Phase measurement of my GPSDO
I have seen differences between both UCT and Oscilloquartz 8663 ocxo’s. The attached plot shows an example. Both boxes use Ublox LEA-6T receiver, surveyed in, AD5680 DAC 18 bit DAC, same level shift circuit and same control circuit. The reference is an LPRO-101. The Oscilloquartz ocxo was purchased used. Both UCT ocxo’s (only the better one is shown) were purchased new and have 100’s of operating hours. I have also seen differences with constant EFC control voltage. The differences limit what performance you can achieve.
Hello all
in the meantime I figured out most of my problems and my GPSDO is working
now with some very ugly prototype code. Today, I wanted to do some ADEV
measurements.
My plan was to compare the 1PPS generated from my GPSDO to the 1PPS of my
Oscilloquartz STAR4; unfortunately I have nothing else (like Rb or so)
which is perhaps more stable. So I try with the STAR4 and see where I get.
However, before I did any meaningful measurements, I wanted to see what the
noise floor of my test equipment is.
Again, unfortunately I have nothing better than a HP 5335A with 1ns
resolution in TIC mode. I measured the noise floor of the TIC as follows:
the 1PPS output of my GPSDO was connected to a resistive power splitter,
and then, one output of the splitter went to channel A of the TIC (START
signal) while the other output from the splitter went first to a long cable
and then to channel B. With this, I achieved about 16ns of delay.
I then used the TIC together with Timelab and measured the ADEV of this
setup.
As far as I understand, if the delay of the cable stays constant (which it
does as long as it is not moved and the temperature stays the same), all I
see in the ADEV plot is the ADEV of my counter itself. Right?
So I let this test run for one hour (collected 3600 samples), and the
result looks terrible. See the attached file. I did the test twice; once I
used the STAR4 GPSDO as external reference for the counter, and once I used
its internal reference, which is a HP 10544A oven. As one can see, the ADEV
at 1sec is between 7e-10 and 8e-10. I don't know yet what numbers I can
expect from my GPSDO, but from datasheets of commercial GPSDOs I saw that
the ADEV shortly after powerup should be in the 1e-11 region. So how does
one measure such low ADEVs?
To me, it appears that the ADEV at 1sec is roughly the counter's
resolution; a bit less due to averaging. If I take averaging over 3600
samples into account, I think I could expect maybe ~1ns/sqrt(3600) =
16.7e-12 as ADEV at 1 second, but we can clearly see that this is not the
case. So there are two interesting questions arising:
a) I think the ADEV is so high because of the quantization error of the
counter. Assume the time interval measured is right at the transition from,
say, 15ns to 16ns, even the smallest amount of noise will produce some
alternating readings of 15ns and 16ns, which, in turn, results in an ADEV
around 1e-9, right? Further, why is this effect not averaged out with
sqrt(# of samples)?
b) if I want to measure 1e-11 or even 1e-12 at 1sec - what resolution does
my counter need? If the above was true, I would expect that a 1ps
resolution (and an even better stability!) was required to measure ADEV of
1e-12, The fact that the (as far as I know) world's most recent,
rocket-science grade counter (some Keysight stuff) has "only" 20ps of
resolution, but people are still able to measure even 1e-14 shows that my
assumption is wrong. So how are the measurement resolution and the ADEV
related to each other? I plan to build my own TIC based on a TDC7200, which
would offer some 55ps of resolution, but how low could I go with that?
Best regards
Tobias
HB9FSX
On Fri, Mar 20, 2020 at 5:34 PM Bob Q bobqhome@live.com wrote:
I have seen differences between both UCT and Oscilloquartz 8663 ocxo’s.
The attached plot shows an example. Both boxes use Ublox LEA-6T receiver,
surveyed in, AD5680 DAC 18 bit DAC, same level shift circuit and same
control circuit. The reference is an LPRO-101. The Oscilloquartz ocxo was
purchased used. Both UCT ocxo’s (only the better one is shown) were
purchased new and have 100’s of operating hours. I have also seen
differences with constant EFC control voltage. The differences limit what
performance you can achieve._______________________________________________
time-nuts mailing list -- time-nuts@lists.febo.com
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http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com
and follow the instructions there.
Hi
What you have measured is the noise floor of a 5335 when trying
to use it to measure ADEV. Anything past the numbers on your plot
will be “past” what the 5335 can “see”. Indeed, even when you get
close to those numbers, things may get a bit weird due to the fact
that you are measuring counter “noise” plus device noise.
Bob
On Apr 2, 2020, at 3:13 PM, Tobias Pluess tpluess@ieee.org wrote:
Hello all
in the meantime I figured out most of my problems and my GPSDO is working
now with some very ugly prototype code. Today, I wanted to do some ADEV
measurements.
