won’t all be on at once. All it takes is one
“bad one” running every so often to really make a mess.
If you grab one of these things and take a look with a spectrum analyzer: You typically see a pretty
big spike at whatever frequency it happens to like today as well as harmonics of that frequency. In
addition, there is a fairly broad set of “crud sidebands” that cover a wide range at a lower level. It
also should be noted that there are lots of ways they do this (even just at 60 KHz). Each one
has it’s own “signature”.
If you are way out on a farm somewhere, you might only have to deal with your own devices. In
a typical urban setting, you have a lot more than just your own devices to worry about.
Not every “bad one” will be equally bad. Some will bother things in a pretty small area. Others just
might cover a pretty wide area.
There’s also somewhat more difficult things that do come up. This or that device is hooked to
something stable. It just happens to have a spur at 60KHz. This is not very common. The issue
here is that the “threat signal” is a pretty stable carrier. Somebody putting up a local “60 KHz home
transmitter” to run their wall clocks? It does indeed happen.
Since this is all a bit random, what works today may not work tomorrow. Equally, that gizmo that
made it impossible for months just might burn out. Putting a lot of time and money into something
like this only to find it destroyed by “something” is not a good outcome.
Are there rules ( ok laws) about this stuff? Sure there are. None of this should be going on. Much
of the compliance testing and certification is done in a pretty informal manner (there is no outside
lab involved). Some folks will always be a bit less worried about this than others.
From doing this in the past, I’d plan for an outdoor antenna. Get it as far from the power lines and
any dwellings as you can. Even with an indoor antenna, getting it as far from power and “other stuff”
is a really good idea.
Fun!!
Bob
On Sep 25, 2025, at 1:33 PM, Adrian Godwin via time-nuts time-nuts@lists.febo.com wrote:
In case it's relevant ..
I'm in Bedford (I'm not the operator of that SDR though).
I've got a little heap of receivers in case they're interesting for
comparison, loan etc. A radio-4-LW-disciplined oscillator, a Thunderbolt, a
heap of KS-24361 ref 0 GPSDOs, and (not connected) an MSF time receiver, a
Pluto SDR and a Rubidium oscillator.
I also have some random MSF and DCF77 clocks around. The MSF clocks work
pretty well but the DCF77 kitchen clock jumps a couple of hours sometimes
then jumps back a couple of days later. The MSF clock used to get upset by
a VGA monitor when it was close by. Doesn't seem to mind modern LCD ones.
I believe the LW-disciplined oscillator is on borrowed time. The device is
fine but the LW transmitter is in danger of being retired any time soon.
On Thu, Sep 25, 2025 at 1:35 PM john.haine--- via time-nuts <
time-nuts@lists.febo.com> wrote:
Many thanks for all the responses! A few random thoughts in response.
-
I'm based in the eastern UK near Cambridge. Anthorn where the only
"nearby" eLoran station, and also MSF, is 370 km / 230 miles. Mainflingen
(DCF77) is 666 km/ 535 miles. DCF77 signals are largely available in the
UK
and most of the cheap RC clocks use them. However in some locations they
are a bit dodgy - for example Bristol University where I sometimes work put
RC clocks in most public areas and they are often wrong or spend all their
time trying to re-sync. MSF and eLoran would work better I suspect though
consumer MSF receivers are less available as the market is more UK-based
while DCF 77 works over most of Europe.
-
Using an online SDR based in Bedford which is 40 miles of so west
of
here eLoran from Anthorn is quite strong but DCF and MSF not observable -
that may be the antennas used as much as anything.
-
Unlike WWVB MSF does not have a phase modulation component, and its
carrier is switched off completely by the keying. Phase locking to its
carrier therefore has to gate the PLL but there is one design at least that
does this.
-
DCF77 has a spread-spectrum phase mod component and its carrier is
not switched off completely. This makes it much more robust if a properly
designed coherent receiver is used and I've seen one design that does this
in software. Unfortunately
-
I wasn't aware that eLoran has a phase mod component, I need to
read
up on its waveform design more. But is being toted as a GPS/GNSS backup
for
timing in Europe where we have a large and unfriendly neighbour.
I'll study the responses in more detail and follow up if this project makes
it to the starting blocks.
Thanks everyone for your input!
- John
-----Original Message-----
From: john.haine--- via time-nuts time-nuts@lists.febo.com
Sent: 22 September 2025 10:20
To: 'Discussion of precise time and frequency measurement'
time-nuts@lists.febo.com
Cc: john.haine@haine-online.net
Subject: [time-nuts] vlf-disciplined OCXO circuit
Does anyone know of designs for disciplined OCXOs that are referenced to
off-air, especially VLF, signals other than GPS/GNSS please? With modules
for the latter being so cheap this might seem pointless but there are some
potential advantages.
-
GPS reception at indoor locations where mechanical clocks need to
be
monitored is often (usually?) unavailable because of shadowing and building
absorbtion and it's usually inconvenient to run a cable.
-
GPS is increasingly likely to be jammed either by criminal elements
or "state actors".
-
VLF (the likes of WWVB / MSF / DCF77 / e-Loran) is more likely to
be
received indoors.
-
E-Loran is being tipped as an off-air time source to back up GPS
and
will become increasingly available.
-
There's the possibility of a multi-standard receiver that might
find
and lock to any available source, and potentially to several.
Obviously there are a lot of very cheap modules around to receive the
signals but these discard the carrier and just output the time-code logic
signal. I have seen a design for an MSF-locked standard in discrete
components and more recently an MSF receiver implemented as
direct-conversion SDR on a Raspberry Pi Pico which phase locks its internal
digital LO to the carrier but I suspect that its phase noise would be
pretty
ropey - really intended as a time, not frequency, source.
-
John Haine.
time-nuts mailing list -- time-nuts@lists.febo.com To unsubscribe send an
email to time-nuts-leave@lists.febo.com
time-nuts mailing list -- time-nuts@lists.febo.com
To unsubscribe send an email to time-nuts-leave@lists.febo.com
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To unsubscribe send an email to time-nuts-leave@lists.febo.com
As it happens, lately I've been working on the following project:
Building a 'retro' decoder for the WWVB time signal, based on a Motorola
6800-series MCU (actually a 6803). It gets its input from a Spectracom
8161, which outputs TTL-level bits detected from the 60 KHz antenna input.
Just this morning I got the basic pulse-width measurement code working
correctly, to detect the 0's, 1's, and Marker pulses.
At the moment, my antenna is a few feet of wire. I'm only about 120 miles
from Ft. Collins, so I get plenty of signal. However, the signal is "messed
with" by the issues that Bob and others have described. So I get a lot of
pulses of the wrong width.
I'll try a small ferrite loop antenna, to see if it cleans things up.
After I can decode WWVB reliably, I'll add code to output the old NASA and
IRIG-B 1-second time codes.
Pete
On Fri, Sep 26, 2025 at 7:29 AM Bob Camp via time-nuts <
time-nuts@lists.febo.com> wrote:
Hi
There are an unfortunate lot of things out there that can take out a 60KHz
VLF signal. For cost
reasons, 60kHz is what a lot of low end switchers run at. They are not
controlled in any way so
that’s a very approximate number. Since they are typically hooked to the
power line, they have a
fine “antenna” to work with. If you count the number of this and that in
the typical home, there might
well be a hundred devices with switchers in them. They won’t all be on at
once. All it takes is one
“bad one” running every so often to really make a mess.
If you grab one of these things and take a look with a spectrum analyzer:
You typically see a pretty
big spike at whatever frequency it happens to like today as well as
harmonics of that frequency. In
addition, there is a fairly broad set of “crud sidebands” that cover a
wide range at a lower level. It
also should be noted that there are lots of ways they do this (even just
at 60 KHz). Each one
has it’s own “signature”.
If you are way out on a farm somewhere, you might only have to deal with
your own devices. In
a typical urban setting, you have a lot more than just your own devices to
worry about.
Not every “bad one” will be equally bad. Some will bother things in a
pretty small area. Others just
might cover a pretty wide area.
There’s also somewhat more difficult things that do come up. This or that
device is hooked to
something stable. It just happens to have a spur at 60KHz. This is not
very common. The issue
here is that the “threat signal” is a pretty stable carrier. Somebody
putting up a local “60 KHz home
transmitter” to run their wall clocks? It does indeed happen.
Since this is all a bit random, what works today may not work tomorrow.
Equally, that gizmo that
made it impossible for months just might burn out. Putting a lot of time
and money into something
like this only to find it destroyed by “something” is not a good outcome.
Are there rules ( ok laws) about this stuff? Sure there are. None of this
should be going on. Much
of the compliance testing and certification is done in a pretty informal
manner (there is no outside
lab involved). Some folks will always be a bit less worried about this
than others.
From doing this in the past, I’d plan for an outdoor antenna. Get it as
far from the power lines and
any dwellings as you can. Even with an indoor antenna, getting it as far
from power and “other stuff”
is a really good idea.
Fun!!
Bob
On Sep 25, 2025, at 1:33 PM, Adrian Godwin via time-nuts <
time-nuts@lists.febo.com> wrote:
In case it's relevant ..
I'm in Bedford (I'm not the operator of that SDR though).
I've got a little heap of receivers in case they're interesting for
comparison, loan etc. A radio-4-LW-disciplined oscillator, a
Thunderbolt, a
heap of KS-24361 ref 0 GPSDOs, and (not connected) an MSF time receiver,
a
Pluto SDR and a Rubidium oscillator.
I also have some random MSF and DCF77 clocks around. The MSF clocks work
pretty well but the DCF77 kitchen clock jumps a couple of hours sometimes
then jumps back a couple of days later. The MSF clock used to get upset
by
a VGA monitor when it was close by. Doesn't seem to mind modern LCD ones.
I believe the LW-disciplined oscillator is on borrowed time. The device
is
fine but the LW transmitter is in danger of being retired any time soon.
On Thu, Sep 25, 2025 at 1:35 PM john.haine--- via time-nuts <
time-nuts@lists.febo.com> wrote:
Many thanks for all the responses! A few random thoughts in response.
-
I'm based in the eastern UK near Cambridge. Anthorn where the
only
"nearby" eLoran station, and also MSF, is 370 km / 230 miles.
Mainflingen
(DCF77) is 666 km/ 535 miles. DCF77 signals are largely available in
the
UK
and most of the cheap RC clocks use them. However in some locations
they
are a bit dodgy - for example Bristol University where I sometimes work
put
RC clocks in most public areas and they are often wrong or spend all
their
time trying to re-sync. MSF and eLoran would work better I suspect
though
consumer MSF receivers are less available as the market is more UK-based
while DCF 77 works over most of Europe.
-
Using an online SDR based in Bedford which is 40 miles of so
west
of
here eLoran from Anthorn is quite strong but DCF and MSF not observable
that may be the antennas used as much as anything.
-
Unlike WWVB MSF does not have a phase modulation component, and
its
carrier is switched off completely by the keying. Phase locking to its
carrier therefore has to gate the PLL but there is one design at least
that
does this.
-
DCF77 has a spread-spectrum phase mod component and its carrier
is
not switched off completely. This makes it much more robust if a
properly
designed coherent receiver is used and I've seen one design that does
this
in software. Unfortunately
-
I wasn't aware that eLoran has a phase mod component, I need to
read
up on its waveform design more. But is being toted as a GPS/GNSS backup
for
timing in Europe where we have a large and unfriendly neighbour.
I'll study the responses in more detail and follow up if this project
makes
it to the starting blocks.
Thanks everyone for your input!
- John
-----Original Message-----
From: john.haine--- via time-nuts time-nuts@lists.febo.com
Sent: 22 September 2025 10:20
To: 'Discussion of precise time and frequency measurement'
time-nuts@lists.febo.com
Cc: john.haine@haine-online.net
Subject: [time-nuts] vlf-disciplined OCXO circuit
Does anyone know of designs for disciplined OCXOs that are referenced to
off-air, especially VLF, signals other than GPS/GNSS please? With
modules
for the latter being so cheap this might seem pointless but there are
some
potential advantages.
-
GPS reception at indoor locations where mechanical clocks need
to
be
monitored is often (usually?) unavailable because of shadowing and
building
absorbtion and it's usually inconvenient to run a cable.
-
GPS is increasingly likely to be jammed either by criminal
elements
or "state actors".
-
VLF (the likes of WWVB / MSF / DCF77 / e-Loran) is more likely
to
be
received indoors.
-
E-Loran is being tipped as an off-air time source to back up GPS
and
will become increasingly available.
-
There's the possibility of a multi-standard receiver that might
find
and lock to any available source, and potentially to several.
Obviously there are a lot of very cheap modules around to receive the
signals but these discard the carrier and just output the time-code
logic
signal. I have seen a design for an MSF-locked standard in discrete
components and more recently an MSF receiver implemented as
direct-conversion SDR on a Raspberry Pi Pico which phase locks its
internal
digital LO to the carrier but I suspect that its phase noise would be
pretty
ropey - really intended as a time, not frequency, source.
-
John Haine.
time-nuts mailing list -- time-nuts@lists.febo.com To unsubscribe send
an
email to time-nuts-leave@lists.febo.com
time-nuts mailing list -- time-nuts@lists.febo.com
To unsubscribe send an email to time-nuts-leave@lists.febo.com
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To unsubscribe send an email to time-nuts-leave@lists.febo.com
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To unsubscribe send an email to time-nuts-leave@lists.febo.com
Way back when I was a consultant I had to troubleshoot a badly executed consumer electronics project that included an AM/FM broadcast receiver amongst other elements. This was installed in the central control panel of a built-in vacuum cleaner system (!), the idea being that radio programmes could be listened to throughout a house. The receiver used a truly awful LF/MF rf module with a high impedance input and zero input selectivity. The "antenna" was a bit of wire hanging out the back of the product enclosure (itself installed into the wall of the house). LF/MF reception was totally impossible in nearly every location due to a loud interference "hash" clearly emanating from the mains. However an ordinary portable radio in the same location usually worked absolutely fine. Eventually found that the short "capacitive" antenna (i.e. the wire) picked up strong impulsive near-E-field interference from things like fluorescent tubes etc, whilst the inductive loop antenna in an ordinary radio was relatively immune from this. The only way we found to solve the problem was to install an external antenna which was pretty ugly with an existing installation in a house.
So a ferrite loop antenna, preferably tuned, may help a lot, as would a crystal filter as generally used in consumer RC clocks. If you can find an application note for the chips they use it should include a circuit.
-----Original Message-----
From: Peter McCollum via time-nuts time-nuts@lists.febo.com
Sent: 26 September 2025 16:10
To: Discussion of precise time and frequency measurement time-nuts@lists.febo.com
Cc: Peter McCollum saipan1959@gmail.com
Subject: [time-nuts] Re: vlf-disciplined OCXO circuit
As it happens, lately I've been working on the following project:
Building a 'retro' decoder for the WWVB time signal, based on a Motorola 6800-series MCU (actually a 6803). It gets its input from a Spectracom 8161, which outputs TTL-level bits detected from the 60 KHz antenna input.
Just this morning I got the basic pulse-width measurement code working correctly, to detect the 0's, 1's, and Marker pulses.
At the moment, my antenna is a few feet of wire. I'm only about 120 miles from Ft. Collins, so I get plenty of signal. However, the signal is "messed with" by the issues that Bob and others have described. So I get a lot of pulses of the wrong width.
I'll try a small ferrite loop antenna, to see if it cleans things up.
