1 pps Accuracy in two locations
Many double-frequency GPS receivers have about 2 mm noise in them, that
for the 1,57542 GHz (19 cm) and 1,2276 GHz (24 cm), so about 1/100 of
the wavelength. It is not very strange that you can measure a fraction
of a cycle, this is about 3,6 degrees on a cycle.
So, to get that resolution you do not need to support full cycles in BW.
If so, our 10 MHz reference oscillator counters would only support
resolution of 100 ns, and that used to be the resolution. Improved
counters used higher clocks, such as 100 MHz to get 10 ns resolution.
Since then we learned to interpolate and can accurately resolve phase of
a cycle in the range of 1000 to 10000 today without too much trouble.
The good old trusty 5370A has 256 in interpolation gain, and that from
200 MHz / 5 ns, but you benefit from it as you measure 5 MHz and 10 MHz
signals... so, for 1 ps resolution you do not need THz bandwidth. I have
counters with about 2 ps RMS trigger jitter and resolution of 800 fs and
200 fs, and their BW is just above 1 GHz.
Cheers,
Magnus
On 2019-12-04 17:56, djl wrote:
Maybe Grace Hopper's aid will help? 11.8 inches = one nanosec, so a ps
is .0118 inches. You really want two clocks not occupying the same
space to be correlated to that accuracy?
Don
On 2019-12-04 08:55, Tom Van Baak wrote:
Martyn,
I'm always being asked to provide equipment that can produce two 1
pps outputs aligned to each other to within a few ps.
They should look at their best 1PPS on a 'scope. You can get ns with
care; I doubt ps is possible. I mean, that's THz BW isn't it?
Can you share with us what their application is?
So they are asking for two of my GNSS frequency standards with 1 pps
outputs.
The 1 pps outputs being derived from the rubidium oscillator (which
is aligned to GPS/GNSS)
The best I think I can achieve is in the low ns range.
Right. It will be ns, not ps. Forget about using GNSS for ps level
timing.
Does anyone know how this can be achieved?
Google for papers by high-end national timing laboratories. Words
like: active temperature stabilized (phase stabilized) bidirectional
optical fiber links.
Very possible, very expensive, quite common now. I'd guess most of the
timing centers in Europe are linked this way.
/tvb
On 12/4/2019 1:40 AM, martyn@ptsyst.com wrote:
Hello,
I'm always being asked to provide equipment that can produce two 1
pps
outputs aligned to each other to within a few ps.
These two 1 pps pulses are not in the same location and could be
100 metres
to a few km away.
So they are asking for two of my GNSS frequency standards with 1 pps
outputs.
The 1 pps outputs being derived from the rubidium oscillator
(which is
aligned to GPS/GNSS)
The best I think I can achieve is in the low ns range.
Does anyone know how this can be achieved?
Regards
Martyn
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On Wed, 4 Dec 2019 09:40:34 -0000
martyn@ptsyst.com wrote:
I'm always being asked to provide equipment that can produce two 1 pps
outputs aligned to each other to within a few ps.
These two 1 pps pulses are not in the same location and could be 100 metres
to a few km away.
As others have written, getting down to a few ps is not feasible, at least
not with the amount of money your customers are likely willing to pay.
To get down to these levels you will need to pull fibres from one location
to another and using special circuitry to activly compensate variation
in length due to temperature changes and vibration, even for burried fibres.
Just to put into perspective what your customers are asking for: in 1ps
light travels 300µm in vacuum/air or ~150µm in fibre/coax. This means,
to get better than 10ps, you have to control the length of everything down
to better than 1mm. Thermal expansion coefficient is somewhere between
1e-6 to 1e-4 for solids. Assuming you have 1e-6 and want to bridge 1km
you would need to keep the temperature of the whole fibre stable to 1°C
in order to keep the timing variation due to length change below 10ps.
And that's not yet accounting for changes in refraction index (i.e. speed
of light) due to temperature changes or a myriad other effects that you
will have to deal with.
Even white rabbit, which is probably the most advanced system you will
likely get your hands on (does two-way timing to compensate for length
of fibre and length variation), gives you about 200ps of uncertainty
from one node to the next (but <10ps rms jitter). Each WR switch/node
costs ~3k€ so isn't exactly cheap either, but you get GBit ethernet
ontop of the time transfer as well.