My plan was to compare the 1PPS generated from my GPSDO to the 1PPS of my
Oscilloquartz STAR4; unfortunately I have nothing else (like Rb or so)
which is perhaps more stable. So I try with the STAR4 and see where I get.
However, before I did any meaningful measurements, I wanted to see what the
noise floor of my test equipment is.
Again, unfortunately I have nothing better than a HP 5335A with 1ns
resolution in TIC mode. I measured the noise floor of the TIC as follows:
the 1PPS output of my GPSDO was connected to a resistive power splitter,
and then, one output of the splitter went to channel A of the TIC (START
signal) while the other output from the splitter went first to a long cable
and then to channel B. With this, I achieved about 16ns of delay.
I then used the TIC together with Timelab and measured the ADEV of this
setup.
As far as I understand, if the delay of the cable stays constant (which it
does as long as it is not moved and the temperature stays the same), all I
see in the ADEV plot is the ADEV of my counter itself. Right?
So I let this test run for one hour (collected 3600 samples), and the
result looks terrible. See the attached file. I did the test twice; once I
used the STAR4 GPSDO as external reference for the counter, and once I used
its internal reference, which is a HP 10544A oven. As one can see, the ADEV
at 1sec is between 7e-10 and 8e-10. I don't know yet what numbers I can
expect from my GPSDO, but from datasheets of commercial GPSDOs I saw that
the ADEV shortly after powerup should be in the 1e-11 region. So how does
one measure such low ADEVs?
To me, it appears that the ADEV at 1sec is roughly the counter's
resolution; a bit less due to averaging. If I take averaging over 3600
samples into account, I think I could expect maybe ~1ns/sqrt(3600) =
16.7e-12 as ADEV at 1 second, but we can clearly see that this is not the
case. So there are two interesting questions arising:
a) I think the ADEV is so high because of the quantization error of the
counter. Assume the time interval measured is right at the transition from,
say, 15ns to 16ns, even the smallest amount of noise will produce some
alternating readings of 15ns and 16ns, which, in turn, results in an ADEV
around 1e-9, right? Further, why is this effect not averaged out with
sqrt(# of samples)?
b) if I want to measure 1e-11 or even 1e-12 at 1sec - what resolution does
my counter need? If the above was true, I would expect that a 1ps
resolution (and an even better stability!) was required to measure ADEV of
1e-12, The fact that the (as far as I know) world's most recent,
rocket-science grade counter (some Keysight stuff) has "only" 20ps of
resolution, but people are still able to measure even 1e-14 shows that my
assumption is wrong. So how are the measurement resolution and the ADEV
related to each other? I plan to build my own TIC based on a TDC7200, which
would offer some 55ps of resolution, but how low could I go with that?
Best regards
Tobias
HB9FSX
On Fri, Mar 20, 2020 at 5:34 PM Bob Q bobqhome@live.com wrote:
I have seen differences between both UCT and Oscilloquartz 8663 ocxo’s.
The attached plot shows an example. Both boxes use Ublox LEA-6T receiver,
surveyed in, AD5680 DAC 18 bit DAC, same level shift circuit and same
control circuit. The reference is an LPRO-101. The Oscilloquartz ocxo was
purchased used. Both UCT ocxo’s (only the better one is shown) were
purchased new and have 100’s of operating hours. I have also seen
differences with constant EFC control voltage. The differences limit what
performance you can achieve._______________________________________________
time-nuts mailing list -- time-nuts@lists.febo.com
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http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com
and follow the instructions there.
<figure-1.png>_______________________________________________
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b) if I want to measure 1e-11 or even 1e-12 at 1sec - what resolution does
my counter need? If the above was true, I would expect that a 1ps
resolution (and an even better stability!) was required to measure ADEV of
1e-12, The fact that the (as far as I know) world's most recent,
rocket-science grade counter (some Keysight stuff) has "only" 20ps of
resolution, but people are still able to measure even 1e-14 shows that my
assumption is wrong. So how are the measurement resolution and the ADEV
related to each other? I plan to build my own TIC based on a TDC7200, which
would offer some 55ps of resolution, but how low could I go with that?
That sounds like a simple question but it's not. There are a few different approaches to look into:
-
Use averaging with your existing counter. Some counters can yield readings in the 1E-12 region at t=1s even though their single-shot jitter is much worse than that. They do this by averaging hundreds or thousands of samples for each reading they report. Whether (and when) this is acceptable is a complex topic in itself, too much so to explain quickly. Search for information on the effects of averaging and dead time on Allan deviation to find the entrance to this fork of the rabbit hole.