After I can decode WWVB reliably, I'll add code to output the old NASA and IRIG-B 1-second time codes.
Pete
On Fri, Sep 26, 2025 at 7:29 AM Bob Camp via time-nuts < time-nuts@lists.febo.com> wrote:
Hi
There are an unfortunate lot of things out there that can take out a
60KHz VLF signal. For cost reasons, 60kHz is what a lot of low end
switchers run at. They are not controlled in any way so that’s a very
approximate number. Since they are typically hooked to the power line,
they have a fine “antenna” to work with. If you count the number of
this and that in the typical home, there might well be a hundred
devices with switchers in them. They won’t all be on at once. All it
takes is one “bad one” running every so often to really make a mess.
If you grab one of these things and take a look with a spectrum analyzer:
You typically see a pretty
big spike at whatever frequency it happens to like today as well as
harmonics of that frequency. In addition, there is a fairly broad set
of “crud sidebands” that cover a wide range at a lower level. It also
should be noted that there are lots of ways they do this (even just at
60 KHz). Each one has it’s own “signature”.
If you are way out on a farm somewhere, you might only have to deal
with your own devices. In a typical urban setting, you have a lot more
than just your own devices to worry about.
Not every “bad one” will be equally bad. Some will bother things in a
pretty small area. Others just might cover a pretty wide area.
There’s also somewhat more difficult things that do come up. This or
that device is hooked to something stable. It just happens to have a
spur at 60KHz. This is not very common. The issue here is that the
“threat signal” is a pretty stable carrier. Somebody putting up a
local “60 KHz home transmitter” to run their wall clocks? It does
indeed happen.
Since this is all a bit random, what works today may not work tomorrow.
Equally, that gizmo that
made it impossible for months just might burn out. Putting a lot of
time and money into something like this only to find it destroyed by
“something” is not a good outcome.
Are there rules ( ok laws) about this stuff? Sure there are. None of
this should be going on. Much of the compliance testing and
certification is done in a pretty informal manner (there is no outside
lab involved). Some folks will always be a bit less worried about this
than others.
From doing this in the past, I’d plan for an outdoor antenna. Get it
as far from the power lines and any dwellings as you can. Even with an
indoor antenna, getting it as far from power and “other stuff”
is a really good idea.
Fun!!
Bob
On Sep 25, 2025, at 1:33 PM, Adrian Godwin via time-nuts <
time-nuts@lists.febo.com> wrote:
In case it's relevant ..
I'm in Bedford (I'm not the operator of that SDR though).
I've got a little heap of receivers in case they're interesting for
comparison, loan etc. A radio-4-LW-disciplined oscillator, a
Thunderbolt, a
heap of KS-24361 ref 0 GPSDOs, and (not connected) an MSF time
receiver,
a
Pluto SDR and a Rubidium oscillator.
I also have some random MSF and DCF77 clocks around. The MSF clocks
work pretty well but the DCF77 kitchen clock jumps a couple of hours
sometimes then jumps back a couple of days later. The MSF clock used
to get upset
by
a VGA monitor when it was close by. Doesn't seem to mind modern LCD ones.
I believe the LW-disciplined oscillator is on borrowed time. The
device
is
fine but the LW transmitter is in danger of being retired any time soon.
On Thu, Sep 25, 2025 at 1:35 PM john.haine--- via time-nuts <
time-nuts@lists.febo.com> wrote:
Many thanks for all the responses! A few random thoughts in response.
-
I'm based in the eastern UK near Cambridge. Anthorn where the
only
"nearby" eLoran station, and also MSF, is 370 km / 230 miles.
Mainflingen
(DCF77) is 666 km/ 535 miles. DCF77 signals are largely available
in
the
UK
and most of the cheap RC clocks use them. However in some
locations
they
are a bit dodgy - for example Bristol University where I sometimes
work
put
RC clocks in most public areas and they are often wrong or spend
all
their
time trying to re-sync. MSF and eLoran would work better I suspect
though
consumer MSF receivers are less available as the market is more
UK-based while DCF 77 works over most of Europe.
-
Using an online SDR based in Bedford which is 40 miles of so
west
of
here eLoran from Anthorn is quite strong but DCF and MSF not
observable
that may be the antennas used as much as anything.
-
Unlike WWVB MSF does not have a phase modulation component, and
its
carrier is switched off completely by the keying. Phase locking to
its carrier therefore has to gate the PLL but there is one design
at least
that
does this.
-
DCF77 has a spread-spectrum phase mod component and its carrier
is
not switched off completely. This makes it much more robust if a
properly
designed coherent receiver is used and I've seen one design that
does
this
in software. Unfortunately
-
I wasn't aware that eLoran has a phase mod component, I need to
read
up on its waveform design more. But is being toted as a GPS/GNSS
backup for timing in Europe where we have a large and unfriendly
neighbour.
I'll study the responses in more detail and follow up if this
project
makes
it to the starting blocks.
Thanks everyone for your input!
- John
-----Original Message-----
From: john.haine--- via time-nuts time-nuts@lists.febo.com
Sent: 22 September 2025 10:20
To: 'Discussion of precise time and frequency measurement'
time-nuts@lists.febo.com
Cc: john.haine@haine-online.net
Subject: [time-nuts] vlf-disciplined OCXO circuit
Does anyone know of designs for disciplined OCXOs that are
referenced to off-air, especially VLF, signals other than GPS/GNSS
please? With
modules
for the latter being so cheap this might seem pointless but there
are
some
potential advantages.
-
GPS reception at indoor locations where mechanical clocks need
to
be
monitored is often (usually?) unavailable because of shadowing and
building
absorbtion and it's usually inconvenient to run a cable.
-
GPS is increasingly likely to be jammed either by criminal
elements
or "state actors".
-
VLF (the likes of WWVB / MSF / DCF77 / e-Loran) is more likely
to
be
received indoors.
-
E-Loran is being tipped as an off-air time source to back up GPS
and
will become increasingly available.
-
There's the possibility of a multi-standard receiver that might
find
and lock to any available source, and potentially to several.
Obviously there are a lot of very cheap modules around to receive
the signals but these discard the carrier and just output the
time-code
logic
signal. I have seen a design for an MSF-locked standard in
discrete components and more recently an MSF receiver implemented
as direct-conversion SDR on a Raspberry Pi Pico which phase locks
its
internal
digital LO to the carrier but I suspect that its phase noise would
be pretty ropey - really intended as a time, not frequency, source.
-
John Haine.
time-nuts mailing list -- time-nuts@lists.febo.com To unsubscribe
send
an
email to time-nuts-leave@lists.febo.com
time-nuts mailing list -- time-nuts@lists.febo.com To unsubscribe
send an email to time-nuts-leave@lists.febo.com
time-nuts mailing list -- time-nuts@lists.febo.com To unsubscribe
send an email to time-nuts-leave@lists.febo.com
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an email to time-nuts-leave@lists.febo.com
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Hi
I think you probably should look at what PHK did a while back on his “VLF setup”.
He addressed the whole “noise comes and goes” thing very well with what he did.
For a single data stream, you just might fit it in a 6803 …. maybe.
Bob
On Sep 26, 2025, at 11:10 AM, Peter McCollum via time-nuts time-nuts@lists.febo.com wrote:
As it happens, lately I've been working on the following project:
Building a 'retro' decoder for the WWVB time signal, based on a Motorola
6800-series MCU (actually a 6803). It gets its input from a Spectracom
8161, which outputs TTL-level bits detected from the 60 KHz antenna input.
Just this morning I got the basic pulse-width measurement code working
correctly, to detect the 0's, 1's, and Marker pulses.
At the moment, my antenna is a few feet of wire. I'm only about 120 miles
from Ft. Collins, so I get plenty of signal. However, the signal is "messed
with" by the issues that Bob and others have described. So I get a lot of
pulses of the wrong width.
I'll try a small ferrite loop antenna, to see if it cleans things up.
After I can decode WWVB reliably, I'll add code to output the old NASA and
IRIG-B 1-second time codes.
Pete
On Fri, Sep 26, 2025 at 7:29 AM Bob Camp via time-nuts <
time-nuts@lists.febo.com> wrote:
Hi
There are an unfortunate lot of things out there that can take out a 60KHz
VLF signal. For cost
reasons, 60kHz is what a lot of low end switchers run at. They are not
controlled in any way so
that’s a very approximate number. Since they are typically hooked to the
power line, they have a
fine “antenna” to work with. If you count the number of this and that in
the typical home, there might
well be a hundred devices with switchers in them. They won’t all be on at
once. All it takes is one
“bad one” running every so often to really make a mess.
If you grab one of these things and take a look with a spectrum analyzer:
You typically see a pretty
big spike at whatever frequency it happens to like today as well as
harmonics of that frequency. In
addition, there is a fairly broad set of “crud sidebands” that cover a
wide range at a lower level. It
also should be noted that there are lots of ways they do this (even just
at 60 KHz). Each one
has it’s own “signature”.
If you are way out on a farm somewhere, you might only have to deal with
your own devices. In
a typical urban setting, you have a lot more than just your own devices to
worry about.
Not every “bad one” will be equally bad. Some will bother things in a
pretty small area. Others just
might cover a pretty wide area.
There’s also somewhat more difficult things that do come up. This or that
device is hooked to
something stable. It just happens to have a spur at 60KHz. This is not
very common. The issue
here is that the “threat signal” is a pretty stable carrier. Somebody
putting up a local “60 KHz home
transmitter” to run their wall clocks? It does indeed happen.
Since this is all a bit random, what works today may not work tomorrow.
Equally, that gizmo that
made it impossible for months just might burn out. Putting a lot of time
and money into something
like this only to find it destroyed by “something” is not a good outcome.
Are there rules ( ok laws) about this stuff? Sure there are. None of this
should be going on. Much
of the compliance testing and certification is done in a pretty informal
manner (there is no outside
lab involved). Some folks will always be a bit less worried about this
than others.
From doing this in the past, I’d plan for an outdoor antenna. Get it as
far from the power lines and
any dwellings as you can. Even with an indoor antenna, getting it as far
from power and “other stuff”
is a really good idea.
Fun!!
Bob
On Sep 25, 2025, at 1:33 PM, Adrian Godwin via time-nuts <
time-nuts@lists.febo.com> wrote:
In case it's relevant ..
I'm in Bedford (I'm not the operator of that SDR though).
I've got a little heap of receivers in case they're interesting for
comparison, loan etc. A radio-4-LW-disciplined oscillator, a
Thunderbolt, a
heap of KS-24361 ref 0 GPSDOs, and (not connected) an MSF time receiver,
a
Pluto SDR and a Rubidium oscillator.
I also have some random MSF and DCF77 clocks around. The MSF clocks work
pretty well but the DCF77 kitchen clock jumps a couple of hours sometimes
then jumps back a couple of days later. The MSF clock used to get upset
by
a VGA monitor when it was close by. Doesn't seem to mind modern LCD ones.
I believe the LW-disciplined oscillator is on borrowed time. The device
is
fine but the LW transmitter is in danger of being retired any time soon.
On Thu, Sep 25, 2025 at 1:35 PM john.haine--- via time-nuts <
time-nuts@lists.febo.com> wrote:
Many thanks for all the responses! A few random thoughts in response.
-
I'm based in the eastern UK near Cambridge. Anthorn where the
only
"nearby" eLoran station, and also MSF, is 370 km / 230 miles.
Mainflingen
(DCF77) is 666 km/ 535 miles. DCF77 signals are largely available in
the
UK
and most of the cheap RC clocks use them. However in some locations
they
are a bit dodgy - for example Bristol University where I sometimes work
put
RC clocks in most public areas and they are often wrong or spend all
their
time trying to re-sync. MSF and eLoran would work better I suspect
though
consumer MSF receivers are less available as the market is more UK-based
while DCF 77 works over most of Europe.
-
Using an online SDR based in Bedford which is 40 miles of so
west
of
here eLoran from Anthorn is quite strong but DCF and MSF not observable
that may be the antennas used as much as anything.
-
Unlike WWVB MSF does not have a phase modulation component, and
its
carrier is switched off completely by the keying. Phase locking to its
carrier therefore has to gate the PLL but there is one design at least
that
does this.
-
DCF77 has a spread-spectrum phase mod component and its carrier
is
not switched off completely. This makes it much more robust if a
properly
designed coherent receiver is used and I've seen one design that does
this
in software. Unfortunately
-
I wasn't aware that eLoran has a phase mod component, I need to
read
up on its waveform design more. But is being toted as a GPS/GNSS backup
for
timing in Europe where we have a large and unfriendly neighbour.
I'll study the responses in more detail and follow up if this project
makes
it to the starting blocks.
Thanks everyone for your input!
- John
-----Original Message-----
From: john.haine--- via time-nuts time-nuts@lists.febo.com
Sent: 22 September 2025 10:20
To: 'Discussion of precise time and frequency measurement'
time-nuts@lists.febo.com
Cc: john.haine@haine-online.net
Subject: [time-nuts] vlf-disciplined OCXO circuit
Does anyone know of designs for disciplined OCXOs that are referenced to
off-air, especially VLF, signals other than GPS/GNSS please? With
modules
for the latter being so cheap this might seem pointless but there are
some
potential advantages.
-
GPS reception at indoor locations where mechanical clocks need
to
be
monitored is often (usually?) unavailable because of shadowing and
building
absorbtion and it's usually inconvenient to run a cable.
-
GPS is increasingly likely to be jammed either by criminal
elements
or "state actors".
-
VLF (the likes of WWVB / MSF / DCF77 / e-Loran) is more likely
to
be
received indoors.
-
E-Loran is being tipped as an off-air time source to back up GPS
and
will become increasingly available.
-
There's the possibility of a multi-standard receiver that might
find
and lock to any available source, and potentially to several.
Obviously there are a lot of very cheap modules around to receive the
signals but these discard the carrier and just output the time-code
logic
signal. I have seen a design for an MSF-locked standard in discrete
components and more recently an MSF receiver implemented as
direct-conversion SDR on a Raspberry Pi Pico which phase locks its
internal
digital LO to the carrier but I suspect that its phase noise would be
pretty
ropey - really intended as a time, not frequency, source.
-
John Haine.
time-nuts mailing list -- time-nuts@lists.febo.com To unsubscribe send
an
email to time-nuts-leave@lists.febo.com
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To unsubscribe send an email to time-nuts-leave@lists.febo.com
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Update on my project, for those that might be interested.
I spent the last day struggling with my MCU code and the Spectracom 8161.
The 8161 seemed to be outputting bad WWVB data, but I couldn't be sure if
it was my poor antenna, a code problem in my MCU, or a problem in the 8161.
Then I remembered that I have one of those C-MAX WWVB receiver modules
providing the TOD for my in-house 'telemetry network'. So I used a scope to
do a side-by-side compare of the C-MAX with the 8161. Sure enough, the 8161
was outputting a LOT of bad data (it would only occasionally do the correct
bits). Studied the 8161 design some more, and realized that its ability to
do the time-code output was dependent on a phase comparison with the VCO
(this is NOT the ovenized VCO 'standard', which is also in the box). But
the VCO is never properly locked to the WWVB carrier, because of the
phase-change issue about 10(?) years ago. The AGC on the 60 KHz receiver
was constantly moving around, therefore messing with the bit detection and
such.
SO, I removed the phase-detector chip (socketed) that handles the AGC and
time-code stuff, and replaced it with a simple AM detector (a diode,
transistor, two R's and a C).
