Accuracy results with Trimble Thunderbolt?
Hello Frank,
you're welcome to pose any questions here.
Concerning your question, there are basically two different parameters
called 'stability' and 'uncertainty'.
I personally don't use the term 'accuracy' any more, because that's
somewhat misleading.
The Stability is a characteristic of oscillators, let it be OCXOs, a
naked GPS receiver, or a combined system of a GPS receiver which
disciplines an OCXO. This stability statistics (Allan Deviation, or
ADEV, e.g. inside LH) describes, how much fluctuation / jitter you
encounter on different time scales (also of your counter) when you use
your TB as a time base, on different Gate Times of your frequency
measurements.
You might study such ADEV diagrams for different GPSDO, OCXO, Rb-, Cs-
and MASER clocks on several time-nuts pages to get a better idea:
http://www.ke5fx.com/gpscomp.htm
For the Trimble TB, these fluctuations are on the order of 10^-10 ..
10^-11 for a short Gate Time of maybe 10msec.. 1sec, and prevent that
you get frequency measurements more precise ('accurate') than that.
If you use averaging, or a longer Gate Time, then these fluctuations go
further down due to the good short term stability of the OCXO inside the
TB, then increase at around the time constant you've chosen (500sec?)
due to the big jitter of the GPS signals, and then go down again, into
the 10^-12.. 10^-13 region at averaging times of hours or days, because
the GPS satellite system is synchronized to a Cs master clock at the
D.O.D. Therefore you can achieve an uncertainty (~ 'accuracy') of about
10^-13 also, but that depends also on the oscillator / clock you have in
your house.
Btw.: The GPS system delivers an in-official uncertainty, because the
D.O.D. clock is not participating in the S.I. representation of the UTC.
This LH parameter 'OSC' will give you an estimate, how close the
internal OCXO is currently synchronized to GPS time, but you always have
to take into account the typical ADEV jitter for your specific
measurement setup.
Frank
On Sat, 21 Mar 2020 14:24:48 +0100
Frank Stellmach frank.stellmach@freenet.de wrote:
you're welcome to pose any questions here.
Definitely! We all started as beginners and learned through asking questions.
Concerning your question, there are basically two different parameters
called 'stability' and 'uncertainty'.
I personally don't use the term 'accuracy' any more, because that's
somewhat misleading.
I recommend here reading [1] and [2]. The terms used in metrology are
confusing at first and one needs time to digest them. A quite a few
things that we are used to do in "normal" life do not work for metrology
anymore. Mostly because we assume that our standard (e.g. the calipers,
the gauges, weights, etc) do not change. Once you enter metrology, your
standard isn't stable anymore and you need to evaluate not only how accurate
it is, but how stable. This all then ends up in an uncertainty of your
measurement, parametrized by a specific set of conditions (temperature,
how long you are measuring, etc).
John Vig's Tutorial is also a good source of information to get started:
http://www.ko4bb.com/getsimple/index.php?id=manuals&dir=02_GPS_Timing/John_Vig_Tutorials_on_Crystal_Oscillators
Btw.: The GPS system delivers an in-official uncertainty, because the
D.O.D. clock is not participating in the S.I. representation of the UTC.
This is not true. While BIPM only allows a single NMI per country to
contribute to TAI/EAL these days, this wasn't case in the past. And
for historic reasons there are a few countries where two entities contribute
to TAI/EAL. The USNO, master over GPS time, is one of those non-NMI
entities contributing. They also used to be in the past the one single
organization that had the most atomic clocks running, though that's slowly
changing now. They still are one of of the organisations that have the most
stable clock ensambles contributing to EAL, though, and will stay so for the
forseeable future.
As for how far they are off, have a look at their circular T entry:
https://webtai.bipm.org/database/canvas.html?utclab=ok&lab=usno&mjd1=57078&mjd2=58919
And compare it to, e.g. PTB:
https://webtai.bipm.org/database/canvas.html?utclab=ok&lab=ptb&mjd1=57078&mjd2=58919
Attila Kinali
[1] "Characterization of Clocks and Oscillators" NIST Technical Note 1337,
by Sullivan, Allan, Howe, Walls, 1990
http://tf.nist.gov/general/pdf/868.pdf
[2] "Handbook of Frequency Stability Analysis" NIST Special Publication 1065,
by Riley, 2008
http://tf.nist.gov/timefreq/general/pdf/2220.pdf
Science is made up of so many things that appear obvious
after they are explained. -- Pardot Kynes
Big thanks to Attila, Frank, and Taka for your responses to my questions
on my Trimble Thunderbolt.
I'm inclined to follow Taka's advice and reset the antenna elevation
mask angle with LH's FE command to something between 10 and 20 degrees.
I'm guessing that this is entered in degrees (i.e. "15"), but I can't
find anything in the LH documentation to verify this, so if it's wrong
I'm happy to be corrected. Also, as noted I set the satellite signal
level mask to 1 with LH's FL command, but short of doing a full reset of
the Thunderbolt I'm having trouble figuring out what the default value
is for this.
I appreciate Attila's and Frank's suggestions on topics to study such as
Allan deviation to dip more toes into metrology. I've started in on
this, though I imagine it will take a while to work through. For now, I
wanted to throw out one follow-up question. I see that ADEV diagrams
plotting Allan deviation against time seem to be a primary tool for
evaluating GPSDO performance. If I wanted to compare the Thunderbolt to
another GPSDO (for example, I also have a Bodnar unit), is there
software not wildly beyond a hobbyist budget that would allow me to
compile and display similar data?