-
Search for the term 'DMTD' and read about that.
-
Search for 'direct digital phase measurement' and read about that.
-
Search for 'tight PLL' and read about that.
Basically, while some counters can perform averaging on a post-detection basis, that's like using the tone control on a radio to reduce static and QRM. It works, sort of, but it's too late in the signal chain at that point to do the job right. You really want to limit the bandwidth before the signal is captured, but since that's almost never practical at RF, the next best thing to do is limit the bandwidth before the signal is "demodulated" (i.e., counted.)
Hence items 2, 3, and 4 above. They either limit the measurement bandwidth prior to detection, lower the frequency itself to keep the counter's inherent jitter from dominating the measurement, or both. You'll have to use one of these methods, or another technique along the same lines, if you want to measure the short-term stability of a good oscillator or GPSDO.
-- john, KE5FX
hi John
yes I know the DMTD method, and indeed I am planing to build my own DMTD
system, something similar to the "Small DMTD system" published by Riley (
https://www.wriley.com/A Small DMTD System.pdf).
However I am unsure whether that will help much in this case, because all
what the DMTD does is to mix the 10MHz signals down to some 1Hz Signal or
so which can be measured more easily, and I already have 1Hz signals (the
1PPS) which I am comparing.
Or do you suggest to use the DMTD and use a higher frequency at its
outputs, say 10Hz or so, and then average for 10 samples to increase the
resolution?
Thanks
Tobias
HB9FSX
On Fri, Apr 3, 2020 at 12:53 AM John Miles john@miles.io wrote:
b) if I want to measure 1e-11 or even 1e-12 at 1sec - what resolution
does
my counter need? If the above was true, I would expect that a 1ps
resolution (and an even better stability!) was required to measure ADEV
of
1e-12, The fact that the (as far as I know) world's most recent,
rocket-science grade counter (some Keysight stuff) has "only" 20ps of
resolution, but people are still able to measure even 1e-14 shows that my
assumption is wrong. So how are the measurement resolution and the ADEV
related to each other? I plan to build my own TIC based on a TDC7200,
which
would offer some 55ps of resolution, but how low could I go with that?
That sounds like a simple question but it's not. There are a few
different approaches to look into:
-
Use averaging with your existing counter. Some counters can yield
readings in the 1E-12 region at t=1s even though their single-shot jitter
is much worse than that. They do this by averaging hundreds or thousands
of samples for each reading they report. Whether (and when) this is
acceptable is a complex topic in itself, too much so to explain quickly.
Search for information on the effects of averaging and dead time on Allan
deviation to find the entrance to this fork of the rabbit hole. -
Search for the term 'DMTD' and read about that.
-
Search for 'direct digital phase measurement' and read about that.
-
Search for 'tight PLL' and read about that.
Basically, while some counters can perform averaging on a post-detection
basis, that's like using the tone control on a radio to reduce static and
QRM. It works, sort of, but it's too late in the signal chain at that
point to do the job right. You really want to limit the bandwidth before
the signal is captured, but since that's almost never practical at RF, the
next best thing to do is limit the bandwidth before the signal is
"demodulated" (i.e., counted.)
Hence items 2, 3, and 4 above. They either limit the measurement
bandwidth prior to detection, lower the frequency itself to keep the
counter's inherent jitter from dominating the measurement, or both. You'll
have to use one of these methods, or another technique along the same
lines, if you want to measure the short-term stability of a good oscillator
or GPSDO.
-- john, KE5FX
time-nuts mailing list -- time-nuts@lists.febo.com
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http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com
and follow the instructions there.
Hi
The quick way to do this is with a single mixer. Take something like an old
10811 and use the coarse tune to set it high in frequency by 5 to 10 Hz.
Then feed it into an RPD-1 mixer and pull out the 5 to 10 Hz audio tone.
That tone is the difference between the 10811 and your device under test.
If the DUT moves 1 Hz, the audio tone changes by 1 Hz.
If you measured the 10 MHz on the DUT, that 1 Hz would be a very small shift
( 0.1 ppm ). At 10 Hz it’s a 10% change. You have “amplified” the change
in frequency by the ratio of 10 MHz to 10 Hz ( so a million X increase ).
IF you could tack that on to the ADEV plot of your 5335 ( no, it’s not that
simple) your 7x10^-10 at 1 second would become more 7x10^-16 at 1
second.
The reason its not quite that simple is that the input circuit on the counter
really does not handle a 10 Hz audio tone as well as it handles a 10 MHz
RF signal. Instead of getting 9 digits a second, you probably will get three
good digits a second and another 6 digits of noise.