It works! After one final tweak to my code, my 6803 MCU gadget is
displaying UTC!
And my simple long-wire antenna is working fine, also.
Pete
On Sat, Sep 27, 2025 at 4:10 AM john.haine@haine-online.net wrote:
Way back when I was a consultant I had to troubleshoot a badly executed
consumer electronics project that included an AM/FM broadcast receiver
amongst other elements. This was installed in the central control panel of
a built-in vacuum cleaner system (!), the idea being that radio programmes
could be listened to throughout a house. The receiver used a truly awful
LF/MF rf module with a high impedance input and zero input selectivity.
The "antenna" was a bit of wire hanging out the back of the product
enclosure (itself installed into the wall of the house). LF/MF reception
was totally impossible in nearly every location due to a loud interference
"hash" clearly emanating from the mains. However an ordinary portable
radio in the same location usually worked absolutely fine. Eventually
found that the short "capacitive" antenna (i.e. the wire) picked up strong
impulsive near-E-field interference from things like fluorescent tubes
etc, whilst the inductive loop antenna in an ordinary radio was relatively
immune from this. The only way we found to solve the problem was to
install an external antenna which was pretty ugly with an existing
installation in a house.
So a ferrite loop antenna, preferably tuned, may help a lot, as would a
crystal filter as generally used in consumer RC clocks. If you can find an
application note for the chips they use it should include a circuit.
-----Original Message-----
From: Peter McCollum via time-nuts time-nuts@lists.febo.com
Sent: 26 September 2025 16:10
To: Discussion of precise time and frequency measurement <
time-nuts@lists.febo.com>
Cc: Peter McCollum saipan1959@gmail.com
Subject: [time-nuts] Re: vlf-disciplined OCXO circuit
As it happens, lately I've been working on the following project:
Building a 'retro' decoder for the WWVB time signal, based on a Motorola
6800-series MCU (actually a 6803). It gets its input from a Spectracom
8161, which outputs TTL-level bits detected from the 60 KHz antenna input.
Just this morning I got the basic pulse-width measurement code working
correctly, to detect the 0's, 1's, and Marker pulses.
At the moment, my antenna is a few feet of wire. I'm only about 120 miles
from Ft. Collins, so I get plenty of signal. However, the signal is "messed
with" by the issues that Bob and others have described. So I get a lot of
pulses of the wrong width.
I'll try a small ferrite loop antenna, to see if it cleans things up.
After I can decode WWVB reliably, I'll add code to output the old NASA and
IRIG-B 1-second time codes.
Pete
On Fri, Sep 26, 2025 at 7:29 AM Bob Camp via time-nuts <
time-nuts@lists.febo.com> wrote:
Hi
There are an unfortunate lot of things out there that can take out a
60KHz VLF signal. For cost reasons, 60kHz is what a lot of low end
switchers run at. They are not controlled in any way so that’s a very
approximate number. Since they are typically hooked to the power line,
they have a fine “antenna” to work with. If you count the number of
this and that in the typical home, there might well be a hundred
devices with switchers in them. They won’t all be on at once. All it
takes is one “bad one” running every so often to really make a mess.
If you grab one of these things and take a look with a spectrum analyzer:
You typically see a pretty
big spike at whatever frequency it happens to like today as well as
harmonics of that frequency. In addition, there is a fairly broad set
of “crud sidebands” that cover a wide range at a lower level. It also
should be noted that there are lots of ways they do this (even just at
60 KHz). Each one has it’s own “signature”.
If you are way out on a farm somewhere, you might only have to deal
with your own devices. In a typical urban setting, you have a lot more
than just your own devices to worry about.
Not every “bad one” will be equally bad. Some will bother things in a
pretty small area. Others just might cover a pretty wide area.
There’s also somewhat more difficult things that do come up. This or
that device is hooked to something stable. It just happens to have a
spur at 60KHz. This is not very common. The issue here is that the
“threat signal” is a pretty stable carrier. Somebody putting up a
local “60 KHz home transmitter” to run their wall clocks? It does
indeed happen.
Since this is all a bit random, what works today may not work tomorrow.
Equally, that gizmo that
made it impossible for months just might burn out. Putting a lot of
time and money into something like this only to find it destroyed by
“something” is not a good outcome.
Are there rules ( ok laws) about this stuff? Sure there are. None of
this should be going on. Much of the compliance testing and
certification is done in a pretty informal manner (there is no outside
lab involved). Some folks will always be a bit less worried about this
than others.
From doing this in the past, I’d plan for an outdoor antenna. Get it
as far from the power lines and any dwellings as you can. Even with an
indoor antenna, getting it as far from power and “other stuff”
is a really good idea.
Fun!!
Bob
On Sep 25, 2025, at 1:33 PM, Adrian Godwin via time-nuts <
time-nuts@lists.febo.com> wrote:
In case it's relevant ..
I'm in Bedford (I'm not the operator of that SDR though).
I've got a little heap of receivers in case they're interesting for
comparison, loan etc. A radio-4-LW-disciplined oscillator, a
Thunderbolt, a
heap of KS-24361 ref 0 GPSDOs, and (not connected) an MSF time
receiver,
a
Pluto SDR and a Rubidium oscillator.
I also have some random MSF and DCF77 clocks around. The MSF clocks
work pretty well but the DCF77 kitchen clock jumps a couple of hours
sometimes then jumps back a couple of days later. The MSF clock used
to get upset
by
a VGA monitor when it was close by. Doesn't seem to mind modern LCD
ones.
I believe the LW-disciplined oscillator is on borrowed time. The
device
is
fine but the LW transmitter is in danger of being retired any time
soon.
On Thu, Sep 25, 2025 at 1:35 PM john.haine--- via time-nuts <
time-nuts@lists.febo.com> wrote:
Many thanks for all the responses! A few random thoughts in response.
-
I'm based in the eastern UK near Cambridge. Anthorn where the
only
"nearby" eLoran station, and also MSF, is 370 km / 230 miles.
Mainflingen
(DCF77) is 666 km/ 535 miles. DCF77 signals are largely available
in
the
UK
and most of the cheap RC clocks use them. However in some
locations
they
are a bit dodgy - for example Bristol University where I sometimes
work
put
RC clocks in most public areas and they are often wrong or spend
all
their
time trying to re-sync. MSF and eLoran would work better I suspect
though
consumer MSF receivers are less available as the market is more
UK-based while DCF 77 works over most of Europe.
-
Using an online SDR based in Bedford which is 40 miles of so
west
of
here eLoran from Anthorn is quite strong but DCF and MSF not
observable
that may be the antennas used as much as anything.
-
Unlike WWVB MSF does not have a phase modulation component,
and
its
carrier is switched off completely by the keying. Phase locking to
its carrier therefore has to gate the PLL but there is one design
at least
that
does this.
-
DCF77 has a spread-spectrum phase mod component and its
carrier
is
not switched off completely. This makes it much more robust if a
properly
designed coherent receiver is used and I've seen one design that
does
this
in software. Unfortunately
-
I wasn't aware that eLoran has a phase mod component, I need
to
read
up on its waveform design more. But is being toted as a GPS/GNSS
backup for timing in Europe where we have a large and unfriendly
neighbour.
I'll study the responses in more detail and follow up if this
project
makes
it to the starting blocks.
Thanks everyone for your input!
- John
-----Original Message-----
From: john.haine--- via time-nuts time-nuts@lists.febo.com
Sent: 22 September 2025 10:20
To: 'Discussion of precise time and frequency measurement'
time-nuts@lists.febo.com
Cc: john.haine@haine-online.net
Subject: [time-nuts] vlf-disciplined OCXO circuit
Does anyone know of designs for disciplined OCXOs that are
referenced to off-air, especially VLF, signals other than GPS/GNSS
please? With
modules
for the latter being so cheap this might seem pointless but there
are
some
potential advantages.
-
GPS reception at indoor locations where mechanical clocks need
to
be
monitored is often (usually?) unavailable because of shadowing and
building
absorbtion and it's usually inconvenient to run a cable.
-
GPS is increasingly likely to be jammed either by criminal
elements
or "state actors".
-
VLF (the likes of WWVB / MSF / DCF77 / e-Loran) is more likely
to
be
received indoors.
-
E-Loran is being tipped as an off-air time source to back up
GPS
and
will become increasingly available.
-
There's the possibility of a multi-standard receiver that
might
find
and lock to any available source, and potentially to several.
Obviously there are a lot of very cheap modules around to receive
the signals but these discard the carrier and just output the
time-code
logic
signal. I have seen a design for an MSF-locked standard in
discrete components and more recently an MSF receiver implemented
as direct-conversion SDR on a Raspberry Pi Pico which phase locks
its
internal
digital LO to the carrier but I suspect that its phase noise would
be pretty ropey - really intended as a time, not frequency, source.
-
John Haine.
time-nuts mailing list -- time-nuts@lists.febo.com To unsubscribe
send
an
email to time-nuts-leave@lists.febo.com
time-nuts mailing list -- time-nuts@lists.febo.com To unsubscribe
send an email to time-nuts-leave@lists.febo.com
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Hi
A crystal filter at the front end of a 60 KHz receiver is a bit of a win / loose sort of thing.
The selection of crystals available at 60 KHz is a bit limited. The typical parts have temperature coefficient’s that are a bit high. This results in a filter with phase delay that varies noticeably over even modest temperature changes. That’s not ideal for a oscillator locking application. For a wall clock that is doing great if it’s only tens of milliseconds off, it’s not an issue.
These days a SDR based approach is the better choice.
Bob
On Sep 27, 2025, at 6:10 AM, john.haine--- via time-nuts time-nuts@lists.febo.com wrote:
Way back when I was a consultant I had to troubleshoot a badly executed consumer electronics project that included an AM/FM broadcast receiver amongst other elements. This was installed in the central control panel of a built-in vacuum cleaner system (!), the idea being that radio programmes could be listened to throughout a house. The receiver used a truly awful LF/MF rf module with a high impedance input and zero input selectivity. The "antenna" was a bit of wire hanging out the back of the product enclosure (itself installed into the wall of the house). LF/MF reception was totally impossible in nearly every location due to a loud interference "hash" clearly emanating from the mains. However an ordinary portable radio in the same location usually worked absolutely fine. Eventually found that the short "capacitive" antenna (i.e. the wire) picked up strong impulsive near-E-field interference from things like fluorescent tubes etc, whilst the inductive loop antenna in an ordinary radio was relatively immune from this. The only way we found to solve the problem was to install an external antenna which was pretty ugly with an existing installation in a house.
So a ferrite loop antenna, preferably tuned, may help a lot, as would a crystal filter as generally used in consumer RC clocks. If you can find an application note for the chips they use it should include a circuit.
-----Original Message-----
From: Peter McCollum via time-nuts time-nuts@lists.febo.com
Sent: 26 September 2025 16:10
To: Discussion of precise time and frequency measurement time-nuts@lists.febo.com
Cc: Peter McCollum saipan1959@gmail.com
Subject: [time-nuts] Re: vlf-disciplined OCXO circuit
As it happens, lately I've been working on the following project:
Building a 'retro' decoder for the WWVB time signal, based on a Motorola 6800-series MCU (actually a 6803). It gets its input from a Spectracom 8161, which outputs TTL-level bits detected from the 60 KHz antenna input.
Just this morning I got the basic pulse-width measurement code working correctly, to detect the 0's, 1's, and Marker pulses.
At the moment, my antenna is a few feet of wire. I'm only about 120 miles from Ft. Collins, so I get plenty of signal. However, the signal is "messed with" by the issues that Bob and others have described. So I get a lot of pulses of the wrong width.
I'll try a small ferrite loop antenna, to see if it cleans things up.
After I can decode WWVB reliably, I'll add code to output the old NASA and IRIG-B 1-second time codes.
Pete
On Fri, Sep 26, 2025 at 7:29 AM Bob Camp via time-nuts < time-nuts@lists.febo.com> wrote:
Hi
There are an unfortunate lot of things out there that can take out a
60KHz VLF signal. For cost reasons, 60kHz is what a lot of low end
switchers run at. They are not controlled in any way so that’s a very
approximate number. Since they are typically hooked to the power line,
they have a fine “antenna” to work with. If you count the number of
this and that in the typical home, there might well be a hundred
devices with switchers in them. They won’t all be on at once. All it
takes is one “bad one” running every so often to really make a mess.
If you grab one of these things and take a look with a spectrum analyzer:
You typically see a pretty
big spike at whatever frequency it happens to like today as well as
harmonics of that frequency. In addition, there is a fairly broad set
of “crud sidebands” that cover a wide range at a lower level. It also
should be noted that there are lots of ways they do this (even just at
60 KHz). Each one has it’s own “signature”.
If you are way out on a farm somewhere, you might only have to deal
with your own devices. In a typical urban setting, you have a lot more
than just your own devices to worry about.
Not every “bad one” will be equally bad. Some will bother things in a
pretty small area. Others just might cover a pretty wide area.
There’s also somewhat more difficult things that do come up. This or
that device is hooked to something stable. It just happens to have a
spur at 60KHz. This is not very common. The issue here is that the
“threat signal” is a pretty stable carrier. Somebody putting up a
local “60 KHz home transmitter” to run their wall clocks? It does
indeed happen.
Since this is all a bit random, what works today may not work tomorrow.
Equally, that gizmo that
made it impossible for months just might burn out. Putting a lot of
time and money into something like this only to find it destroyed by
“something” is not a good outcome.
Are there rules ( ok laws) about this stuff? Sure there are. None of
this should be going on. Much of the compliance testing and
certification is done in a pretty informal manner (there is no outside
lab involved). Some folks will always be a bit less worried about this
than others.
From doing this in the past, I’d plan for an outdoor antenna. Get it
as far from the power lines and any dwellings as you can. Even with an
indoor antenna, getting it as far from power and “other stuff”
is a really good idea.
Fun!!
Bob
On Sep 25, 2025, at 1:33 PM, Adrian Godwin via time-nuts <
time-nuts@lists.febo.com> wrote:
In case it's relevant ..
I'm in Bedford (I'm not the operator of that SDR though).
I've got a little heap of receivers in case they're interesting for
comparison, loan etc. A radio-4-LW-disciplined oscillator, a
Thunderbolt, a
heap of KS-24361 ref 0 GPSDOs, and (not connected) an MSF time
receiver,
a
Pluto SDR and a Rubidium oscillator.
I also have some random MSF and DCF77 clocks around. The MSF clocks
work pretty well but the DCF77 kitchen clock jumps a couple of hours
sometimes then jumps back a couple of days later. The MSF clock used
to get upset
by
a VGA monitor when it was close by. Doesn't seem to mind modern LCD ones.
I believe the LW-disciplined oscillator is on borrowed time. The
device
is
fine but the LW transmitter is in danger of being retired any time soon.
On Thu, Sep 25, 2025 at 1:35 PM john.haine--- via time-nuts <
time-nuts@lists.febo.com> wrote:
Many thanks for all the responses! A few random thoughts in response.
-
I'm based in the eastern UK near Cambridge. Anthorn where the
only
"nearby" eLoran station, and also MSF, is 370 km / 230 miles.
Mainflingen
(DCF77) is 666 km/ 535 miles. DCF77 signals are largely available
in
the
UK
and most of the cheap RC clocks use them. However in some
locations
they
are a bit dodgy - for example Bristol University where I sometimes
work
put
RC clocks in most public areas and they are often wrong or spend
all
their
time trying to re-sync. MSF and eLoran would work better I suspect
though
consumer MSF receivers are less available as the market is more
UK-based while DCF 77 works over most of Europe.