So they are asking for two of my GNSS frequency standards with 1 pps
outputs.
State of the art time transfer using GPS, I am aware of, is what BIPM has
demonstrated two years ago using iPPP and, IIRC a base line of a few 100km
of 200ps uncertainty. But that's using calibrated GPS receivers in a
temperature controlled environment with lots of post-processing.
I.e. it's not real-time. For shorter baselines, I expect the uncertainty
to come down a bit. But I would not assume for it to go below 100ps without
verifying first, even at very short baseliens of a few 10s of meters.
If you have money to spare and line of sight between the locations,
you could employ something like the NIST free space laser system
https://www.nist.gov/programs-projects/optical-two-way-time-frequency-transfer
But beware: it uses multiple frequency combs which cost as much as a very
nice car. Each of them. But they get to sub-100fs stability, which means
you could get to low-ps uncertainty, if done correctly.
Rule of thumb: if you need time transfer better than 1ns you need to think
about what you are doing, even within the same room. If you leave the
building, going below 1ns is going to be hard. Doing better than 1ns within
a neighborhood, while possible, will need some serious equipment and proper
planing. Any factor of 10 better will drive up your cost by a factor 100 to
1000.
Rule of thumb #2: if you need to control lengths to better than 10µm, your
mechanics guy will throw a fit as he will most likely be unable to manufacture
to that precision, unless you are building something tiny.
Attila Kinali
--
Science is made up of so many things that appear obvious
after they are explained. -- Pardot Kynes
If anybody knows how to do this, I bet it would be the people who do
long-baseline
interferometry at millimeter-wave frequencies, such as at the eVLA in New
Mexico.
Dana
On Wed, Jan 15, 2020 at 4:35 PM Attila Kinali attila@kinali.ch wrote:
On Wed, 4 Dec 2019 09:40:34 -0000
martyn@ptsyst.com wrote:
I'm always being asked to provide equipment that can produce two 1 pps
outputs aligned to each other to within a few ps.
These two 1 pps pulses are not in the same location and could be 100
metres
to a few km away.
As others have written, getting down to a few ps is not feasible, at least
not with the amount of money your customers are likely willing to pay.
To get down to these levels you will need to pull fibres from one location
to another and using special circuitry to activly compensate variation
in length due to temperature changes and vibration, even for burried
fibres.
Just to put into perspective what your customers are asking for: in 1ps
light travels 300µm in vacuum/air or ~150µm in fibre/coax. This means,
to get better than 10ps, you have to control the length of everything down
to better than 1mm. Thermal expansion coefficient is somewhere between
1e-6 to 1e-4 for solids. Assuming you have 1e-6 and want to bridge 1km
you would need to keep the temperature of the whole fibre stable to 1°C
in order to keep the timing variation due to length change below 10ps.
And that's not yet accounting for changes in refraction index (i.e. speed
of light) due to temperature changes or a myriad other effects that you
will have to deal with.
Even white rabbit, which is probably the most advanced system you will
likely get your hands on (does two-way timing to compensate for length
of fibre and length variation), gives you about 200ps of uncertainty
from one node to the next (but <10ps rms jitter). Each WR switch/node
costs ~3k€ so isn't exactly cheap either, but you get GBit ethernet
ontop of the time transfer as well.
So they are asking for two of my GNSS frequency standards with 1 pps
outputs.
State of the art time transfer using GPS, I am aware of, is what BIPM has
demonstrated two years ago using iPPP and, IIRC a base line of a few 100km
of 200ps uncertainty. But that's using calibrated GPS receivers in a
temperature controlled environment with lots of post-processing.
I.e. it's not real-time. For shorter baselines, I expect the uncertainty
to come down a bit. But I would not assume for it to go below 100ps without
verifying first, even at very short baseliens of a few 10s of meters.
If you have money to spare and line of sight between the locations,
you could employ something like the NIST free space laser system
https://www.nist.gov/programs-projects/optical-two-way-time-frequency-transfer
But beware: it uses multiple frequency combs which cost as much as a very
nice car. Each of them. But they get to sub-100fs stability, which means
you could get to low-ps uncertainty, if done correctly.