The good news is that an op amp used as a preamp ( to get you up to maybe
32 V p-p rather than a volt or so) and another op amp or three as limiters will
get you up around 6 or 7 good digits. Toss in a cap or two as a high pass
and low pass filter ( DC offsets can be a problem ….) and you have a working
device that gets into the parts in 10^-13 with your 5335.
It all can be done with point to point wiring. No need for a PCB layout. Be
careful that the +/- 18V supplies to the op amp both go on and off at the
same time ….
Bob
On Apr 3, 2020, at 5:13 AM, Tobias Pluess tpluess@ieee.org wrote:
hi John
yes I know the DMTD method, and indeed I am planing to build my own DMTD
system, something similar to the "Small DMTD system" published by Riley (
https://www.wriley.com/A Small DMTD System.pdf).
However I am unsure whether that will help much in this case, because all
what the DMTD does is to mix the 10MHz signals down to some 1Hz Signal or
so which can be measured more easily, and I already have 1Hz signals (the
1PPS) which I am comparing.
Or do you suggest to use the DMTD and use a higher frequency at its
outputs, say 10Hz or so, and then average for 10 samples to increase the
resolution?
Thanks
Tobias
HB9FSX
On Fri, Apr 3, 2020 at 12:53 AM John Miles john@miles.io wrote:
b) if I want to measure 1e-11 or even 1e-12 at 1sec - what resolution
does
my counter need? If the above was true, I would expect that a 1ps
resolution (and an even better stability!) was required to measure ADEV
of
1e-12, The fact that the (as far as I know) world's most recent,
rocket-science grade counter (some Keysight stuff) has "only" 20ps of
resolution, but people are still able to measure even 1e-14 shows that my
assumption is wrong. So how are the measurement resolution and the ADEV
related to each other? I plan to build my own TIC based on a TDC7200,
which
would offer some 55ps of resolution, but how low could I go with that?
That sounds like a simple question but it's not. There are a few
different approaches to look into:
-
Use averaging with your existing counter. Some counters can yield
readings in the 1E-12 region at t=1s even though their single-shot jitter
is much worse than that. They do this by averaging hundreds or thousands
of samples for each reading they report. Whether (and when) this is
acceptable is a complex topic in itself, too much so to explain quickly.
Search for information on the effects of averaging and dead time on Allan
deviation to find the entrance to this fork of the rabbit hole. -
Search for the term 'DMTD' and read about that.
-
Search for 'direct digital phase measurement' and read about that.
-
Search for 'tight PLL' and read about that.
Basically, while some counters can perform averaging on a post-detection
basis, that's like using the tone control on a radio to reduce static and
QRM. It works, sort of, but it's too late in the signal chain at that
point to do the job right. You really want to limit the bandwidth before
the signal is captured, but since that's almost never practical at RF, the
next best thing to do is limit the bandwidth before the signal is
"demodulated" (i.e., counted.)
Hence items 2, 3, and 4 above. They either limit the measurement
bandwidth prior to detection, lower the frequency itself to keep the
counter's inherent jitter from dominating the measurement, or both. You'll
have to use one of these methods, or another technique along the same
lines, if you want to measure the short-term stability of a good oscillator
or GPSDO.
-- john, KE5FX
time-nuts mailing list -- time-nuts@lists.febo.com
To unsubscribe, go to
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and follow the instructions there.
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and follow the instructions there.
Caution: opamps make terrible limiters- their overload behavior is
generally ugly
and unpredictable. It's much better to use a genuine level comparator, and
wire it
up so that it has a modest amount of hysteresis.
Dana
On Fri, Apr 3, 2020 at 6:45 AM Bob kb8tq kb8tq@n1k.org wrote:
Hi
The quick way to do this is with a single mixer. Take something like an old
10811 and use the coarse tune to set it high in frequency by 5 to 10 Hz.
Then feed it into an RPD-1 mixer and pull out the 5 to 10 Hz audio tone.
That tone is the difference between the 10811 and your device under
test.
If the DUT moves 1 Hz, the audio tone changes by 1 Hz.
If you measured the 10 MHz on the DUT, that 1 Hz would be a very small
shift
( 0.1 ppm ). At 10 Hz it’s a 10% change. You have “amplified” the change
in frequency by the ratio of 10 MHz to 10 Hz ( so a million X increase ).
IF you could tack that on to the ADEV plot of your 5335 ( no, it’s not
that
simple) your 7x10^-10 at 1 second would become more 7x10^-16 at 1
second.
The reason its not quite that simple is that the input circuit on the
counter
really does not handle a 10 Hz audio tone as well as it handles a 10 MHz
RF signal. Instead of getting 9 digits a second, you probably will get
three
good digits a second and another 6 digits of noise.