-
Using an online SDR based in Bedford which is 40 miles of so
west
of
here eLoran from Anthorn is quite strong but DCF and MSF not
observable
that may be the antennas used as much as anything.
-
Unlike WWVB MSF does not have a phase modulation component, and
its
carrier is switched off completely by the keying. Phase locking to
its carrier therefore has to gate the PLL but there is one design
at least
that
does this.
-
DCF77 has a spread-spectrum phase mod component and its carrier
is
not switched off completely. This makes it much more robust if a
properly
designed coherent receiver is used and I've seen one design that
does
this
in software. Unfortunately
-
I wasn't aware that eLoran has a phase mod component, I need to
read
up on its waveform design more. But is being toted as a GPS/GNSS
backup for timing in Europe where we have a large and unfriendly
neighbour.
I'll study the responses in more detail and follow up if this
project
makes
it to the starting blocks.
Thanks everyone for your input!
- John
-----Original Message-----
From: john.haine--- via time-nuts time-nuts@lists.febo.com
Sent: 22 September 2025 10:20
To: 'Discussion of precise time and frequency measurement'
time-nuts@lists.febo.com
Cc: john.haine@haine-online.net
Subject: [time-nuts] vlf-disciplined OCXO circuit
Does anyone know of designs for disciplined OCXOs that are
referenced to off-air, especially VLF, signals other than GPS/GNSS
please? With
modules
for the latter being so cheap this might seem pointless but there
are
some
potential advantages.
-
GPS reception at indoor locations where mechanical clocks need
to
be
monitored is often (usually?) unavailable because of shadowing and
building
absorbtion and it's usually inconvenient to run a cable.
-
GPS is increasingly likely to be jammed either by criminal
elements
or "state actors".
-
VLF (the likes of WWVB / MSF / DCF77 / e-Loran) is more likely
to
be
received indoors.
-
E-Loran is being tipped as an off-air time source to back up GPS
and
will become increasingly available.
-
There's the possibility of a multi-standard receiver that might
find
and lock to any available source, and potentially to several.
Obviously there are a lot of very cheap modules around to receive
the signals but these discard the carrier and just output the
time-code
logic
signal. I have seen a design for an MSF-locked standard in
discrete components and more recently an MSF receiver implemented
as direct-conversion SDR on a Raspberry Pi Pico which phase locks
its
internal
digital LO to the carrier but I suspect that its phase noise would
be pretty ropey - really intended as a time, not frequency, source.
-
John Haine.
time-nuts mailing list -- time-nuts@lists.febo.com To unsubscribe
send
an
email to time-nuts-leave@lists.febo.com
time-nuts mailing list -- time-nuts@lists.febo.com To unsubscribe
send an email to time-nuts-leave@lists.febo.com
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To unsubscribe send an email to time-nuts-leave@lists.febo.com
suggest any narrowband receiver down there to be prepended by a wideband
noise blanker.
Lightning crashes, mains connected appliances being turned on and off.....
The act of narrowing the bandwidth stretches out those big pulses.
Usually, best option to to blank or limit the input where it is still
wideband, (at the front end) and that way minimize the blanking time.
Your mileage may vary with the antenna bandwidth possibly being the
limiting factor- a tuned magnetic antenna will be quite narrow, and put
a limit on the pulse risetimes.
The bandwidth of the antenna is stil likely to be 10x at least the
bandwidth of the baseband, so an front end noise blanker is still a
useful thing.
For how to build a good noise blanker there are plenty of web links
around. essentialy a edge detector firing (timed) a blank, mute or clip,
or whatever width works best.
-glen
On 28/09/2025 21:54, Bob Camp via time-nuts wrote:
The thing about SDR is always that they have zero selectivity prior to the RF amp and mixer. If you have a lot of interference around it's good to have selectivity before you get into anything potentially non-linear, and it also makes the ADC's job easier. Surely if the crystal has known characteristics it should be possible to compensate for the varying phase shift? Or, given that it's going to discipline an OCXO anyway, put it in an oven too.
-----Original Message-----
From: Bob Camp via time-nuts time-nuts@lists.febo.com
Sent: 28 September 2025 12:54
To: Discussion of precise time and frequency measurement time-nuts@lists.febo.com
Cc: Bob Camp kb8tq@n1k.org
Subject: [time-nuts] Re: vlf-disciplined OCXO circuit
Hi
A crystal filter at the front end of a 60 KHz receiver is a bit of a win / loose sort of thing.
The selection of crystals available at 60 KHz is a bit limited. The typical parts have temperature coefficient’s that are a bit high. This results in a filter with phase delay that varies noticeably over even modest temperature changes. That’s not ideal for a oscillator locking application. For a wall clock that is doing great if it’s only tens of milliseconds off, it’s not an issue.
These days a SDR based approach is the better choice.
Bob
On Sep 27, 2025, at 6:10 AM, john.haine--- via time-nuts time-nuts@lists.febo.com wrote:
Way back when I was a consultant I had to troubleshoot a badly executed consumer electronics project that included an AM/FM broadcast receiver amongst other elements. This was installed in the central control panel of a built-in vacuum cleaner system (!), the idea being that radio programmes could be listened to throughout a house. The receiver used a truly awful LF/MF rf module with a high impedance input and zero input selectivity. The "antenna" was a bit of wire hanging out the back of the product enclosure (itself installed into the wall of the house). LF/MF reception was totally impossible in nearly every location due to a loud interference "hash" clearly emanating from the mains. However an ordinary portable radio in the same location usually worked absolutely fine. Eventually found that the short "capacitive" antenna (i.e. the wire) picked up strong impulsive near-E-field interference from things like fluorescent tubes etc, whilst the inductive loop antenna in an ordinary radio was relatively immune from this. The only way we found to solve the problem was to install an external antenna which was pretty ugly with an existing installation in a house.
So a ferrite loop antenna, preferably tuned, may help a lot, as would a crystal filter as generally used in consumer RC clocks. If you can find an application note for the chips they use it should include a circuit.
-----Original Message-----
From: Peter McCollum via time-nuts time-nuts@lists.febo.com
Sent: 26 September 2025 16:10
To: Discussion of precise time and frequency measurement
time-nuts@lists.febo.com
Cc: Peter McCollum saipan1959@gmail.com
Subject: [time-nuts] Re: vlf-disciplined OCXO circuit
As it happens, lately I've been working on the following project:
Building a 'retro' decoder for the WWVB time signal, based on a Motorola 6800-series MCU (actually a 6803). It gets its input from a Spectracom 8161, which outputs TTL-level bits detected from the 60 KHz antenna input.
Just this morning I got the basic pulse-width measurement code working correctly, to detect the 0's, 1's, and Marker pulses.
At the moment, my antenna is a few feet of wire. I'm only about 120 miles from Ft. Collins, so I get plenty of signal. However, the signal is "messed with" by the issues that Bob and others have described. So I get a lot of pulses of the wrong width.
I'll try a small ferrite loop antenna, to see if it cleans things up.
After I can decode WWVB reliably, I'll add code to output the old NASA and IRIG-B 1-second time codes.
Pete
On Fri, Sep 26, 2025 at 7:29 AM Bob Camp via time-nuts < time-nuts@lists.febo.com> wrote:
Hi
There are an unfortunate lot of things out there that can take out a
60KHz VLF signal. For cost reasons, 60kHz is what a lot of low end
switchers run at. They are not controlled in any way so that’s a very
approximate number. Since they are typically hooked to the power
line, they have a fine “antenna” to work with. If you count the
number of this and that in the typical home, there might well be a
hundred devices with switchers in them. They won’t all be on at once.
All it takes is one “bad one” running every so often to really make a mess.
If you grab one of these things and take a look with a spectrum analyzer:
You typically see a pretty
big spike at whatever frequency it happens to like today as well as
harmonics of that frequency. In addition, there is a fairly broad set
of “crud sidebands” that cover a wide range at a lower level. It also
should be noted that there are lots of ways they do this (even just
at
60 KHz). Each one has it’s own “signature”.
If you are way out on a farm somewhere, you might only have to deal
with your own devices. In a typical urban setting, you have a lot
more than just your own devices to worry about.
Not every “bad one” will be equally bad. Some will bother things in a
pretty small area. Others just might cover a pretty wide area.
There’s also somewhat more difficult things that do come up. This or
that device is hooked to something stable. It just happens to have
a spur at 60KHz. This is not very common. The issue here is that the
“threat signal” is a pretty stable carrier. Somebody putting up a
local “60 KHz home transmitter” to run their wall clocks? It does
indeed happen.
Since this is all a bit random, what works today may not work tomorrow.
Equally, that gizmo that
made it impossible for months just might burn out. Putting a lot of
time and money into something like this only to find it destroyed by
“something” is not a good outcome.
Are there rules ( ok laws) about this stuff? Sure there are. None of
this should be going on. Much of the compliance testing and
certification is done in a pretty informal manner (there is no
outside lab involved). Some folks will always be a bit less worried
about this than others.
From doing this in the past, I’d plan for an outdoor antenna. Get it
as far from the power lines and any dwellings as you can. Even with
an indoor antenna, getting it as far from power and “other stuff”
is a really good idea.
Fun!!
Bob
On Sep 25, 2025, at 1:33 PM, Adrian Godwin via time-nuts <
time-nuts@lists.febo.com> wrote:
In case it's relevant ..
I'm in Bedford (I'm not the operator of that SDR though).
I've got a little heap of receivers in case they're interesting for
comparison, loan etc. A radio-4-LW-disciplined oscillator, a
Thunderbolt, a
heap of KS-24361 ref 0 GPSDOs, and (not connected) an MSF time
receiver,
a
Pluto SDR and a Rubidium oscillator.
I also have some random MSF and DCF77 clocks around. The MSF clocks
work pretty well but the DCF77 kitchen clock jumps a couple of hours
sometimes then jumps back a couple of days later. The MSF clock used
to get upset
by
a VGA monitor when it was close by. Doesn't seem to mind modern LCD ones.
I believe the LW-disciplined oscillator is on borrowed time. The
device
is
fine but the LW transmitter is in danger of being retired any time soon.
On Thu, Sep 25, 2025 at 1:35 PM john.haine--- via time-nuts <
time-nuts@lists.febo.com> wrote:
Many thanks for all the responses! A few random thoughts in response.
-
I'm based in the eastern UK near Cambridge. Anthorn where the
only
"nearby" eLoran station, and also MSF, is 370 km / 230 miles.
Mainflingen
(DCF77) is 666 km/ 535 miles. DCF77 signals are largely available
in
the
UK
and most of the cheap RC clocks use them. However in some
locations
they
are a bit dodgy - for example Bristol University where I sometimes
work
put
RC clocks in most public areas and they are often wrong or spend
all
their
time trying to re-sync. MSF and eLoran would work better I suspect
though
consumer MSF receivers are less available as the market is more
UK-based while DCF 77 works over most of Europe.
-
Using an online SDR based in Bedford which is 40 miles of so
west
of
here eLoran from Anthorn is quite strong but DCF and MSF not
observable
that may be the antennas used as much as anything.
-
Unlike WWVB MSF does not have a phase modulation component, and
its
carrier is switched off completely by the keying. Phase locking to
its carrier therefore has to gate the PLL but there is one design
at least
that
does this.
-
DCF77 has a spread-spectrum phase mod component and its carrier
is
not switched off completely. This makes it much more robust if a
properly
designed coherent receiver is used and I've seen one design that
does
this
in software. Unfortunately
-
I wasn't aware that eLoran has a phase mod component, I need to
read
up on its waveform design more. But is being toted as a GPS/GNSS
backup for timing in Europe where we have a large and unfriendly
neighbour.
I'll study the responses in more detail and follow up if this
project
makes
it to the starting blocks.
Thanks everyone for your input!
- John
-----Original Message-----
From: john.haine--- via time-nuts time-nuts@lists.febo.com
Sent: 22 September 2025 10:20
To: 'Discussion of precise time and frequency measurement'
time-nuts@lists.febo.com
Cc: john.haine@haine-online.net
Subject: [time-nuts] vlf-disciplined OCXO circuit
Does anyone know of designs for disciplined OCXOs that are
referenced to off-air, especially VLF, signals other than GPS/GNSS
please? With
modules
for the latter being so cheap this might seem pointless but there
are
some
potential advantages.
-
GPS reception at indoor locations where mechanical clocks need
to
be
monitored is often (usually?) unavailable because of shadowing and
building
absorbtion and it's usually inconvenient to run a cable.
-
GPS is increasingly likely to be jammed either by criminal
elements
or "state actors".
-
VLF (the likes of WWVB / MSF / DCF77 / e-Loran) is more likely
to
be
received indoors.
-
E-Loran is being tipped as an off-air time source to back up GPS
and
will become increasingly available.
-
There's the possibility of a multi-standard receiver that might
find
and lock to any available source, and potentially to several.
Obviously there are a lot of very cheap modules around to receive
the signals but these discard the carrier and just output the
time-code
logic
signal. I have seen a design for an MSF-locked standard in
discrete components and more recently an MSF receiver implemented
as direct-conversion SDR on a Raspberry Pi Pico which phase locks
its
internal
digital LO to the carrier but I suspect that its phase noise would
be pretty ropey - really intended as a time, not frequency, source.
-
John Haine.
time-nuts mailing list -- time-nuts@lists.febo.com To unsubscribe
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an
email to time-nuts-leave@lists.febo.com
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WHat's your plan for an 60kHz antenna ?
If a ferrite loaded loop, you can get far better selectivity by building
it as a double tuned circuit :
- with the loop antenna being one tank, and something like a potcore
inductor being the 2nd coupled tank. - this will give you a nice flattop-ish response with steeper skirts
than a single mag loop antenna
I think the SDR using a single $10 STM32 microcontroller with a 2 Msps
12 bit ADC would be ideal as a back end.. run the ADC at something like
240 kHz
You'l
l need some sort of aliasing filter ahead as AM radio stations may alias
into the passband, depending on choice of sampling rate.
-glen
On 29/09/2025 07:52, john.haine--- via time-nuts wrote:
The thing about SDR is always that they have zero selectivity prior to the RF amp and mixer. If you have a lot of interference around it's good to have selectivity before you get into anything potentially non-linear, and it also makes the ADC's job easier. Surely if the crystal has known characteristics it should be possible to compensate for the varying phase shift? Or, given that it's going to discipline an OCXO anyway, put it in an oven too.
-----Original Message-----
Hi
At VLF, you can run straight into an ADC converter. No mixer, and not much of an RF amp. Until you get a noise spike that saturates the (maybe) 5V input range on the converter ….. it pretty much just works. If you put a narrowband filter in front of the ADC, you then should come up with a noise source to “dither” the input. A 24 bit converter can give you a lot of dynamic range and really good IMD performance.
Do you want the local AM broadcast station(s) blasting in? Probably not. In most areas, that’s a pretty broad filter. That’s good because the narrower the filter, the more it’s going to mess up the carrier phase as temperature and humidity change. Build the filter (or tuned antenna) with ferrite based inductors and it might get really crazy.
Once you get into the ADC, you can implement a really narrow filter and have essentially no impact on carrier phase (in terms of stability).