Rule of thumb: if you need time transfer better than 1ns you need to think
about what you are doing, even within the same room. If you leave the
building, going below 1ns is going to be hard. Doing better than 1ns within
a neighborhood, while possible, will need some serious equipment and proper
planing. Any factor of 10 better will drive up your cost by a factor 100 to
1000.
Rule of thumb #2: if you need to control lengths to better than 10µm, your
mechanics guy will throw a fit as he will most likely be unable to
manufacture
to that precision, unless you are building something tiny.
Attila Kinali
--
Science is made up of so many things that appear obvious
after they are explained. -- Pardot Kynes
time-nuts mailing list -- time-nuts@lists.febo.com
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and follow the instructions there.
On Wed, 15 Jan 2020 16:39:45 -0600
Dana Whitlow k8yumdoober@gmail.com wrote:
If anybody knows how to do this, I bet it would be the people who do
long-baseline
interferometry at millimeter-wave frequencies, such as at the eVLA in New
Mexico.
You probably know that much better than I do, but I thought that
VLBI applications usually did all the magic in post-processing.
Ie record everything with highly stable local clocks (aka hydrogen masers),
then feed everything to a large cluster of computers and let it figure out
what the actual time offsets between the stations were.
Attila Kinali
--
<JaberWorky> The bad part of Zurich is where the degenerates
throw DARK chocolate at you.
That is true. Is post-processing not an option here?
Dana
On Wed, Jan 15, 2020 at 5:05 PM Attila Kinali attila@kinali.ch wrote:
On Wed, 15 Jan 2020 16:39:45 -0600
Dana Whitlow k8yumdoober@gmail.com wrote:
If anybody knows how to do this, I bet it would be the people who do
long-baseline
interferometry at millimeter-wave frequencies, such as at the eVLA in New
Mexico.
You probably know that much better than I do, but I thought that
VLBI applications usually did all the magic in post-processing.
Ie record everything with highly stable local clocks (aka hydrogen masers),
then feed everything to a large cluster of computers and let it figure out
what the actual time offsets between the stations were.
Attila Kinali
--
<JaberWorky> The bad part of Zurich is where the degenerates
throw DARK chocolate at you.
time-nuts mailing list -- time-nuts@lists.febo.com
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and follow the instructions there.
On 1/15/20 2:34 PM, Attila Kinali wrote:
On Wed, 4 Dec 2019 09:40:34 -0000
martyn@ptsyst.com wrote:
I'm always being asked to provide equipment that can produce two 1 pps
outputs aligned to each other to within a few ps.
These two 1 pps pulses are not in the same location and could be 100 metres
to a few km away.
As others have written, getting down to a few ps is not feasible, at least
not with the amount of money your customers are likely willing to pay.
To get down to these levels you will need to pull fibres from one location
to another and using special circuitry to activly compensate variation
in length due to temperature changes and vibration, even for burried fibres.
Just to put into perspective what your customers are asking for: in 1ps
light travels 300µm in vacuum/air or ~150µm in fibre/coax. This means,
to get better than 10ps, you have to control the length of everything down
to better than 1mm. Thermal expansion coefficient is somewhere between
1e-6 to 1e-4 for solids. Assuming you have 1e-6 and want to bridge 1km
you would need to keep the temperature of the whole fibre stable to 1°C
in order to keep the timing variation due to length change below 10ps.
And that's not yet accounting for changes in refraction index (i.e. speed
of light) due to temperature changes or a myriad other effects that you
will have to deal with.
Even white rabbit, which is probably the most advanced system you will
likely get your hands on (does two-way timing to compensate for length
of fibre and length variation), gives you about 200ps of uncertainty
from one node to the next (but <10ps rms jitter). Each WR switch/node
costs ~3k€ so isn't exactly cheap either, but you get GBit ethernet
ontop of the time transfer as well.
So they are asking for two of my GNSS frequency standards with 1 pps
outputs.
State of the art time transfer using GPS, I am aware of, is what BIPM has
demonstrated two years ago using iPPP and, IIRC a base line of a few 100km
of 200ps uncertainty. But that's using calibrated GPS receivers in a
temperature controlled environment with lots of post-processing.