The good news is that an op amp used as a preamp ( to get you up to maybe
32 V p-p rather than a volt or so) and another op amp or three as limiters
will
get you up around 6 or 7 good digits. Toss in a cap or two as a high pass
and low pass filter ( DC offsets can be a problem ….) and you have a
working
device that gets into the parts in 10^-13 with your 5335.
It all can be done with point to point wiring. No need for a PCB layout.
Be
careful that the +/- 18V supplies to the op amp both go on and off at
the
same time ….
Bob
On Apr 3, 2020, at 5:13 AM, Tobias Pluess tpluess@ieee.org wrote:
hi John
yes I know the DMTD method, and indeed I am planing to build my own DMTD
system, something similar to the "Small DMTD system" published by Riley (
https://www.wriley.com/A Small DMTD System.pdf).
However I am unsure whether that will help much in this case, because all
what the DMTD does is to mix the 10MHz signals down to some 1Hz Signal or
so which can be measured more easily, and I already have 1Hz signals (the
1PPS) which I am comparing.
Or do you suggest to use the DMTD and use a higher frequency at its
outputs, say 10Hz or so, and then average for 10 samples to increase the
resolution?
Thanks
Tobias
HB9FSX
On Fri, Apr 3, 2020 at 12:53 AM John Miles john@miles.io wrote:
b) if I want to measure 1e-11 or even 1e-12 at 1sec - what resolution
does
my counter need? If the above was true, I would expect that a 1ps
resolution (and an even better stability!) was required to measure ADEV
of
1e-12, The fact that the (as far as I know) world's most recent,
rocket-science grade counter (some Keysight stuff) has "only" 20ps of
resolution, but people are still able to measure even 1e-14 shows that
my
assumption is wrong. So how are the measurement resolution and the ADEV
related to each other? I plan to build my own TIC based on a TDC7200,
which
would offer some 55ps of resolution, but how low could I go with that?
That sounds like a simple question but it's not. There are a few
different approaches to look into:
- Use averaging with your existing counter. Some counters can yield
readings in the 1E-12 region at t=1s even though their single-shot
jitter
is much worse than that. They do this by averaging hundreds or
thousands
of samples for each reading they report. Whether (and when) this is
acceptable is a complex topic in itself, too much so to explain quickly.
Search for information on the effects of averaging and dead time on
Allan
deviation to find the entrance to this fork of the rabbit hole.
-
Search for the term 'DMTD' and read about that.
-
Search for 'direct digital phase measurement' and read about that.
-
Search for 'tight PLL' and read about that.
Basically, while some counters can perform averaging on a post-detection
basis, that's like using the tone control on a radio to reduce static
and
QRM. It works, sort of, but it's too late in the signal chain at that
point to do the job right. You really want to limit the bandwidth
before
the signal is captured, but since that's almost never practical at RF,
the
next best thing to do is limit the bandwidth before the signal is
"demodulated" (i.e., counted.)
Hence items 2, 3, and 4 above. They either limit the measurement
bandwidth prior to detection, lower the frequency itself to keep the
counter's inherent jitter from dominating the measurement, or both.
You'll
have to use one of these methods, or another technique along the same
lines, if you want to measure the short-term stability of a good
oscillator
or GPSDO.
-- john, KE5FX
time-nuts mailing list -- time-nuts@lists.febo.com
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and follow the instructions there.
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To unsubscribe, go to
and follow the instructions there.
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Hi
If you are doing a limiter for a 5 Hz sine wave, a “normal” comparator is very much
not what you want to use. The slew rates involved are simply way to far below what
it is targeted to do. Effectively, it’s what’s in the counter input circuit that has already
failed miserably trying to do this. Think of this as a “slow sine to square converter”.
For all the ugly details the discussions a few years back on the Colins style limiter
get into the this and that. Bruce did a nice write up at:
http://www.ko4bb.com/getsimple/index.php?id=bruces-zero-crossing-detectors
Bob
On Apr 3, 2020, at 9:29 AM, Dana Whitlow k8yumdoober@gmail.com wrote:
Caution: opamps make terrible limiters- their overload behavior is
generally ugly
and unpredictable. It's much better to use a genuine level comparator, and
wire it
up so that it has a modest amount of hysteresis.
Dana
On Fri, Apr 3, 2020 at 6:45 AM Bob kb8tq kb8tq@n1k.org wrote:
Hi
The quick way to do this is with a single mixer. Take something like an old
10811 and use the coarse tune to set it high in frequency by 5 to 10 Hz.
Then feed it into an RPD-1 mixer and pull out the 5 to 10 Hz audio tone.