Bob
On Sep 28, 2025, at 5:52 PM, john.haine--- via time-nuts time-nuts@lists.febo.com wrote:
The thing about SDR is always that they have zero selectivity prior to the RF amp and mixer. If you have a lot of interference around it's good to have selectivity before you get into anything potentially non-linear, and it also makes the ADC's job easier. Surely if the crystal has known characteristics it should be possible to compensate for the varying phase shift? Or, given that it's going to discipline an OCXO anyway, put it in an oven too.
-----Original Message-----
From: Bob Camp via time-nuts time-nuts@lists.febo.com
Sent: 28 September 2025 12:54
To: Discussion of precise time and frequency measurement time-nuts@lists.febo.com
Cc: Bob Camp kb8tq@n1k.org
Subject: [time-nuts] Re: vlf-disciplined OCXO circuit
Hi
A crystal filter at the front end of a 60 KHz receiver is a bit of a win / loose sort of thing.
The selection of crystals available at 60 KHz is a bit limited. The typical parts have temperature coefficient’s that are a bit high. This results in a filter with phase delay that varies noticeably over even modest temperature changes. That’s not ideal for a oscillator locking application. For a wall clock that is doing great if it’s only tens of milliseconds off, it’s not an issue.
These days a SDR based approach is the better choice.
Bob
On Sep 27, 2025, at 6:10 AM, john.haine--- via time-nuts time-nuts@lists.febo.com wrote:
Way back when I was a consultant I had to troubleshoot a badly executed consumer electronics project that included an AM/FM broadcast receiver amongst other elements. This was installed in the central control panel of a built-in vacuum cleaner system (!), the idea being that radio programmes could be listened to throughout a house. The receiver used a truly awful LF/MF rf module with a high impedance input and zero input selectivity. The "antenna" was a bit of wire hanging out the back of the product enclosure (itself installed into the wall of the house). LF/MF reception was totally impossible in nearly every location due to a loud interference "hash" clearly emanating from the mains. However an ordinary portable radio in the same location usually worked absolutely fine. Eventually found that the short "capacitive" antenna (i.e. the wire) picked up strong impulsive near-E-field interference from things like fluorescent tubes etc, whilst the inductive loop antenna in an ordinary radio was relatively immune from this. The only way we found to solve the problem was to install an external antenna which was pretty ugly with an existing installation in a house.
So a ferrite loop antenna, preferably tuned, may help a lot, as would a crystal filter as generally used in consumer RC clocks. If you can find an application note for the chips they use it should include a circuit.
-----Original Message-----
From: Peter McCollum via time-nuts time-nuts@lists.febo.com
Sent: 26 September 2025 16:10
To: Discussion of precise time and frequency measurement
time-nuts@lists.febo.com
Cc: Peter McCollum saipan1959@gmail.com
Subject: [time-nuts] Re: vlf-disciplined OCXO circuit
As it happens, lately I've been working on the following project:
Building a 'retro' decoder for the WWVB time signal, based on a Motorola 6800-series MCU (actually a 6803). It gets its input from a Spectracom 8161, which outputs TTL-level bits detected from the 60 KHz antenna input.
Just this morning I got the basic pulse-width measurement code working correctly, to detect the 0's, 1's, and Marker pulses.
At the moment, my antenna is a few feet of wire. I'm only about 120 miles from Ft. Collins, so I get plenty of signal. However, the signal is "messed with" by the issues that Bob and others have described. So I get a lot of pulses of the wrong width.
I'll try a small ferrite loop antenna, to see if it cleans things up.
After I can decode WWVB reliably, I'll add code to output the old NASA and IRIG-B 1-second time codes.
Pete
On Fri, Sep 26, 2025 at 7:29 AM Bob Camp via time-nuts < time-nuts@lists.febo.com> wrote:
Hi
There are an unfortunate lot of things out there that can take out a
60KHz VLF signal. For cost reasons, 60kHz is what a lot of low end
switchers run at. They are not controlled in any way so that’s a very
approximate number. Since they are typically hooked to the power
line, they have a fine “antenna” to work with. If you count the
number of this and that in the typical home, there might well be a
hundred devices with switchers in them. They won’t all be on at once.
All it takes is one “bad one” running every so often to really make a mess.
If you grab one of these things and take a look with a spectrum analyzer:
You typically see a pretty
big spike at whatever frequency it happens to like today as well as
harmonics of that frequency. In addition, there is a fairly broad set
of “crud sidebands” that cover a wide range at a lower level. It also
should be noted that there are lots of ways they do this (even just
at
60 KHz). Each one has it’s own “signature”.
If you are way out on a farm somewhere, you might only have to deal
with your own devices. In a typical urban setting, you have a lot
more than just your own devices to worry about.
Not every “bad one” will be equally bad. Some will bother things in a
pretty small area. Others just might cover a pretty wide area.
There’s also somewhat more difficult things that do come up. This or
that device is hooked to something stable. It just happens to have
a spur at 60KHz. This is not very common. The issue here is that the
“threat signal” is a pretty stable carrier. Somebody putting up a
local “60 KHz home transmitter” to run their wall clocks? It does
indeed happen.
Since this is all a bit random, what works today may not work tomorrow.
Equally, that gizmo that
made it impossible for months just might burn out. Putting a lot of
time and money into something like this only to find it destroyed by
“something” is not a good outcome.
Are there rules ( ok laws) about this stuff? Sure there are. None of
this should be going on. Much of the compliance testing and
certification is done in a pretty informal manner (there is no
outside lab involved). Some folks will always be a bit less worried
about this than others.
From doing this in the past, I’d plan for an outdoor antenna. Get it
as far from the power lines and any dwellings as you can. Even with
an indoor antenna, getting it as far from power and “other stuff”
is a really good idea.
Fun!!
Bob
On Sep 25, 2025, at 1:33 PM, Adrian Godwin via time-nuts <
time-nuts@lists.febo.com> wrote:
In case it's relevant ..
I'm in Bedford (I'm not the operator of that SDR though).
I've got a little heap of receivers in case they're interesting for
comparison, loan etc. A radio-4-LW-disciplined oscillator, a
Thunderbolt, a
heap of KS-24361 ref 0 GPSDOs, and (not connected) an MSF time
receiver,
a
Pluto SDR and a Rubidium oscillator.
I also have some random MSF and DCF77 clocks around. The MSF clocks
work pretty well but the DCF77 kitchen clock jumps a couple of hours
sometimes then jumps back a couple of days later. The MSF clock used
to get upset
by
a VGA monitor when it was close by. Doesn't seem to mind modern LCD ones.
I believe the LW-disciplined oscillator is on borrowed time. The
device
is
fine but the LW transmitter is in danger of being retired any time soon.
On Thu, Sep 25, 2025 at 1:35 PM john.haine--- via time-nuts <
time-nuts@lists.febo.com> wrote:
Many thanks for all the responses! A few random thoughts in response.
-
I'm based in the eastern UK near Cambridge. Anthorn where the
only
"nearby" eLoran station, and also MSF, is 370 km / 230 miles.
Mainflingen
(DCF77) is 666 km/ 535 miles. DCF77 signals are largely available
in
the
UK
and most of the cheap RC clocks use them. However in some
locations
they
are a bit dodgy - for example Bristol University where I sometimes
work
put
RC clocks in most public areas and they are often wrong or spend
all
their
time trying to re-sync. MSF and eLoran would work better I suspect
though
consumer MSF receivers are less available as the market is more
UK-based while DCF 77 works over most of Europe.
-
Using an online SDR based in Bedford which is 40 miles of so
west
of
here eLoran from Anthorn is quite strong but DCF and MSF not
observable
that may be the antennas used as much as anything.
-
Unlike WWVB MSF does not have a phase modulation component, and
its
carrier is switched off completely by the keying. Phase locking to
its carrier therefore has to gate the PLL but there is one design
at least
that
does this.
-
DCF77 has a spread-spectrum phase mod component and its carrier
is
not switched off completely. This makes it much more robust if a
properly
designed coherent receiver is used and I've seen one design that
does
this
in software. Unfortunately
-
I wasn't aware that eLoran has a phase mod component, I need to
read
up on its waveform design more. But is being toted as a GPS/GNSS
backup for timing in Europe where we have a large and unfriendly
neighbour.
I'll study the responses in more detail and follow up if this
project
makes
it to the starting blocks.
Thanks everyone for your input!
- John
-----Original Message-----
From: john.haine--- via time-nuts time-nuts@lists.febo.com
Sent: 22 September 2025 10:20
To: 'Discussion of precise time and frequency measurement'
time-nuts@lists.febo.com
Cc: john.haine@haine-online.net
Subject: [time-nuts] vlf-disciplined OCXO circuit
Does anyone know of designs for disciplined OCXOs that are
referenced to off-air, especially VLF, signals other than GPS/GNSS
please? With
modules
for the latter being so cheap this might seem pointless but there
are
some
potential advantages.
-
GPS reception at indoor locations where mechanical clocks need
to
be
monitored is often (usually?) unavailable because of shadowing and
building
absorbtion and it's usually inconvenient to run a cable.
-
GPS is increasingly likely to be jammed either by criminal
elements
or "state actors".
-
VLF (the likes of WWVB / MSF / DCF77 / e-Loran) is more likely
to
be
received indoors.
-
E-Loran is being tipped as an off-air time source to back up GPS
and
will become increasingly available.
-
There's the possibility of a multi-standard receiver that might
find
and lock to any available source, and potentially to several.
Obviously there are a lot of very cheap modules around to receive
the signals but these discard the carrier and just output the
time-code
logic
signal. I have seen a design for an MSF-locked standard in
discrete components and more recently an MSF receiver implemented
as direct-conversion SDR on a Raspberry Pi Pico which phase locks
its
internal
digital LO to the carrier but I suspect that its phase noise would
be pretty ropey - really intended as a time, not frequency, source.
-
John Haine.
time-nuts mailing list -- time-nuts@lists.febo.com To unsubscribe
send
an
email to time-nuts-leave@lists.febo.com
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A wonderful evening!
Quite a few interesting things have been written on this thread,
but a few of the original questions remain unaswered.
On Mon, 22 Sep 2025 10:19:56 +0100
"john.haine--- via time-nuts" time-nuts@lists.febo.com wrote:
Does anyone know of designs for disciplined OCXOs that are referenced to
off-air, especially VLF, signals other than GPS/GNSS please? With modules
for the latter being so cheap this might seem pointless but there are some
potential advantages.
Yes, there have been several. But most of these have gone out of
fashion with the arrival of cheap GPS modules. Especially in the
German amateur radio community, various DCF77 disciplined oscillator
projects were shared.
E.g.:
"Baumappe zum DCF77-gesteuerten 10MHz-Frequenznomral",
by Funkamateur, 2009.
https://www.box73.de/file_dl/bausaetze/BX-176.pdf
Which is based on the design by DL1SNG and DL1FAC that was published
in Funkamateur December 2008
https://web.archive.org/web/20240609113503if_/https://www.worldradiohistory.com/INTERNATIONAL/FunkAmateur/FunkAmateur-2008-12.pdf
There are a few others, that work kind of in a similar way.
A more modern design is
"Performance Analysis and Receiver Architectures of DCF77
Radio-Controlled Clocks", by Daniel Engeler, 2012
https://doi.org/10.1109/TUFFC.2012.2272
Daniel used an FPGA to do tracking of the signal including the
PSK signal. One of the goals of this was to use this as a demonstrator
on how to decode and track time signals. The paper contains quite
a bit of analysis of different receiver strategies and how they
perform. So it's definitely worth a read.
Another interesting one is
"Software-Defined Radio Decoding of DCF77: Time and Frequency
Dissemination with a Sound Card" by Jean-Michel Friedt,
Clément Eustache, Emile Carry and Enrico Rubiola, 2018
https://doi.org/10.1002/2017RS006420
where they use a soundcard and GnuRadio to decode the signal.
- GPS reception at indoor locations where mechanical clocks need to be
monitored is often (usually?) unavailable because of shadowing and building
absorbtion and it's usually inconvenient to run a cable.
While this is true, the disadvantage of VLF transmissions is that
they have a very high variability during the day and from day to day.
Not only does the signal strength vary a lot (up to several 10s of dB)
but also the timing varies by 100s of µs to 10s of ms, depending on
where you are, relative to the station. So, if you need time accurate
to a few µs (which is a pretty normal requirement for data centers
these days), then VLF isn't going to cut it. In these cases it is
easier to just wire up an antenna to the roof and get <100ns from
GPS, then distribute time inside the building, either as a dedicated
pulse signal (often used for wall clocks) or over the network via
NTP or PTP.
- GPS is increasingly likely to be jammed either by criminal elements
or "state actors".
While this is true, most of the jaming in the US is actually accidental
jamming from faulty RF equipment (like active antennas for TV reception).
Though, GPS jammers of disgruntled drivers who don't want their boss
to know they are going for a coffee break, are on the rise.
But, while jamming is getting more prevalent, it's by far not common.
A lot of our infrastructure does depend on GNSS reception. And most
of it works pretty well.
In eastern Europe, things are a little bit different at the moment.
(see, e.g. https://www.flightradar24.com/data/gps-jamming )
I am not fully aware what the different users of time do to cope
with this situation, but given that there has been very few
articles covering the situation, they seem to cope quite fine.
- E-Loran is being tipped as an off-air time source to back up GPS and
will become increasingly available.
For all practical purposes, Loran is dead. While e-Loran does try
to make a come-back, it has failed to achieve that in the past 10 years.
Given that the reliability of GNSS is good enough for most applications
that use it, an additional expensive service isn't going to get
investors. And the states do not seem to be interested in it.
There is an European iniciative for reliable time distribution
on a continental scale, but as far as I know, VLF transmissions
are not being considered.
- There's the possibility of a multi-standard receiver that might find
and lock to any available source, and potentially to several.
I have thought of building a multi-standard receiver myself
a couple of years ago. But quite honestly, the availability,
reliability and accuracy of GNSS makes the other time signals
just not worthwhile. At least not as a product. Ok, that's not
100% correct. Companies like Meinberg still sell DCF77/MSF/WWVB
systems. I am not quite sure what their purpose is, but apparently
people are paying for them. But I wouldn't develop such a new
product for VLF reception these days, as the demand is rather low
and does not justify the development cost. Though, it could be a
lot of fun as a hobby project.
Obviously there are a lot of very cheap modules around to receive the
signals but these discard the carrier and just output the time-code logic
signal. I have seen a design for an MSF-locked standard in discrete
components and more recently an MSF receiver implemented as
direct-conversion SDR on a Raspberry Pi Pico which phase locks its internal
digital LO to the carrier but I suspect that its phase noise would be pretty
ropey - really intended as a time, not frequency, source.
These modules are mostly built based on chips used for alarm clocks.
These are fine as long as they aren't more than a few ms off. Hence
they don't use the carrier. And that's also the reason why they are
so cheap: the chips they use are produced in the millions and a few
of those end up on boards used as time references for computers and
such.
If you want to build your own VLF receiver, I would go the way PHK
described: Use a losely tuned antenna, add some pre-amplifier.
A simple JFET stage as a high impedance first stage for the
antenna and a low noise opamp should do the trick. Maybe add an
intermediate BJT based amplifier for additional low noise gain.
If you want to receive only one or two specific transmitters, you
might want to add some filtering too. Amplify the signal up to the
point where you can feed it to an ADC of sorts. Anything above 4bits
is enough, unless you are dealing with a very weak signal that you
need to dig out of the noise, then go for as many bits as possible.
Feed the ADC samples into a µC that does all the magic DSP stuff.