I.e. it's not real-time. For shorter baselines, I expect the uncertainty
to come down a bit. But I would not assume for it to go below 100ps without
verifying first, even at very short baseliens of a few 10s of meters.
If you have money to spare and line of sight between the locations,
you could employ something like the NIST free space laser system
https://www.nist.gov/programs-projects/optical-two-way-time-frequency-transfer
But beware: it uses multiple frequency combs which cost as much as a very
nice car. Each of them. But they get to sub-100fs stability, which means
you could get to low-ps uncertainty, if done correctly.
Rule of thumb: if you need time transfer better than 1ns you need to think
about what you are doing, even within the same room. If you leave the
building, going below 1ns is going to be hard. Doing better than 1ns within
a neighborhood, while possible, will need some serious equipment and proper
planing. Any factor of 10 better will drive up your cost by a factor 100 to
1000.
Even free space optical would have to deal with propagation speed
variations due to temperature and humidity. 1ns is 300 mm. 1ps is 0.3
mm. Out of 1km is is 0.3 ppm.
40C variation in temperature changes the refractive index by about 50
ppm. There's also a humidity effect, and a CO2 effect. At VHF
frequencies, it's enough to allow "tropospheric ducting" to allow
communications well beyond line of sight.
And, of course, measuring those chances at L-band GPS signals is a way
to characterize the atmosphere using occultation - COSMIC and COSMIC 2
do this.
Gravitational effects on the beam path are probably not an issue.
But ultimately, this is the kind of problem that LIGO and other long
path interferometers face (and why they use pipes with a vacuum).
Rule of thumb #2: if you need to control lengths to better than 10µm, your
mechanics guy will throw a fit as he will most likely be unable to manufacture
to that precision, unless you are building something tiny.
Attila Kinali
VLA and ALMA aren't exactly VLBI - they're all in the same (general)
place and they distribute LO and timing signals to all the antennas.
Larry D'Addario, recently retired from JPL and now down at Caltech,
described the setup at ALMA in 2009 - He's talking 1 microsecond, maybe
100 nanoseconds.
https://library.nrao.edu/public/memos/alma/memo298.pdf
A more recent reference for ALMA (2018) describes a scheme using pairs
of lasers which are transmitted via fiber heterodyned at the user end to
regenerate a Local Oscillator - that gives you good frequency accuracy,
but I don't know about absolute phase. They use fiber "line stretchers"
to compensate for the fiber delay. But I didn't see a performance spec.
You can hunt if you like.
https://www.iram.fr/IRAMFR/ARC/documents/cycle6/ALMA_Cycle6_Technical_Handbook.pdf
DSN does antenna arraying on receive as well. DSN uses fiber links in
temperature controlled pipes underground with two way measurements.
Calhoun 2007 gives a performance of about 1E-15 at 100 seconds, which is
0.1 ps. But that's a pretty darn elaborate system (costing the
equivalent of many houses, I suspect, with Cryogenic Sapphire
Oscillators for cleanup loops)
M. Calhoun, S. Huang and R. L. Tjoelker, "Stable Photonic Links for
Frequency and Time Transfer in the Deep-Space Network and Antenna
Arrays," in Proceedings of the IEEE, vol. 95, no. 10, pp. 1931-1946,
Oct. 2007.
doi: 10.1109/JPROC.2007.905048
For radio astronomy, one can do a lot of post processing to "back out"
the time shifts - if there's some bright source you can use that as a
focusing target. It's not like CygA or CasA are going to be moving
(other than from Earth rotation and movement)
On 1/15/20 4:12 PM, Dana Whitlow wrote:
That is true. Is post-processing not an option here?
Dana
On Wed, Jan 15, 2020 at 5:05 PM Attila Kinali attila@kinali.ch wrote:
On Wed, 15 Jan 2020 16:39:45 -0600
Dana Whitlow k8yumdoober@gmail.com wrote:
If anybody knows how to do this, I bet it would be the people who do
long-baseline
interferometry at millimeter-wave frequencies, such as at the eVLA in New
Mexico.
You probably know that much better than I do, but I thought that
VLBI applications usually did all the magic in post-processing.