That tone is the difference between the 10811 and your device under
test.
If the DUT moves 1 Hz, the audio tone changes by 1 Hz.
If you measured the 10 MHz on the DUT, that 1 Hz would be a very small
shift
( 0.1 ppm ). At 10 Hz it’s a 10% change. You have “amplified” the change
in frequency by the ratio of 10 MHz to 10 Hz ( so a million X increase ).
IF you could tack that on to the ADEV plot of your 5335 ( no, it’s not
that
simple) your 7x10^-10 at 1 second would become more 7x10^-16 at 1
second.
The reason its not quite that simple is that the input circuit on the
counter
really does not handle a 10 Hz audio tone as well as it handles a 10 MHz
RF signal. Instead of getting 9 digits a second, you probably will get
three
good digits a second and another 6 digits of noise.
The good news is that an op amp used as a preamp ( to get you up to maybe
32 V p-p rather than a volt or so) and another op amp or three as limiters
will
get you up around 6 or 7 good digits. Toss in a cap or two as a high pass
and low pass filter ( DC offsets can be a problem ….) and you have a
working
device that gets into the parts in 10^-13 with your 5335.
It all can be done with point to point wiring. No need for a PCB layout.
Be
careful that the +/- 18V supplies to the op amp both go on and off at
the
same time ….
Bob
On Apr 3, 2020, at 5:13 AM, Tobias Pluess tpluess@ieee.org wrote:
hi John
yes I know the DMTD method, and indeed I am planing to build my own DMTD
system, something similar to the "Small DMTD system" published by Riley (
https://www.wriley.com/A Small DMTD System.pdf).
However I am unsure whether that will help much in this case, because all
what the DMTD does is to mix the 10MHz signals down to some 1Hz Signal or
so which can be measured more easily, and I already have 1Hz signals (the
1PPS) which I am comparing.
Or do you suggest to use the DMTD and use a higher frequency at its
outputs, say 10Hz or so, and then average for 10 samples to increase the
resolution?
Thanks
Tobias
HB9FSX
On Fri, Apr 3, 2020 at 12:53 AM John Miles john@miles.io wrote:
b) if I want to measure 1e-11 or even 1e-12 at 1sec - what resolution
does
my counter need? If the above was true, I would expect that a 1ps
resolution (and an even better stability!) was required to measure ADEV
of
1e-12, The fact that the (as far as I know) world's most recent,
rocket-science grade counter (some Keysight stuff) has "only" 20ps of
resolution, but people are still able to measure even 1e-14 shows that
my
assumption is wrong. So how are the measurement resolution and the ADEV
related to each other? I plan to build my own TIC based on a TDC7200,
which
would offer some 55ps of resolution, but how low could I go with that?
That sounds like a simple question but it's not. There are a few
different approaches to look into:
- Use averaging with your existing counter. Some counters can yield
readings in the 1E-12 region at t=1s even though their single-shot
jitter
is much worse than that. They do this by averaging hundreds or
thousands
of samples for each reading they report. Whether (and when) this is
acceptable is a complex topic in itself, too much so to explain quickly.
Search for information on the effects of averaging and dead time on
Allan
deviation to find the entrance to this fork of the rabbit hole.
-
Search for the term 'DMTD' and read about that.
-
Search for 'direct digital phase measurement' and read about that.
-
Search for 'tight PLL' and read about that.
Basically, while some counters can perform averaging on a post-detection
basis, that's like using the tone control on a radio to reduce static
and
QRM. It works, sort of, but it's too late in the signal chain at that
point to do the job right. You really want to limit the bandwidth
before
the signal is captured, but since that's almost never practical at RF,
the
next best thing to do is limit the bandwidth before the signal is
"demodulated" (i.e., counted.)
Hence items 2, 3, and 4 above. They either limit the measurement
bandwidth prior to detection, lower the frequency itself to keep the
counter's inherent jitter from dominating the measurement, or both.
You'll
have to use one of these methods, or another technique along the same
lines, if you want to measure the short-term stability of a good
oscillator
or GPSDO.
-- john, KE5FX
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Hi Bob
knowing that my counter's noise floor is terrible (even though I still
don't understand why) I tried to measure the ADEV and MDEV of my GPSDO
against another GPSDO.
From the graphs, everything below tau=10s is, I would say, rubbish. But I
tend to mistrust these complete results, as I have no means of finding out
whether my reference is so bad or my own GPSDO. The reference is an eBay
GPSDO, and as we all know, these are sometimes of doubtful pedigree.
But still, below the 10s tau, the ADEV and MDEV are so close to the noise
floor that I would say this measurement is useless.