How you want to do it, depends a bit on how complicated you want
it to be and which signal you are tracking. Average the signal
for at least 24h and steer a good OCXO with it.
Attila Kinali
--
The driving force behind research is the question: "Why?"
There are things we don't understand and things we always
wonder about. And that's why we do research.
-- Kobayashi Makoto
Bob Camp via time-nuts writes:
At VLF, you can run straight into an ADC converter. No mixer, and
not much of an RF amp. Until you get a noise spike that saturates
the (maybe) 5V input range on the converter …..
So this gets to choice of antenna.
The best, flattest wide-band antennas are indisputably "E-field
probes", basically a piece of conductor connected to an amplifier
with as high input impedance as you can make it..
It is almost trivial to build an E-field probe which is flat from
DC to north of a GHz. I'm personally partial to Chris Trask's
designs ("Complementary Push-Pull Amplifiers for Active Antennas:
A Critical Review") but there are many others.
But because they are wideband, they also pick up "static" and in
particular the insanely wide spectrum[1] of nearby lightning strikes,
which are the major cause of the big transients you talk about.
Below a MHz one can also use "M-field probes", which is a coil
attached to an amplifier with a balanced input, commonly known as
a loop-antenna.
The kind of noise spikes you talk about only happen in loop-antenna
if somebody quenches the superconducting magnet in the MR-scanner
next door.
A major difference between loop-antennas and e-field probes is
that loop-antennas have a figure-of-eight sensitivity pattern.
This is great if, like me, you have a hundreds of kW LF transmitter
in the next town over, but less great if you want to receive several
signals from all over at the same time.
Loop-antennas can also be tuned to a particular frequency band
by adding a capacitor in parallel to the coil, and you can get
amazing "amplification" by using a high impedance input amplifier
because it operates near-resonance. The downside is that it
takes forever for the resonance to die out again, which is
why it is almost only used in the "run forever on an AAA battery"
radio-controlled clocks, which only need a ~3Hz bandwidth.
I have experimented with both E-field and M-field probes in the VLF
band and I far prefer (untuned) M-field probes.
Poul-Henning
[1] Zero to many kA in less than 5 microsecond, you do the FFT.
--
Poul-Henning Kamp | UNIX since Zilog Zeus 3.20
phk@FreeBSD.ORG | TCP/IP since RFC 956
FreeBSD committer | BSD since 4.3-tahoe
Never attribute to malice what can adequately be explained by incompetence.
Yes, but you DO need a bunch o'bits in the ADC (depending on that input filter). However, you could have a fairly wide filter, chosen for small tempco of phase shift, and that would help a lot. But, for instance, if you try to do it with the mighty $20 RTL-SDR, which is 8 bits I/Q and 2 MHz BW, AM/MW stations will be a problem.
On Sun, 28 Sep 2025 07:54:01 -0400, Bob Camp via time-nuts time-nuts@lists.febo.com wrote:
Hi
A crystal filter at the front end of a 60 KHz receiver is a bit of a win / loose sort of thing.
The selection of crystals available at 60 KHz is a bit limited. The typical parts have temperature coefficient’s that are a bit high. This results in a filter with phase delay that varies noticeably over even modest temperature changes. That’s not ideal for a oscillator locking application. For a wall clock that is doing great if it’s only tens of milliseconds off, it’s not an issue.
These days a SDR based approach is the better choice.
Bob
On Sep 27, 2025, at 6:10 AM, john.haine--- via time-nuts wrote:
Way back when I was a consultant I had to troubleshoot a badly executed consumer electronics project that included an AM/FM broadcast receiver amongst other elements. This was installed in the central control panel of a built-in vacuum cleaner system (!), the idea being that radio programmes could be listened to throughout a house. The receiver used a truly awful LF/MF rf module with a high impedance input and zero input selectivity. The "antenna" was a bit of wire hanging out the back of the product enclosure (itself installed into the wall of the house). LF/MF reception was totally impossible in nearly every location due to a loud interference "hash" clearly emanating from the mains. However an ordinary portable radio in the same location usually worked absolutely fine. Eventually found that the short "capacitive" antenna (i.e. the wire) picked up strong impulsive near-E-field interference from things like fluorescent tubes etc, whilst the inductive loop antenna in an ordinary radio was relatively immune from this. The only way we found to solve the problem was to install an external antenna which was pretty ugly with an existing installation in a house.
So a ferrite loop antenna, preferably tuned, may help a lot, as would a crystal filter as generally used in consumer RC clocks. If you can find an application note for the chips they use it should include a circuit.
-----Original Message-----
From: Peter McCollum via time-nuts
Sent: 26 September 2025 16:10
To: Discussion of precise time and frequency measurement
Cc: Peter McCollum
Subject: [time-nuts] Re: vlf-disciplined OCXO circuit
As it happens, lately I've been working on the following project:
Building a 'retro' decoder for the WWVB time signal, based on a Motorola 6800-series MCU (actually a 6803). It gets its input from a Spectracom 8161, which outputs TTL-level bits detected from the 60 KHz antenna input.
Just this morning I got the basic pulse-width measurement code working correctly, to detect the 0's, 1's, and Marker pulses.
At the moment, my antenna is a few feet of wire. I'm only about 120 miles from Ft. Collins, so I get plenty of signal. However, the signal is "messed with" by the issues that Bob and others have described. So I get a lot of pulses of the wrong width.
I'll try a small ferrite loop antenna, to see if it cleans things up.
After I can decode WWVB reliably, I'll add code to output the old NASA and IRIG-B 1-second time codes.
Pete
On Fri, Sep 26, 2025 at 7:29 AM Bob Camp via time-nuts < time-nuts@lists.febo.com> wrote:
Hi
There are an unfortunate lot of things out there that can take out a
60KHz VLF signal. For cost reasons, 60kHz is what a lot of low end
switchers run at. They are not controlled in any way so that’s a very
approximate number. Since they are typically hooked to the power line,
they have a fine “antenna” to work with. If you count the number of
this and that in the typical home, there might well be a hundred
devices with switchers in them. They won’t all be on at once. All it
takes is one “bad one” running every so often to really make a mess.
If you grab one of these things and take a look with a spectrum analyzer:
You typically see a pretty
big spike at whatever frequency it happens to like today as well as
harmonics of that frequency. In addition, there is a fairly broad set
of “crud sidebands” that cover a wide range at a lower level. It also
should be noted that there are lots of ways they do this (even just at
60 KHz). Each one has it’s own “signature”.
If you are way out on a farm somewhere, you might only have to deal
with your own devices. In a typical urban setting, you have a lot more
than just your own devices to worry about.
Not every “bad one” will be equally bad. Some will bother things in a
pretty small area. Others just might cover a pretty wide area.
There’s also somewhat more difficult things that do come up. This or
that device is hooked to something stable. It just happens to have a
spur at 60KHz. This is not very common. The issue here is that the
“threat signal” is a pretty stable carrier. Somebody putting up a
local “60 KHz home transmitter” to run their wall clocks? It does
indeed happen.
Since this is all a bit random, what works today may not work tomorrow.
Equally, that gizmo that
made it impossible for months just might burn out. Putting a lot of
time and money into something like this only to find it destroyed by
“something” is not a good outcome.
Are there rules ( ok laws) about this stuff? Sure there are. None of
this should be going on. Much of the compliance testing and
certification is done in a pretty informal manner (there is no outside
lab involved). Some folks will always be a bit less worried about this
than others.
From doing this in the past, I’d plan for an outdoor antenna. Get it
as far from the power lines and any dwellings as you can. Even with an
indoor antenna, getting it as far from power and “other stuff”
is a really good idea.
Fun!!
Bob
On Sep 25, 2025, at 1:33 PM, Adrian Godwin via time-nuts <
time-nuts@lists.febo.com> wrote:
In case it's relevant ..
I'm in Bedford (I'm not the operator of that SDR though).
I've got a little heap of receivers in case they're interesting for
comparison, loan etc. A radio-4-LW-disciplined oscillator, a
Thunderbolt, a
heap of KS-24361 ref 0 GPSDOs, and (not connected) an MSF time
receiver,
a
Pluto SDR and a Rubidium oscillator.
I also have some random MSF and DCF77 clocks around. The MSF clocks
work pretty well but the DCF77 kitchen clock jumps a couple of hours
sometimes then jumps back a couple of days later. The MSF clock used
to get upset
by
a VGA monitor when it was close by. Doesn't seem to mind modern LCD ones.
I believe the LW-disciplined oscillator is on borrowed time. The
device
is
fine but the LW transmitter is in danger of being retired any time soon.
On Thu, Sep 25, 2025 at 1:35 PM john.haine--- via time-nuts <
time-nuts@lists.febo.com> wrote:
Many thanks for all the responses! A few random thoughts in response.
- I'm based in the eastern UK near Cambridge. Anthorn where the
only
"nearby" eLoran station, and also MSF, is 370 km / 230 miles.
Mainflingen
(DCF77) is 666 km/ 535 miles. DCF77 signals are largely available
in
the
UK
and most of the cheap RC clocks use them. However in some
locations
they
are a bit dodgy - for example Bristol University where I sometimes
work
put
RC clocks in most public areas and they are often wrong or spend
all
their
time trying to re-sync. MSF and eLoran would work better I suspect
though
consumer MSF receivers are less available as the market is more
UK-based while DCF 77 works over most of Europe.
- Using an online SDR based in Bedford which is 40 miles of so
west
of
here eLoran from Anthorn is quite strong but DCF and MSF not
observable
that may be the antennas used as much as anything.
- Unlike WWVB MSF does not have a phase modulation component, and
its
carrier is switched off completely by the keying. Phase locking to
its carrier therefore has to gate the PLL but there is one design
at least
that
does this.
- DCF77 has a spread-spectrum phase mod component and its carrier
is
not switched off completely. This makes it much more robust if a
properly
designed coherent receiver is used and I've seen one design that
does
this
in software. Unfortunately
- I wasn't aware that eLoran has a phase mod component, I need to
read
up on its waveform design more. But is being toted as a GPS/GNSS
backup for timing in Europe where we have a large and unfriendly
neighbour.
I'll study the responses in more detail and follow up if this
project
makes
it to the starting blocks.
Thanks everyone for your input!
- John
-----Original Message-----
From: john.haine--- via time-nuts
Sent: 22 September 2025 10:20
To: 'Discussion of precise time and frequency measurement'
Cc: john.haine@haine-online.net
Subject: [time-nuts] vlf-disciplined OCXO circuit
Does anyone know of designs for disciplined OCXOs that are
referenced to off-air, especially VLF, signals other than GPS/GNSS
please? With
modules
for the latter being so cheap this might seem pointless but there
are
some
potential advantages.
- GPS reception at indoor locations where mechanical clocks need
to
be
monitored is often (usually?) unavailable because of shadowing and
building
absorbtion and it's usually inconvenient to run a cable.
- GPS is increasingly likely to be jammed either by criminal
elements
or "state actors".
- VLF (the likes of WWVB / MSF / DCF77 / e-Loran) is more likely
to
be
received indoors.
- E-Loran is being tipped as an off-air time source to back up GPS
and
will become increasingly available. - There's the possibility of a multi-standard receiver that might
find
and lock to any available source, and potentially to several.
Obviously there are a lot of very cheap modules around to receive
the signals but these discard the carrier and just output the
time-code
logic
signal. I have seen a design for an MSF-locked standard in
discrete components and more recently an MSF receiver implemented
as direct-conversion SDR on a Raspberry Pi Pico which phase locks
its
internal
digital LO to the carrier but I suspect that its phase noise would
be pretty ropey - really intended as a time, not frequency, source.
- John Haine.
time-nuts mailing list -- time-nuts@lists.febo.com To unsubscribe
send
an
email to time-nuts-leave@lists.febo.com
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On Sep 29, 2025, at 3:31 AM, Poul-Henning Kamp phk@phk.freebsd.dk wrote:
So this gets to choice of antenna.
Hi
Assuming we have the bits and pieces worked out (yes that’s a big assumption): Next up is how this fits together.
A disciplined oscillator can be looked at in many ways. The most common is to look at it as a frequency reference. It is not unusual to use ADEV to describe what it’s doing. There are a bunch of other objectives you might have and ways to look at things. Let’s keep it somewhat simple for now. This already is going to be a long post.
Your OCXO has an ADEV curve. Let’s pick a fairly common one and use it.
2x10^-12 at 1 second
1x10^-12 at 10 seconds
1x10^-12 at 100 seconds
5x10^-12 at 1K seconds (maybe)
keeps going up past 1K seconds
That’s what an HP 10811 that has been on power for a few months in a very stable environment might be doing. In a not quite so stable environment, you might be above 1x10^-11 at 1K seconds. Net is, you get the famous “bathtub” shaped curve.
Your disciplining source “as received” has an ADEV plot. Typically there is a number at 1 second and it proceeds down a curve from there. It may go as 1/tau or as 1/squrt(tau). This eventually hits a limit and the slope changes.
The typical objective is to combine the “best” of the OCXO curve with the curve from the disciplining source. The hope is that you get a resulting curve that is quite good. Ideally, the performance of the disciplining source needs to be better than the OCXO at a reasonable tau. If the tau gets to long, the OCXO is heading up the right hand side of the bathtub curve.
A GNSS module might give you a 1 second ADEV anywhere from 1x10^-8 to 1x10^-10 depending on the specific part you are using. It is likely that it goes down at 1/tau out past 10K seconds. If you start at 1x10^-9, you will hit 1x10^-11 at 100 seconds. You will cross the OCXO ADEV curve shown above before you get to 1,000 seconds.
A VLF carrier is a much less stable source. It also likely drops off as 1/sqrt(tau). If it’s 1x10^-6 at 1 second, that’s way better than anything you would need for a wall clock. If it’s 1x10^-7 and drops with a power law (square root), you would be at 1x10^-9 at 10,000 seconds.
The cross over between the OCXO and disciplining source curves is done by your control loop. The time constants are very long. Doing this with anything other than (custom) code running in an MCU of some sort is not a good way to go.
You very much need to do the best job you can on your receiver if you are going to get any reasonable sort of result. A delta of 1x10-7 at 1 second would come out to 2.2 degrees at 60KHz.
Bob
Hi
Since this is 60 KHz, you can get a whole lot of bits on the ADC and still not be spending an insane amount of money. There are devices that do quite well. Start moving to parts that have clock rates above about a MHz and the cost does indeed rise at an alarming rate :)
One “side benefit” of all the man made crud down at VLF: Assuming you are in a normal urban environment, it’s there 24/7/365. Your setup does not need to get to anywhere close to the “thermal noise floor”. Part of that impacts how you hook to the antenna. The other part impacts how you rig up the ADC.
As you go looking at SDR’s showing their reception on the web, you do need to pay attention to just what you have tapped into. The combination of antenna plus SDR design will have an impact on what you do or don’t see on their plots. It’s not unusual for them show coverage down to frequencies that they really do not handle very well.
Bob
On Sep 29, 2025, at 10:01 AM, Jim Lux via time-nuts time-nuts@lists.febo.com wrote:
Yes, but you DO need a bunch o'bits in the ADC (depending on that input filter). However, you could have a fairly wide filter, chosen for small tempco of phase shift, and that would help a lot. But, for instance, if you try to do it with the mighty $20 RTL-SDR, which is 8 bits I/Q and 2 MHz BW, AM/MW stations will be a problem.