Ie record everything with highly stable local clocks (aka hydrogen masers),
then feed everything to a large cluster of computers and let it figure out
what the actual time offsets between the stations were.
Attila Kinali
--
<JaberWorky> The bad part of Zurich is where the degenerates
throw DARK chocolate at you.
time-nuts mailing list -- time-nuts@lists.febo.com
To unsubscribe, go to
http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com
and follow the instructions there.
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and follow the instructions there.
Hi,
On 2020-01-15 23:34, Attila Kinali wrote:
On Wed, 4 Dec 2019 09:40:34 -0000
martyn@ptsyst.com wrote:
I'm always being asked to provide equipment that can produce two 1 pps
outputs aligned to each other to within a few ps.
These two 1 pps pulses are not in the same location and could be 100 metres
to a few km away.
As others have written, getting down to a few ps is not feasible, at least
not with the amount of money your customers are likely willing to pay.
To get down to these levels you will need to pull fibres from one location
to another and using special circuitry to activly compensate variation
in length due to temperature changes and vibration, even for burried fibres.
Just to put into perspective what your customers are asking for: in 1ps
light travels 300µm in vacuum/air or ~150µm in fibre/coax.
Let me correct that a little.
For fibre the relative dielectrics of the silica glass is just about
2.25 giving the index just about 1.5, which then gives the 300 um / 1.5
to about 200 um. I am known to indicate the length of 1 ns in fibre
betwen my index finger and thumb, roughly 2 dm, giving the delay for 1 m
to be about 5 ns, letting the round-trip-time for 1 m be 10 ns which is
a very handy number for rule of thumb conversions for fibre. If you look
in more detail, the actual property depend on the wavelength being used
and the temperature of the fibre, as this changes the actual delay.
While first degree compensation is trivial in two-way systems, you end
up having calibration issues.
Coax is less easy. If you have the normal RG58 crap, it aligns to about
the same numbers as fiber, as the dielectrics is about the same.
However, for more phase-stable cables with lower dielectric loss one
simply has less dielectrics to start with, such as foam or other form of
support for center conductor. That gives the relative dielectric go
towards 1 and thus the velocity factor with that. It's much more a "it
depends".
Other than that, I agree with the general analysis of Attila, it is
close to my experience, and I've been working on these things
commercially for over 10 years now. If you want to know how things works
(or rather not work) in a telecom, it is even more painful than this.
So end conclusion being, if you required precision of 1 ps from a timing
system, you are likely going to have one very expensive system and it
will be a pain to operate, it may be worth considering if you are doing
it the right way. I've seen requirements in the 10s of ps for a fixed
system setup, but that is while challenging kind of doable, but then
that requires quite a bit of additional control loops and knowing what
one does.
Cheers,
Magnus
Except for some low tempco single mode fibers the delay tempco is on the order of 10ppm/K:
https://library.nrao.edu/public/memos/edtn/EDTN_168.pdf
Bruce
On 16 January 2020 at 23:29 Magnus Danielson magnus@rubidium.se wrote:
Hi,
On 2020-01-15 23:34, Attila Kinali wrote:
On Wed, 4 Dec 2019 09:40:34 -0000
martyn@ptsyst.com wrote:
I'm always being asked to provide equipment that can produce two 1 pps
outputs aligned to each other to within a few ps.
These two 1 pps pulses are not in the same location and could be 100 metres
to a few km away.
As others have written, getting down to a few ps is not feasible, at least
not with the amount of money your customers are likely willing to pay.
To get down to these levels you will need to pull fibres from one location
to another and using special circuitry to activly compensate variation
in length due to temperature changes and vibration, even for burried fibres.
Just to put into perspective what your customers are asking for: in 1ps
light travels 300µm in vacuum/air or ~150µm in fibre/coax.
Let me correct that a little.
For fibre the relative dielectrics of the silica glass is just about
2.25 giving the index just about 1.5, which then gives the 300 um / 1.5
to about 200 um. I am known to indicate the length of 1 ns in fibre
betwen my index finger and thumb, roughly 2 dm, giving the delay for 1 m
to be about 5 ns, letting the round-trip-time for 1 m be 10 ns which is
a very handy number for rule of thumb conversions for fibre. If you look
in more detail, the actual property depend on the wavelength being used
and the temperature of the fibre, as this changes the actual delay.