But it still does not explain why my 5335A is so bad.
Tobias
HB9FSX
On Thu, Apr 2, 2020 at 10:17 PM Bob kb8tq kb8tq@n1k.org wrote:
Hi
What you have measured is the noise floor of a 5335 when trying
to use it to measure ADEV. Anything past the numbers on your plot
will be “past” what the 5335 can “see”. Indeed, even when you get
close to those numbers, things may get a bit weird due to the fact
that you are measuring counter “noise” plus device noise.
Bob
On Apr 2, 2020, at 3:13 PM, Tobias Pluess tpluess@ieee.org wrote:
Hello all
in the meantime I figured out most of my problems and my GPSDO is working
now with some very ugly prototype code. Today, I wanted to do some ADEV
measurements.
My plan was to compare the 1PPS generated from my GPSDO to the 1PPS of my
Oscilloquartz STAR4; unfortunately I have nothing else (like Rb or so)
which is perhaps more stable. So I try with the STAR4 and see where I
get.
However, before I did any meaningful measurements, I wanted to see what
the
noise floor of my test equipment is.
Again, unfortunately I have nothing better than a HP 5335A with 1ns
resolution in TIC mode. I measured the noise floor of the TIC as follows:
the 1PPS output of my GPSDO was connected to a resistive power splitter,
and then, one output of the splitter went to channel A of the TIC (START
signal) while the other output from the splitter went first to a long
cable
and then to channel B. With this, I achieved about 16ns of delay.
I then used the TIC together with Timelab and measured the ADEV of this
setup.
As far as I understand, if the delay of the cable stays constant (which
it
does as long as it is not moved and the temperature stays the same), all
I
see in the ADEV plot is the ADEV of my counter itself. Right?
So I let this test run for one hour (collected 3600 samples), and the
result looks terrible. See the attached file. I did the test twice; once
I
used the STAR4 GPSDO as external reference for the counter, and once I
used
its internal reference, which is a HP 10544A oven. As one can see, the
ADEV
at 1sec is between 7e-10 and 8e-10. I don't know yet what numbers I can
expect from my GPSDO, but from datasheets of commercial GPSDOs I saw that
the ADEV shortly after powerup should be in the 1e-11 region. So how does
one measure such low ADEVs?
To me, it appears that the ADEV at 1sec is roughly the counter's
resolution; a bit less due to averaging. If I take averaging over 3600
samples into account, I think I could expect maybe ~1ns/sqrt(3600) =
16.7e-12 as ADEV at 1 second, but we can clearly see that this is not the
case. So there are two interesting questions arising:
a) I think the ADEV is so high because of the quantization error of the
counter. Assume the time interval measured is right at the transition
from,
say, 15ns to 16ns, even the smallest amount of noise will produce some
alternating readings of 15ns and 16ns, which, in turn, results in an ADEV
around 1e-9, right? Further, why is this effect not averaged out with
sqrt(# of samples)?
b) if I want to measure 1e-11 or even 1e-12 at 1sec - what resolution
does
my counter need? If the above was true, I would expect that a 1ps
resolution (and an even better stability!) was required to measure ADEV
of
1e-12, The fact that the (as far as I know) world's most recent,
rocket-science grade counter (some Keysight stuff) has "only" 20ps of
resolution, but people are still able to measure even 1e-14 shows that my
assumption is wrong. So how are the measurement resolution and the ADEV
related to each other? I plan to build my own TIC based on a TDC7200,
which
would offer some 55ps of resolution, but how low could I go with that?
Best regards
Tobias
HB9FSX
On Fri, Mar 20, 2020 at 5:34 PM Bob Q bobqhome@live.com wrote:
I have seen differences between both UCT and Oscilloquartz 8663 ocxo’s.
The attached plot shows an example. Both boxes use Ublox LEA-6T
receiver,
surveyed in, AD5680 DAC 18 bit DAC, same level shift circuit and same
control circuit. The reference is an LPRO-101. The Oscilloquartz ocxo
was
purchased used. Both UCT ocxo’s (only the better one is shown) were
purchased new and have 100’s of operating hours. I have also seen
differences with constant EFC control voltage. The differences limit
what
performance you can
achieve._______________________________________________
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<figure-1.png>_______________________________________________
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Hi again Bob,
yes you describe a simple DMTD measurement. But could you tell me what the
difference is between that and comparing the 1PPS pulses?
I mean, I could set the 10811 high in frequency by just 1Hz, and then it
would result in two 1Hz signals which are then compared.
Which is essentially the same as comparing two 1PPS signals, isn't it?
Ok there is a minor difference: since the 1PPS signals are divided down
from 10MHz, their noise is also divided down, which is not the case for the
DMTD.