On Sun, 28 Sep 2025 07:54:01 -0400, Bob Camp via time-nuts time-nuts@lists.febo.com wrote:
Hi
A crystal filter at the front end of a 60 KHz receiver is a bit of a win / loose sort of thing.
The selection of crystals available at 60 KHz is a bit limited. The typical parts have temperature coefficient’s that are a bit high. This results in a filter with phase delay that varies noticeably over even modest temperature changes. That’s not ideal for a oscillator locking application. For a wall clock that is doing great if it’s only tens of milliseconds off, it’s not an issue.
These days a SDR based approach is the better choice.
Bob
On Sep 27, 2025, at 6:10 AM, john.haine--- via time-nuts wrote:
Way back when I was a consultant I had to troubleshoot a badly executed consumer electronics project that included an AM/FM broadcast receiver amongst other elements. This was installed in the central control panel of a built-in vacuum cleaner system (!), the idea being that radio programmes could be listened to throughout a house. The receiver used a truly awful LF/MF rf module with a high impedance input and zero input selectivity. The "antenna" was a bit of wire hanging out the back of the product enclosure (itself installed into the wall of the house). LF/MF reception was totally impossible in nearly every location due to a loud interference "hash" clearly emanating from the mains. However an ordinary portable radio in the same location usually worked absolutely fine. Eventually found that the short "capacitive" antenna (i.e. the wire) picked up strong impulsive near-E-field interference from things like fluorescent tubes etc, whilst the inductive loop antenna in an ordinary radio was relatively immune from this. The only way we found to solve the problem was to install an external antenna which was pretty ugly with an existing installation in a house.
So a ferrite loop antenna, preferably tuned, may help a lot, as would a crystal filter as generally used in consumer RC clocks. If you can find an application note for the chips they use it should include a circuit.
-----Original Message-----
From: Peter McCollum via time-nuts
Sent: 26 September 2025 16:10
To: Discussion of precise time and frequency measurement
Cc: Peter McCollum
Subject: [time-nuts] Re: vlf-disciplined OCXO circuit
As it happens, lately I've been working on the following project:
Building a 'retro' decoder for the WWVB time signal, based on a Motorola 6800-series MCU (actually a 6803). It gets its input from a Spectracom 8161, which outputs TTL-level bits detected from the 60 KHz antenna input.
Just this morning I got the basic pulse-width measurement code working correctly, to detect the 0's, 1's, and Marker pulses.
At the moment, my antenna is a few feet of wire. I'm only about 120 miles from Ft. Collins, so I get plenty of signal. However, the signal is "messed with" by the issues that Bob and others have described. So I get a lot of pulses of the wrong width.
I'll try a small ferrite loop antenna, to see if it cleans things up.
After I can decode WWVB reliably, I'll add code to output the old NASA and IRIG-B 1-second time codes.
Pete
On Fri, Sep 26, 2025 at 7:29 AM Bob Camp via time-nuts < time-nuts@lists.febo.com> wrote:
Hi
There are an unfortunate lot of things out there that can take out a
60KHz VLF signal. For cost reasons, 60kHz is what a lot of low end
switchers run at. They are not controlled in any way so that’s a very
approximate number. Since they are typically hooked to the power line,
they have a fine “antenna” to work with. If you count the number of
this and that in the typical home, there might well be a hundred
devices with switchers in them. They won’t all be on at once. All it
takes is one “bad one” running every so often to really make a mess.
If you grab one of these things and take a look with a spectrum analyzer:
You typically see a pretty
big spike at whatever frequency it happens to like today as well as
harmonics of that frequency. In addition, there is a fairly broad set
of “crud sidebands” that cover a wide range at a lower level. It also
should be noted that there are lots of ways they do this (even just at
60 KHz). Each one has it’s own “signature”.
If you are way out on a farm somewhere, you might only have to deal
with your own devices. In a typical urban setting, you have a lot more
than just your own devices to worry about.
Not every “bad one” will be equally bad. Some will bother things in a
pretty small area. Others just might cover a pretty wide area.
There’s also somewhat more difficult things that do come up. This or
that device is hooked to something stable. It just happens to have a
spur at 60KHz. This is not very common. The issue here is that the
“threat signal” is a pretty stable carrier. Somebody putting up a
local “60 KHz home transmitter” to run their wall clocks? It does
indeed happen.
Since this is all a bit random, what works today may not work tomorrow.
Equally, that gizmo that
made it impossible for months just might burn out. Putting a lot of
time and money into something like this only to find it destroyed by
“something” is not a good outcome.
Are there rules ( ok laws) about this stuff? Sure there are. None of
this should be going on. Much of the compliance testing and
certification is done in a pretty informal manner (there is no outside
lab involved). Some folks will always be a bit less worried about this
than others.
From doing this in the past, I’d plan for an outdoor antenna. Get it
as far from the power lines and any dwellings as you can. Even with an
indoor antenna, getting it as far from power and “other stuff”
is a really good idea.
Fun!!
Bob
On Sep 25, 2025, at 1:33 PM, Adrian Godwin via time-nuts <
time-nuts@lists.febo.com> wrote:
In case it's relevant ..
I'm in Bedford (I'm not the operator of that SDR though).
I've got a little heap of receivers in case they're interesting for
comparison, loan etc. A radio-4-LW-disciplined oscillator, a
Thunderbolt, a
heap of KS-24361 ref 0 GPSDOs, and (not connected) an MSF time
receiver,
a
Pluto SDR and a Rubidium oscillator.
I also have some random MSF and DCF77 clocks around. The MSF clocks
work pretty well but the DCF77 kitchen clock jumps a couple of hours
sometimes then jumps back a couple of days later. The MSF clock used
to get upset
by
a VGA monitor when it was close by. Doesn't seem to mind modern LCD ones.
I believe the LW-disciplined oscillator is on borrowed time. The
device
is
fine but the LW transmitter is in danger of being retired any time soon.
On Thu, Sep 25, 2025 at 1:35 PM john.haine--- via time-nuts <
time-nuts@lists.febo.com> wrote:
Many thanks for all the responses! A few random thoughts in response.
- I'm based in the eastern UK near Cambridge. Anthorn where the
only
"nearby" eLoran station, and also MSF, is 370 km / 230 miles.
Mainflingen
(DCF77) is 666 km/ 535 miles. DCF77 signals are largely available
in
the
UK
and most of the cheap RC clocks use them. However in some
locations
they
are a bit dodgy - for example Bristol University where I sometimes
work
put
RC clocks in most public areas and they are often wrong or spend
all
their
time trying to re-sync. MSF and eLoran would work better I suspect
though
consumer MSF receivers are less available as the market is more
UK-based while DCF 77 works over most of Europe.
- Using an online SDR based in Bedford which is 40 miles of so
west
of
here eLoran from Anthorn is quite strong but DCF and MSF not
observable
that may be the antennas used as much as anything.
- Unlike WWVB MSF does not have a phase modulation component, and
its
carrier is switched off completely by the keying. Phase locking to
its carrier therefore has to gate the PLL but there is one design
at least
that
does this.
- DCF77 has a spread-spectrum phase mod component and its carrier
is
not switched off completely. This makes it much more robust if a
properly
designed coherent receiver is used and I've seen one design that
does
this
in software. Unfortunately
- I wasn't aware that eLoran has a phase mod component, I need to
read
up on its waveform design more. But is being toted as a GPS/GNSS
backup for timing in Europe where we have a large and unfriendly
neighbour.
I'll study the responses in more detail and follow up if this
project
makes
it to the starting blocks.
Thanks everyone for your input!
- John
-----Original Message-----
From: john.haine--- via time-nuts
Sent: 22 September 2025 10:20
To: 'Discussion of precise time and frequency measurement'
Cc: john.haine@haine-online.net
Subject: [time-nuts] vlf-disciplined OCXO circuit
Does anyone know of designs for disciplined OCXOs that are
referenced to off-air, especially VLF, signals other than GPS/GNSS
please? With
modules
for the latter being so cheap this might seem pointless but there
are
some
potential advantages.
- GPS reception at indoor locations where mechanical clocks need
to
be
monitored is often (usually?) unavailable because of shadowing and
building
absorbtion and it's usually inconvenient to run a cable.
- GPS is increasingly likely to be jammed either by criminal
elements
or "state actors".
- VLF (the likes of WWVB / MSF / DCF77 / e-Loran) is more likely
to
be
received indoors.
- E-Loran is being tipped as an off-air time source to back up GPS
and
will become increasingly available. - There's the possibility of a multi-standard receiver that might
find
and lock to any available source, and potentially to several.
Obviously there are a lot of very cheap modules around to receive
the signals but these discard the carrier and just output the
time-code
logic
signal. I have seen a design for an MSF-locked standard in
discrete components and more recently an MSF receiver implemented
as direct-conversion SDR on a Raspberry Pi Pico which phase locks
its
internal
digital LO to the carrier but I suspect that its phase noise would
be pretty ropey - really intended as a time, not frequency, source.
- John Haine.
time-nuts mailing list -- time-nuts@lists.febo.com To unsubscribe
send
an
email to time-nuts-leave@lists.febo.com
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Slight digression here:
The radiated field from lightning has peaks around 80 MHz, likely due to the size of the "jumps" in the leader development and the current in the return stroke. This has been used to good effect by putting multiple VHF receivers out there and doing a sort of Time of Arrival mapping, and you can see the stroke develop within the cloud. As you can imagine (time nuts alert!) this requires very good timing of the pulse detection. I first saw this at a conference in ~2000, so it's been around a while.
The spectrum of the actual stroke is pretty broad, and depending on what's carrying it, may or may not radiate. As PHK said, 2 microsecond rise time from 10-90%, 50 microsecond fall time to 50% is the standard "lightning impulse shape", in the form 1- exp(-alpha*t))exp(-betat) (aka "double exponential"), with a series of pulses in one stroke.
Speaking of FFTs, that's one way they do analysis of antennas and systems with lightning is to use something like NEC to model the system at a variety of frequencies, and then combine them using an inverse FFT, weighted by the spectrum of the lightning.
On Mon, 29 Sep 2025 07:31:30 +0000, Poul-Henning Kamp via time-nuts time-nuts@lists.febo.com wrote:
Bob Camp via time-nuts writes:
At VLF, you can run straight into an ADC converter. No mixer, and
not much of an RF amp. Until you get a noise spike that saturates
the (maybe) 5V input range on the converter …..
So this gets to choice of antenna.
The best, flattest wide-band antennas are indisputably "E-field
probes", basically a piece of conductor connected to an amplifier
with as high input impedance as you can make it..
It is almost trivial to build an E-field probe which is flat from
DC to north of a GHz. I'm personally partial to Chris Trask's
designs ("Complementary Push-Pull Amplifiers for Active Antennas:
A Critical Review") but there are many others.
But because they are wideband, they also pick up "static" and in
particular the insanely wide spectrum[1] of nearby lightning strikes,
which are the major cause of the big transients you talk about.
Below a MHz one can also use "M-field probes", which is a coil
attached to an amplifier with a balanced input, commonly known as
a loop-antenna.
The kind of noise spikes you talk about only happen in loop-antenna
if somebody quenches the superconducting magnet in the MR-scanner
next door.
A major difference between loop-antennas and e-field probes is
that loop-antennas have a figure-of-eight sensitivity pattern.
This is great if, like me, you have a hundreds of kW LF transmitter
in the next town over, but less great if you want to receive several
signals from all over at the same time.
Loop-antennas can also be tuned to a particular frequency band
by adding a capacitor in parallel to the coil, and you can get
amazing "amplification" by using a high impedance input amplifier
because it operates near-resonance. The downside is that it
takes forever for the resonance to die out again, which is
why it is almost only used in the "run forever on an AAA battery"
radio-controlled clocks, which only need a ~3Hz bandwidth.
I have experimented with both E-field and M-field probes in the VLF
band and I far prefer (untuned) M-field probes.
Poul-Henning
[1] Zero to many kA in less than 5 microsecond, you do the FFT.
--
Poul-Henning Kamp | UNIX since Zilog Zeus 3.20
phk@FreeBSD.ORG | TCP/IP since RFC 956
FreeBSD committer | BSD since 4.3-tahoe
Never attribute to malice what can adequately be explained by incompetence.
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Bob Camp via time-nuts writes:
Since this is 60 KHz, you can get a whole lot of bits on the ADC and still not be spending an insane amount of money.
The three 12 bit, 500 ksps ADCs in a Rasberry Pico2 is plenty.
It comes with two ARM cores (or RiscV if you prefer), so you can
use one for the realtime processing and one for the interesting
stuff.
And I think they cost something like $5 qty=1 ?
It is probably fast enough that you can write the receiver software
in micropython instead of C :-)
Dont talk, just do it :-)
--
Poul-Henning Kamp | UNIX since Zilog Zeus 3.20
phk@FreeBSD.ORG | TCP/IP since RFC 956
FreeBSD committer | BSD since 4.3-tahoe
Never attribute to malice what can adequately be explained by incompetence.
suggest any narrowband receiver down there to be prepended by a wideband
noise blanker.
Lightning crashes, mains connected appliances being turned on and off.....
The act of narrowing the bandwidth stretches out those big pulses.
Usually, best option to to blank or limit the input where it is still
wideband, (at the front end) and that way minimize the blanking time.
Your mileage may vary with the antenna bandwidth possibly being the
limiting factor- a tuned magnetic antenna will be quite narrow, and put
a limit on the pulse risetimes.
The bandwidth of the antenna is stil likely to be 10x at least the
bandwidth of the baseband, so an front end noise blanker is still a
useful thing.
For how to build a good noise blanker there are plenty of web links
around. essentialy a edge detector firing (timed) a blank, mute or clip,
or whatever width works best.
-glen
On 28/09/2025 21:54, Bob Camp via time-nuts wrote:
glen english LIST via time-nuts writes:
I dont want to single you out Glen, but you and others in this
thread are extrapolating your experiences from higher frequency
bands to a lower frequency band without really knowing what you are
talking about.
VLF is not like HF.
At HF frequences M-field antennas are not an option due to the
inter-winding capacitance.
VLF is not even like MW.
It is much easier to propagate a radio signal at 500-1000 kHz than
it is at 40-125 kHz. The wave length really, REALLY, matters here.
And in particular M-field antennas are totally different from E-field
antennas.
You only need to tune your loop-antennas to save power in wall-clocks
or to reduce cost in consumer radios.
To receive time signals, and in particular Loran-C, you do not need to,
and should not tune your loop antenna.
Please do me the favour of just winding a small loop-antenna:
https://phk.freebsd.dk/loran-c/Antenna/
Hook it up to some microcontroller's ADC input, collect som data
and have some fun with it?
But at least stop preaching as if the VLF band is just the lower
end of the HF band, because it isn't.
--
Poul-Henning Kamp | UNIX since Zilog Zeus 3.20
phk@FreeBSD.ORG | TCP/IP since RFC 956
FreeBSD committer | BSD since 4.3-tahoe
Never attribute to malice what can adequately be explained by incompetence.
Hi Paul
good points Paul.
I develop VLF underground proximity systems professionally.. and these
are not simple single axis loop systems.
between 25kHz and 80 kHz.... so I have a reasonable handle on it.
Maybe people make the mistake of trying to acheive too high a Q antenna
----and smear the information that the LORANC signal is trying to impart.
I dont know much about LORANC, but I would expect that there is a direct
relationship between bandwidth and uncertainty for a specific number of
pulses observed.