While first degree compensation is trivial in two-way systems, you end
up having calibration issues.
Coax is less easy. If you have the normal RG58 crap, it aligns to about
the same numbers as fiber, as the dielectrics is about the same.
However, for more phase-stable cables with lower dielectric loss one
simply has less dielectrics to start with, such as foam or other form of
support for center conductor. That gives the relative dielectric go
towards 1 and thus the velocity factor with that. It's much more a "it
depends".
Other than that, I agree with the general analysis of Attila, it is
close to my experience, and I've been working on these things
commercially for over 10 years now. If you want to know how things works
(or rather not work) in a telecom, it is even more painful than this.
So end conclusion being, if you required precision of 1 ps from a timing
system, you are likely going to have one very expensive system and it
will be a pain to operate, it may be worth considering if you are doing
it the right way. I've seen requirements in the 10s of ps for a fixed
system setup, but that is while challenging kind of doable, but then
that requires quite a bit of additional control loops and knowing what
one does.
Cheers,
Magnus
time-nuts mailing list -- time-nuts@lists.febo.com
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and follow the instructions there.
PSOF fiber has a much lower tempco:
https://accelconf.web.cern.ch/accelconf/p01/PAPERS/MPPH011.PDF
Bruce
On 17 January 2020 at 00:08 Bruce Griffiths bruce.griffiths@xtra.co.nz wrote:
Except for some low tempco single mode fibers the delay tempco is on the order of 10ppm/K:
https://library.nrao.edu/public/memos/edtn/EDTN_168.pdf
Bruce
On 16 January 2020 at 23:29 Magnus Danielson magnus@rubidium.se wrote:
Hi,
On 2020-01-15 23:34, Attila Kinali wrote:
On Wed, 4 Dec 2019 09:40:34 -0000
martyn@ptsyst.com wrote:
I'm always being asked to provide equipment that can produce two 1 pps
outputs aligned to each other to within a few ps.
These two 1 pps pulses are not in the same location and could be 100 metres
to a few km away.
As others have written, getting down to a few ps is not feasible, at least
not with the amount of money your customers are likely willing to pay.
To get down to these levels you will need to pull fibres from one location
to another and using special circuitry to activly compensate variation
in length due to temperature changes and vibration, even for burried fibres.
Just to put into perspective what your customers are asking for: in 1ps
light travels 300µm in vacuum/air or ~150µm in fibre/coax.
Let me correct that a little.
For fibre the relative dielectrics of the silica glass is just about
2.25 giving the index just about 1.5, which then gives the 300 um / 1.5
to about 200 um. I am known to indicate the length of 1 ns in fibre
betwen my index finger and thumb, roughly 2 dm, giving the delay for 1 m
to be about 5 ns, letting the round-trip-time for 1 m be 10 ns which is
a very handy number for rule of thumb conversions for fibre. If you look
in more detail, the actual property depend on the wavelength being used
and the temperature of the fibre, as this changes the actual delay.
While first degree compensation is trivial in two-way systems, you end
up having calibration issues.
Coax is less easy. If you have the normal RG58 crap, it aligns to about
the same numbers as fiber, as the dielectrics is about the same.
However, for more phase-stable cables with lower dielectric loss one
simply has less dielectrics to start with, such as foam or other form of
support for center conductor. That gives the relative dielectric go
towards 1 and thus the velocity factor with that. It's much more a "it
depends".
Other than that, I agree with the general analysis of Attila, it is
close to my experience, and I've been working on these things
commercially for over 10 years now. If you want to know how things works
(or rather not work) in a telecom, it is even more painful than this.
So end conclusion being, if you required precision of 1 ps from a timing
system, you are likely going to have one very expensive system and it
will be a pain to operate, it may be worth considering if you are doing
it the right way. I've seen requirements in the 10s of ps for a fixed
system setup, but that is while challenging kind of doable, but then
that requires quite a bit of additional control loops and knowing what
one does.
Cheers,
Magnus
time-nuts mailing list -- time-nuts@lists.febo.com
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and follow the instructions there.
time-nuts mailing list -- time-nuts@lists.febo.com
To unsubscribe, go to http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com
and follow the instructions there.