Thanks again,
Frank
Frank,
The software that most of us use to create Allan deviation plots is free
and not that hard to use.
You will need ascii numerical phase or frequency data, e.g., from a time
interval or frequency counter.
- Stable32, by Bill Riley
See:
https://ieee-uffc.org/frequency-control/frequency-control-software/stable32/
Documentation and a goldmine of T&F information: http://www.wriley.com/
- TimeLab, by John Miles
http://www.ke5fx.com/timelab/readme.htm
Documentation and tutorials:
http://www.miles.io/PhaseStation_53100A_user_manual.pdf
- Allan deviation for Python, by Anders Wallin
https://pypi.org/project/AllanTools/
- Command line tools for ADEV
adev_lib.c, adev4.c, adev5.c in my http://leapsecond.com/tools/ directory.
- Excel, gnuplot, etc.
Any software that generates log-log plots can be used to make ADEV
plots. You first calculate the statistics using CLI tools and then use
the GUI to make the plots. This gives maximum flexibility in plotting
clarity and style but requires more work than canned packages like
Stable32 or TimeLab.
Normally I recommend TimeLab to new users, but I'm run into VBA and
Excel wizards who are proficient in that environment.
- Plotter, by Ulrich Bangert
Still used by some time nuts, maybe "not recommended for new design",
since Ulrich is no longer with us.
/tvb
On 3/25/2020 1:44 PM, Frank wrote:
Big thanks to Attila, Frank, and Taka for your responses to my
questions on my Trimble Thunderbolt.
I'm inclined to follow Taka's advice and reset the antenna elevation
mask angle with LH's FE command to something between 10 and 20
degrees. I'm guessing that this is entered in degrees (i.e. "15"), but
I can't find anything in the LH documentation to verify this, so if
it's wrong I'm happy to be corrected. Also, as noted I set the
satellite signal level mask to 1 with LH's FL command, but short of
doing a full reset of the Thunderbolt I'm having trouble figuring out
what the default value is for this.
I appreciate Attila's and Frank's suggestions on topics to study such
as Allan deviation to dip more toes into metrology. I've started in on
this, though I imagine it will take a while to work through. For now,
I wanted to throw out one follow-up question. I see that ADEV diagrams
plotting Allan deviation against time seem to be a primary tool for
evaluating GPSDO performance. If I wanted to compare the Thunderbolt
to another GPSDO (for example, I also have a Bodnar unit), is there
software not wildly beyond a hobbyist budget that would allow me to
compile and display similar data?
Thanks again,
Frank
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Tom,
Thanks very much, that's a great list and will keep me busy for some time.
At the hobbyist level, are there time interval counters or frequency
counters that are particularly popular, and/or that pair well with the
listed software?
Frank
On 3/25/20 2:30 PM, Tom Van Baak wrote:
Frank,
The software that most of us use to create Allan deviation plots is
free and not that hard to use.
You will need ascii numerical phase or frequency data, e.g., from a
time interval or frequency counter.
- Stable32, by Bill Riley
See:
https://ieee-uffc.org/frequency-control/frequency-control-software/stable32/
Documentation and a goldmine of T&F information: http://www.wriley.com/
- TimeLab, by John Miles
http://www.ke5fx.com/timelab/readme.htm
Documentation and tutorials:
http://www.miles.io/PhaseStation_53100A_user_manual.pdf
- Allan deviation for Python, by Anders Wallin
https://pypi.org/project/AllanTools/
- Command line tools for ADEV
adev_lib.c, adev4.c, adev5.c in my http://leapsecond.com/tools/
directory.
- Excel, gnuplot, etc.
Any software that generates log-log plots can be used to make ADEV
plots. You first calculate the statistics using CLI tools and then use
the GUI to make the plots. This gives maximum flexibility in plotting
clarity and style but requires more work than canned packages like
Stable32 or TimeLab.
Normally I recommend TimeLab to new users, but I'm run into VBA and
Excel wizards who are proficient in that environment.
- Plotter, by Ulrich Bangert
Still used by some time nuts, maybe "not recommended for new design",
since Ulrich is no longer with us.
/tvb
On 3/25/2020 1:44 PM, Frank wrote:
Big thanks to Attila, Frank, and Taka for your responses to my
questions on my Trimble Thunderbolt.
I'm inclined to follow Taka's advice and reset the antenna elevation
mask angle with LH's FE command to something between 10 and 20
degrees. I'm guessing that this is entered in degrees (i.e. "15"),
but I can't find anything in the LH documentation to verify this, so
if it's wrong I'm happy to be corrected. Also, as noted I set the
satellite signal level mask to 1 with LH's FL command, but short of
doing a full reset of the Thunderbolt I'm having trouble figuring out
what the default value is for this.
I appreciate Attila's and Frank's suggestions on topics to study such
as Allan deviation to dip more toes into metrology. I've started in
on this, though I imagine it will take a while to work through. For
now, I wanted to throw out one follow-up question. I see that ADEV
diagrams plotting Allan deviation against time seem to be a primary
tool for evaluating GPSDO performance. If I wanted to compare the
Thunderbolt to another GPSDO (for example, I also have a Bodnar
unit), is there software not wildly beyond a hobbyist budget that
would allow me to compile and display similar data?
Thanks again,
Frank