Care to elaborate with some numbers on it ?
regards,
glen.
On 30/09/2025 19:42, Poul-Henning Kamp wrote:
glen english LIST via time-nuts writes:
I dont want to single you out Glen, but you and others in this
thread are extrapolating your experiences from higher frequency
bands to a lower frequency band without really knowing what you are
talking about.
VLF is not like HF.
At HF frequences M-field antennas are not an option due to the
inter-winding capacitance.
VLF is not even like MW.
It is much easier to propagate a radio signal at 500-1000 kHz than
it is at 40-125 kHz. The wave length really, REALLY, matters here.
And in particular M-field antennas are totally different from E-field
antennas.
You only need to tune your loop-antennas to save power in wall-clocks
or to reduce cost in consumer radios.
To receive time signals, and in particular Loran-C, you do not need to,
and should not tune your loop antenna.
glen english LIST writes:
----and smear the information that the LORANC signal is trying to impart.
I dont know much about LORANC, but I would expect that there is a direct
relationship between bandwidth and uncertainty for a specific number of
pulses observed.
Care to elaborate with some numbers on it ?
Ideally a Loran-C antenna should pass 85-115 kHz with high flatness,
but in practice few did and 90-110kHz seems to have been the "normal".
As the bandwidth narrows it causes two problems:
1: It makes it harder to identify the 3rd positive zero-crossing
This is a serious problem for "delay and add" type Loran-C detectors[1]
If the receiver always get it wrong the same way, ie: always picks
the second or the fourth crossing, this is "mostly harmless": The
second crossing has much worse S/N, the fourth may run into nightwave
problems, if you are very close to the transmitter.
If the receiver sometimes get one and sometimes get another, you
get a 10µs navigation error, roughly two nautical miles, which is
very noticeable near land but not so much on the high seas.
2: It changes the absolute timing of the 3rd positive zero-crossing.
This is only a problem for time-receivers, but if the antenna filters
are stable, it will be calibrated out with other electronic and
cable delays.
Serious Loran-C monitor or time receivers would often have from one
to five tunable high-Q notch filters to take out near-band CW
signals.
Poul-Henning
[1] If you add a 5 microsecond delayed copy of the Loran-C pulse
the sign of the result has a very characteristic 2-1-1-2 time
signature centered on the 3rd zero-crossing. Most microprocessor
based Loran-C signals did this, because it eliminates the need for
fast ADC's and simplifies the RF part too.
--
Poul-Henning Kamp | UNIX since Zilog Zeus 3.20
phk@FreeBSD.ORG | TCP/IP since RFC 956
FreeBSD committer | BSD since 4.3-tahoe
Never attribute to malice what can adequately be explained by incompetence.
Hi
Since this seems to be headed off in a couple of directions:
Loran due to its very broad bandwidth pulse signal (check out any SDR near a Loran transmitter …) is a different signal than what WWVB / MSF put out. The why is pretty simple, the system was designed for navigation. To get accurate location you needed a good edge to lock on to. Its is not “spread spectrum”, but it has a lot of the same ideas lurking behind the design of the signal. The net result needed to be a measurement in the (sub) microsecond range.
Propagation changes a surprising amount in various VLF “bands”. What you see at 100KHz is not what you see at 60 KHz. Go to 20 KHz or 10 KHz it also changes. One could spend a lot of time wandering down the propagation rabbit hole. We don’t get to move the transmitter frequency. Let’s move on rather than go there right now.
Back in the day when this all was “how you do it” a lot of folks ran this sort of gear. Both WWVB and Loran devices showed up in the US. The normal conclusion was that WWVB was far more trouble / lower performance than Loran. Loran usually won that race by a couple orders of magnitude. For a “wall clock” time source, you used WWVB.
If you wanted to go crazy you could go one step further. NBS mailed out a monthly report mailed (as in what the mail man delivered) to you from NIST. After some looking at logs you could work out what had been going on weeks ago.
Did some of this gear use ferrite antennas at 60 KHz? It sure did. You might sit down and chat with the design guys at various companies. The answer they all gave you was: It’s more compact and some customers are after that. For the customer's application it works ok. It’s not the best choice stability wise.
If you decide to “roll your own” antenna, a horizontal wire simply does not work at these frequencies. That’s part of why those various internet SDR sites show very little “down there”. A vertical works ok, but it has a crazy high impedance at the base. Some sort of preamp is typically used simply to convert to a rational cable or twisted pair impedance. Air core loops come in a variety of configurations and each has its own “fun”.
Bob
On Sep 30, 2025, at 6:01 AM, glen english LIST via time-nuts time-nuts@lists.febo.com wrote:
Hi Paul
good points Paul.
I develop VLF underground proximity systems professionally.. and these are not simple single axis loop systems.
between 25kHz and 80 kHz.... so I have a reasonable handle on it.
Maybe people make the mistake of trying to acheive too high a Q antenna
----and smear the information that the LORANC signal is trying to impart.
I dont know much about LORANC, but I would expect that there is a direct relationship between bandwidth and uncertainty for a specific number of pulses observed.
Care to elaborate with some numbers on it ?
regards,
glen.
On 30/09/2025 19:42, Poul-Henning Kamp wrote:
glen english LIST via time-nuts writes:
I dont want to single you out Glen, but you and others in this
thread are extrapolating your experiences from higher frequency
bands to a lower frequency band without really knowing what you are
talking about.
VLF is not like HF.
At HF frequences M-field antennas are not an option due to the
inter-winding capacitance.
VLF is not even like MW.
It is much easier to propagate a radio signal at 500-1000 kHz than
it is at 40-125 kHz. The wave length really, REALLY, matters here.
And in particular M-field antennas are totally different from E-field
antennas.
You only need to tune your loop-antennas to save power in wall-clocks
or to reduce cost in consumer radios.
To receive time signals, and in particular Loran-C, you do not need to,
and should not tune your loop antenna.
time-nuts mailing list -- time-nuts@lists.febo.com
To unsubscribe send an email to time-nuts-leave@lists.febo.com
Bob Camp writes:
Loran due to its very broad bandwidth pulse signal [...]
[...]
Its is not 'spread spectrum', [...]
Make up you mind Bob :-)
Loran-C is a spread spectrum signal by accident, even if that term
only got a firm definition a decade later.
The first thing was the power-bill. Many of the early Loran-C
stations were powered by diesel generators, so they wanted
as much peak amplitude for as little power as possible.
That dictated a signel with brief pulses, spaced widely apart, A
typical dual-rated Lorsta would consume 50kW power and transmit 250
pulses per second, each pulse peaking between one and two megawatt.
For the hams: "Loran-splatter" happened because "Sidebands 20db
down" literally means "no more than 20kW" for a Loran-C transmitter.
The next problem was the "night-wave crash".
When you receive Loran-C at night, you will get both the direct
signal, and a reflection from the ionosphere.
If you watch this:
https://phk.freebsd.dk/AducLoran/animation2.gif
You can see the reflected night-wave dance around in the tail.
The path difference between the direct "ground-wave" and the reflected
"sky-wave" can be very, very small, and the skywave will often have
higher amplitude than the ground-wave.
See the plot under "6731" here for instance:
https://phk.freebsd.dk/loran-c/Chains/
But the "sky-wave" always travels further, and therefore arrive
later, and there is no way it can ever arrive at or before the 3rd
positive zero-crossing, which is therefore the official timing
reference.
In theory, they could have designed the signal to end after the
third positive zero-crossing, and they sort of did, but it is
not easy to implement in practice.
At these frequencies the only viable omnidirectional antenna is a
"top-hat-capacitor" where the antenna tower and a good fraction of
the "barduns" from the top to ground make up one electrode, and the
"counter-poise", a similar net of copperwire on the ground, is the
other electrode.
You dont make a big capacitor that way, so you need voltage measured
in tens of kilovolts, but you still get nowhere, unless you also add
a bit of inductance to get a resonance going.
So the "half-wave-generators" in the transmitter dump energy from
a high voltage capacitor into the antenna system at carefully timed
moments, to get a resonance going, and then as soon as they have
created the important 3rd positve half-cycle, their job is done.
The receiver still needs some way to tell which is the 3rd positive
zero-crossing, so in practice the signal amplitude is controlled
all the way to the top, so that the peak amplitudes can be used
to identify the 3rd positive zero crossing, but all the energy
is dumped into the antenna in the first 10 half-waves.
Because the signal is pulsed, the resulting spectrum consists of a
lot of closely spaced frequencies, and because the pulses have
"coded" polarities so you can tell them apart, the spectrum gets
even more complex when averaged over a GRI or FRI,
Theoretical plots here:
https://phk.freebsd.dk/loran-c/theoretical_spectrum/
Propagation changes a surprising amount in various VLF 'bands'.
What you see at 100KHz is not what you see at 60 KHz.
This was a very important consideration for Loran-C: They did
not want Loran-C to reach too far, because navigation would
not be useful and it would just interfere with other stations.
100kHz is not exactly "line of sight" but it does stop after
some hundreds of kilometers.
60kHz on goes halfway around the globe, which allows WWVB
to cover CONUS with a single transmitter. (The first transtlantic
radio-telephone link used 60 kHz in one direction and 66kHz in the
other.)
Omega on the other hand had only 8 (9?) transmitters world wide,
so they wanted frequencies where the earth/ocean and ionosphere
forms a waveguide, that happens below 30-ish kHz.
And dont even get me started about the 76Hz and 86Hz radio
transmitters. Yes: "Hz". Not "kHz" :-)
Poul-Henning
--
Poul-Henning Kamp | UNIX since Zilog Zeus 3.20
phk@FreeBSD.ORG | TCP/IP since RFC 956
FreeBSD committer | BSD since 4.3-tahoe
Never attribute to malice what can adequately be explained by incompetence.
That explaination by PHK about propagation at LORAC freqs has a few
errors, but is mostly right, good enough for most people .
BTW , let's get our terminology right :
VLF is < 30kHz . LF is 30kHz-300kHz.
and 60kHz and 100kHz are actually not all that different in propagation.
AND, let's clarify we are talking about TIME problems here, not location
determination.
Ideal antennas for DCF77 and WWV and LORANC are different
Considering time :
- DCF77, ~ 3kHz gives good fidelity of the PM component. (a bit is
120 cycles of carrier) - LORANC - ideally 30kHz of BW for best fidelity, likely difficult to
acheive good SNRs over this full bandwidth over short periods in the
suburbs .
works with reduces bandwidth with increased smearing of those sharp pulses
- WWVB - PM service, ~ 4Hz will provide good fidelity. (1 bit per
second)
But what is good fidelity must be defined by the user.
All these LF time signals will suffer degrees of multipath interference
due to the presence of the ionoosphere
There will be a crossover point where increasing the bandwidth which
reduces smearing of the transitions of the data is undone by effects of
multipath and man man interference.
Narrow bandwidth is beneficial in that it increases SNR (improving
precision) , and reduces effects of off channel interference
Narrow bandwidth smears the data transitions (reduces precision)
Narrow bandwidth from high Q antennas produces delays that need to be
removed. (reducing accuracy)
and then there may be some temperature and static magnetic field
variations that will perturb the narrowband tuned circuit .
There's some advantage here to having a ncie flat top response with the
double tuned circuit, but fundamentals like thermal drift, aging etc of
components, stray fields all limit accuracy.
I would suggest a electrostatically shielded loop (to be able to null
out local dominant interferers and a dust core (low thermal
sensitivity) in a double tuned circuit.
By all means use a wideband E field probe (to maximize bandwidth) if you
are in the middle of nowhere . I have to work with 3 x 50kW AM broadcast
transmitters 1 mile away.....
if the time signals were usable down under, I might consider making
something and publishing.
Poul, I dont consider LORANC-C spread spectrum at all .
It does not fit the accepted characterisation of being a direct sequence
modulated, frequency hopped or chirp waveform.
It's just a wide bandwidth pulse waveform.
-glen
On 1/10/2025 07:39, Poul-Henning Kamp wrote:
Bob Camp writes:
Loran due to its very broad bandwidth pulse signal [...]
[...]
Its is not 'spread spectrum', [...]
Make up you mind Bob :-)
Loran-C is a spread spectrum signal by accident, even if that term
only got a firm definition a decade later.
The first thing was the power-bill. Many of the early Loran-C
stations were powered by diesel generators, so they wanted
as much peak amplitude for as little power as possible.
That dictated a signel with brief pulses, spaced widely apart, A
typical dual-rated Lorsta would consume 50kW power and transmit 250
pulses per second, each pulse peaking between one and two megawatt.
For the hams: "Loran-splatter" happened because "Sidebands 20db
down" literally means "no more than 20kW" for a Loran-C transmitter.
The next problem was the "night-wave crash".
When you receive Loran-C at night, you will get both the direct
signal, and a reflection from the ionosphere.
If you watch this:
https://phk.freebsd.dk/AducLoran/animation2.gif
You can see the reflected night-wave dance around in the tail.
The path difference between the direct "ground-wave" and the reflected
"sky-wave" can be very, very small, and the skywave will often have
higher amplitude than the ground-wave.
See the plot under "6731" here for instance:
https://phk.freebsd.dk/loran-c/Chains/
But the "sky-wave" always travels further, and therefore arrive
later, and there is no way it can ever arrive at or before the 3rd
positive zero-crossing, which is therefore the official timing
reference.
In theory, they could have designed the signal to end after the
third positive zero-crossing, and they sort of did, but it is
not easy to implement in practice.
At these frequencies the only viable omnidirectional antenna is a
"top-hat-capacitor" where the antenna tower and a good fraction of
the "barduns" from the top to ground make up one electrode, and the
"counter-poise", a similar net of copperwire on the ground, is the
other electrode.
You dont make a big capacitor that way, so you need voltage measured
in tens of kilovolts, but you still get nowhere, unless you also add
a bit of inductance to get a resonance going.
So the "half-wave-generators" in the transmitter dump energy from
a high voltage capacitor into the antenna system at carefully timed
moments, to get a resonance going, and then as soon as they have
created the important 3rd positve half-cycle, their job is done.
The receiver still needs some way to tell which is the 3rd positive
zero-crossing, so in practice the signal amplitude is controlled
all the way to the top, so that the peak amplitudes can be used
to identify the 3rd positive zero crossing, but all the energy
is dumped into the antenna in the first 10 half-waves.
Because the signal is pulsed, the resulting spectrum consists of a
lot of closely spaced frequencies, and because the pulses have
"coded" polarities so you can tell them apart, the spectrum gets
even more complex when averaged over a GRI or FRI,
Theoretical plots here:
https://phk.freebsd.dk/loran-c/theoretical_spectrum/
Propagation changes a surprising amount in various VLF 'bands'.
What you see at 100KHz is not what you see at 60 KHz.
This was a very important consideration for Loran-C: They did
not want Loran-C to reach too far, because navigation would
not be useful and it would just interfere with other stations.
100kHz is not exactly "line of sight" but it does stop after
some hundreds of kilometers.
60kHz on goes halfway around the globe, which allows WWVB
to cover CONUS with a single transmitter. (The first transtlantic
radio-telephone link used 60 kHz in one direction and 66kHz in the
other.)
Omega on the other hand had only 8 (9?) transmitters world wide,
so they wanted frequencies where the earth/ocean and ionosphere
forms a waveguide, that happens below 30-ish kHz.
And dont even get me started about the 76Hz and 86Hz radio
transmitters. Yes: "Hz". Not "kHz" :-)
Poul-Henning