However, in the end I am comparing signals in the 1Hz to 5Hz or 10Hz
region, and apparently, the 5335A is not suitable for those, at least not
with the desired stability, is it?
Tobias
On Fri, Apr 3, 2020 at 1:45 PM Bob kb8tq kb8tq@n1k.org wrote:
Hi
The quick way to do this is with a single mixer. Take something like an old
10811 and use the coarse tune to set it high in frequency by 5 to 10 Hz.
Then feed it into an RPD-1 mixer and pull out the 5 to 10 Hz audio tone.
That tone is the difference between the 10811 and your device under
test.
If the DUT moves 1 Hz, the audio tone changes by 1 Hz.
If you measured the 10 MHz on the DUT, that 1 Hz would be a very small
shift
( 0.1 ppm ). At 10 Hz it’s a 10% change. You have “amplified” the change
in frequency by the ratio of 10 MHz to 10 Hz ( so a million X increase ).
IF you could tack that on to the ADEV plot of your 5335 ( no, it’s not
that
simple) your 7x10^-10 at 1 second would become more 7x10^-16 at 1
second.
The reason its not quite that simple is that the input circuit on the
counter
really does not handle a 10 Hz audio tone as well as it handles a 10 MHz
RF signal. Instead of getting 9 digits a second, you probably will get
three
good digits a second and another 6 digits of noise.
The good news is that an op amp used as a preamp ( to get you up to maybe
32 V p-p rather than a volt or so) and another op amp or three as limiters
will
get you up around 6 or 7 good digits. Toss in a cap or two as a high pass
and low pass filter ( DC offsets can be a problem ….) and you have a
working
device that gets into the parts in 10^-13 with your 5335.
It all can be done with point to point wiring. No need for a PCB layout.
Be
careful that the +/- 18V supplies to the op amp both go on and off at
the
same time ….
Bob
On Apr 3, 2020, at 5:13 AM, Tobias Pluess tpluess@ieee.org wrote:
hi John
yes I know the DMTD method, and indeed I am planing to build my own DMTD
system, something similar to the "Small DMTD system" published by Riley (
https://www.wriley.com/A Small DMTD System.pdf).
However I am unsure whether that will help much in this case, because all
what the DMTD does is to mix the 10MHz signals down to some 1Hz Signal or
so which can be measured more easily, and I already have 1Hz signals (the
1PPS) which I am comparing.
Or do you suggest to use the DMTD and use a higher frequency at its
outputs, say 10Hz or so, and then average for 10 samples to increase the
resolution?
Thanks
Tobias
HB9FSX
On Fri, Apr 3, 2020 at 12:53 AM John Miles john@miles.io wrote:
b) if I want to measure 1e-11 or even 1e-12 at 1sec - what resolution
does
my counter need? If the above was true, I would expect that a 1ps
resolution (and an even better stability!) was required to measure ADEV
of
1e-12, The fact that the (as far as I know) world's most recent,
rocket-science grade counter (some Keysight stuff) has "only" 20ps of
resolution, but people are still able to measure even 1e-14 shows that
my
assumption is wrong. So how are the measurement resolution and the ADEV
related to each other? I plan to build my own TIC based on a TDC7200,
which
would offer some 55ps of resolution, but how low could I go with that?
That sounds like a simple question but it's not. There are a few
different approaches to look into:
- Use averaging with your existing counter. Some counters can yield
readings in the 1E-12 region at t=1s even though their single-shot
jitter
is much worse than that. They do this by averaging hundreds or
thousands
of samples for each reading they report. Whether (and when) this is
acceptable is a complex topic in itself, too much so to explain quickly.
Search for information on the effects of averaging and dead time on
Allan
deviation to find the entrance to this fork of the rabbit hole.
-
Search for the term 'DMTD' and read about that.
-
Search for 'direct digital phase measurement' and read about that.
-
Search for 'tight PLL' and read about that.
Basically, while some counters can perform averaging on a post-detection
basis, that's like using the tone control on a radio to reduce static
and
QRM. It works, sort of, but it's too late in the signal chain at that
point to do the job right. You really want to limit the bandwidth
before
the signal is captured, but since that's almost never practical at RF,
the
next best thing to do is limit the bandwidth before the signal is
"demodulated" (i.e., counted.)
Hence items 2, 3, and 4 above. They either limit the measurement
bandwidth prior to detection, lower the frequency itself to keep the
counter's inherent jitter from dominating the measurement, or both.
You'll
have to use one of these methods, or another technique along the same
lines, if you want to measure the short-term stability of a good
oscillator
or GPSDO.
-- john, KE5FX
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