Discussion:
Trueposition Antenna Location
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Mark Goldberg
2018-11-30 04:22:44 UTC
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Permalink
I have one of the TruePosition GPSDOs. They already are not the best in
terms of signal sensitivity, but the antenna is now located where it sees a
narrow segment of open sky. It generally sees no more than 4 or 5 sats and
goes into holdover often.

I just use the 10 MHz output as a frequency reference.

Attached, (hopefully if I did it right) are images of the Lady Heather
screen and an ADEV plot referenced to a Bodnar GPSDO.

How much could I expect ADEV to improve if I move the antenna to a better
location with clear view of the sky? The things I am testing need to be
better than 8e-10, so this at just under 3e-10 may be "good enough". Moving
the antenna would involve drilling a hole in the outside wall, so it is not
easy.

Any suggestions are welcome.

Thanks,

Mark
Bob kb8tq
2018-11-30 14:58:33 UTC
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Permalink
Hi

There are lots more issues to GPSDO stability than just antenna placement. Indeed
you are correct that a location that keeps you out of holdover is a must. Ideally you
want to be able to lock on to 4 sats at all times. That generally equates to being able
to “see” something in the > 6 range all the time. Once you get the antenna set up
well, you can start fiddling with the rest of the normal GPSDO issues.

Equally, there are a bunch of qualifiers any time you do an ADEV measurement. The
most basic is that it’s a comparison. You get the combined ADEV of your two standards.
That’s fine if you have a hydrogen maser and are looking at a GPSDO :). Comparing one
GPSDO to another will always get into the “which one is better?” debate.

One way to separate data is by comparing three devices. That assumes you *have* three
fairly good devices to compare. In this case a reasonably good OCXO could do the trick.
If a telecom Rb is handy, it also would be a good thing to toss into the mix.

All that said, there are GPSDO’s out there that will plot out roughly an order of magnitude
better than what you are seeing. That’s when they are compared to something much
better (and it drops out of the plot).

Bob
Post by Mark Goldberg
I have one of the TruePosition GPSDOs. They already are not the best in
terms of signal sensitivity, but the antenna is now located where it sees a
narrow segment of open sky. It generally sees no more than 4 or 5 sats and
goes into holdover often.
I just use the 10 MHz output as a frequency reference.
Attached, (hopefully if I did it right) are images of the Lady Heather
screen and an ADEV plot referenced to a Bodnar GPSDO.
How much could I expect ADEV to improve if I move the antenna to a better
location with clear view of the sky? The things I am testing need to be
better than 8e-10, so this at just under 3e-10 may be "good enough". Moving
the antenna would involve drilling a hole in the outside wall, so it is not
easy.
Any suggestions are welcome.
Thanks,
Mark
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Attila Kinali
2018-11-30 15:54:45 UTC
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On Thu, 29 Nov 2018 21:22:44 -0700
Post by Mark Goldberg
How much could I expect ADEV to improve if I move the antenna to a better
location with clear view of the sky? The things I am testing need to be
better than 8e-10, so this at just under 3e-10 may be "good enough". Moving
the antenna would involve drilling a hole in the outside wall, so it is not
As Bob already wrote, antenna position is only one thing. But if your
receiver is going into holdover at all, then you are already in a place
where you will not have good enough reception to get down to your requirement.
Antenna position, ie sky view and multipath, are the two things that
kill most of the performance of any GPSDO. Bad sky view can result
in several µs of phase variation and multipath in several 100ns.

After that, you will need a local oscillator with sufficient stability
to average over a long period. To get below 1e-10 you will need to average
over at least 100s (given good receiver and perfect antenna position),
better make it 1000s to 10ks. This means, the local oscillator has to be
stable enough to stay below 1e-10 over that period. Which means you
need to use a good OCXO at the very least, in a temperature stable
environment. No-one of the GPSDOs you mentioned fullfills that requirement.
(The NV47* of the TruePosition is decent, but falls slightly short.
Leo's GPSDO uses a TCXO)

I would recommend you to get either a Thunderbolt (first choice)
or a Star4 (second choice) from ebay. Alternatively, get one
of Nick Sayer's GPSDO, modify it to use a good OCXO and crank
up its loop time constant.
Do not get one of the Trimble UCCM, they have quite horrible
frequency jumps.



Attila Kinali
--
It is upon moral qualities that a society is ultimately founded. All
the prosperity and technological sophistication in the world is of no
use without that foundation.
-- Miss Matheson, The Diamond Age, Neal Stephenson

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Mark Goldberg
2018-11-30 16:19:19 UTC
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Permalink
Post by Attila Kinali
On Thu, 29 Nov 2018 21:22:44 -0700
Post by Mark Goldberg
How much could I expect ADEV to improve if I move the antenna to a better
location with clear view of the sky? The things I am testing need to be
better than 8e-10, so this at just under 3e-10 may be "good enough".
Moving
Post by Mark Goldberg
the antenna would involve drilling a hole in the outside wall, so it is
not
As Bob already wrote, antenna position is only one thing. But if your
receiver is going into holdover at all, then you are already in a place
where you will not have good enough reception to get down to your requirement.
Antenna position, ie sky view and multipath, are the two things that
kill most of the performance of any GPSDO. Bad sky view can result
in several µs of phase variation and multipath in several 100ns.
I definitely have a bad sky view. The antenna is in a ground floor window
of a two story home with a metal covered RV parking area next to it. I can
only see maybe 30 degrees of sky. I am in Arizona. The view is basically
from about straight up to 60 degrees from the horizon to the south.
Post by Attila Kinali
After that, you will need a local oscillator with sufficient stability
to average over a long period. To get below 1e-10 you will need to average
over at least 100s (given good receiver and perfect antenna position),
better make it 1000s to 10ks.
I made the measurements using the setup described at:

https://sites.google.com/site/spectrumlabtesting/

and the other sites linked there. There could be some anomalies due to the
peak frequency measurement algorithm.
Post by Attila Kinali
This means, the local oscillator has to be
stable enough to stay below 1e-10 over that period. Which means you
need to use a good OCXO at the very least, in a temperature stable
environment. No-one of the GPSDOs you mentioned fullfills that requirement.
(The NV47* of the TruePosition is decent, but falls slightly short.
Leo's GPSDO uses a TCXO)
I would recommend you to get either a Thunderbolt (first choice)
or a Star4 (second choice) from ebay. Alternatively, get one
of Nick Sayer's GPSDO, modify it to use a good OCXO and crank
up its loop time constant.
Do not get one of the Trimble UCCM, they have quite horrible
frequency jumps.
Unfortunately, I have what I have for now. I have basically three devices,
the TruePosition and Bodnar GPSDOs, and an 8642A generator with the OCXO
option. None are optimal I know, I just want to do the best with what I
have.

Thanks for the suggestions though.

Regards,

Mark
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Mark Goldberg
2018-12-02 16:16:50 UTC
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I broke down and drilled a couple holes in the wall and moved the antennas
for both GPSDOs outside with a better view of the sky, but still somewhat
shielded by the house and a metal RV cover. The number of sats is way up
and the TruePosition is in normal mode most of the time. The ADEV is not
improved and actually measured worse. I took data for twice as long at two
sample rates, 0.6816 sec and 5.453 sec. The ADEV from the slower sample
rates is lower, possibly due to the measurement method, but they all are in
the same order of magnitude. I've named the Heather screen captures for the
inside and outside locations and shown one combined Timelab capture.

Regards,

Mark
Bob kb8tq
2018-12-02 21:15:22 UTC
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Permalink
Hi

Looking at the data, I’m guessing the slower sample rate was done last. If so,
you may not be letting the GPSDO’s “warm up” long enough. Most designs
take days (if not weeks) to get to their ultimate stability.

Bob
Post by Mark Goldberg
I broke down and drilled a couple holes in the wall and moved the antennas
for both GPSDOs outside with a better view of the sky, but still somewhat
shielded by the house and a metal RV cover. The number of sats is way up
and the TruePosition is in normal mode most of the time. The ADEV is not
improved and actually measured worse. I took data for twice as long at two
sample rates, 0.6816 sec and 5.453 sec. The ADEV from the slower sample
rates is lower, possibly due to the measurement method, but they all are in
the same order of magnitude. I've named the Heather screen captures for the
inside and outside locations and shown one combined Timelab capture.
Regards,
Mark
<TruePos_Outside_Ant_Heather.png><TruePos_Inside_Outside_Compare_Ant_Allan.png><TruePos_Inside_Ant_Heather.png>_______________________________________________
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Mark Goldberg
2018-12-03 05:30:51 UTC
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I ran them again today and the results are almost identical. The slower
sample rate still shows a lower ADEV. I am not sure what the algorithm to
determine the peak frequency is but it is described as a linear
interpolation between the two highest peaks on the FFT. I could see how
that could be subject to math errors. For the slower sample rate, the
frequency bins are smaller. I have not researched how all this could affect
the peak frequency and possibly introduce noise (or by truncation remove
small amounts of noise).

I will run it a few times in the next few days and will also try to move
the antennas to a location with lower multipath.

Thanks for the suggestions,

Mark
Post by Bob kb8tq
Hi
Looking at the data, I’m guessing the slower sample rate was done last. If so,
you may not be letting the GPSDO’s “warm up” long enough. Most designs
take days (if not weeks) to get to their ultimate stability.
Bob
Post by Mark Goldberg
I broke down and drilled a couple holes in the wall and moved the
antennas
Post by Mark Goldberg
for both GPSDOs outside with a better view of the sky, but still somewhat
shielded by the house and a metal RV cover. The number of sats is way up
and the TruePosition is in normal mode most of the time. The ADEV is not
improved and actually measured worse. I took data for twice as long at
two
Post by Mark Goldberg
sample rates, 0.6816 sec and 5.453 sec. The ADEV from the slower sample
rates is lower, possibly due to the measurement method, but they all are
in
Post by Mark Goldberg
the same order of magnitude. I've named the Heather screen captures for
the
Post by Mark Goldberg
inside and outside locations and shown one combined Timelab capture.
Regards,
Mark
<TruePos_Outside_Ant_Heather.png><TruePos_Inside_Outside_Compare_Ant_Allan.png><TruePos_Inside_Ant_Heather.png>_______________________________________________
Post by Mark Goldberg
To unsubscribe, go to
http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com
Post by Mark Goldberg
and follow the instructions there.
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Bob kb8tq
2018-12-03 13:26:46 UTC
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Hi

ADEV is pretty well documented and the results should *not* be dependent on the
sample rate. The proper approach to dropping the sample rate is decimation of the
data. I think you may want to look at the way you are doing ADEV.

Bob
Post by Mark Goldberg
I ran them again today and the results are almost identical. The slower
sample rate still shows a lower ADEV. I am not sure what the algorithm to
determine the peak frequency is but it is described as a linear
interpolation between the two highest peaks on the FFT. I could see how
that could be subject to math errors. For the slower sample rate, the
frequency bins are smaller. I have not researched how all this could affect
the peak frequency and possibly introduce noise (or by truncation remove
small amounts of noise).
I will run it a few times in the next few days and will also try to move
the antennas to a location with lower multipath.
Thanks for the suggestions,
Mark
Post by Bob kb8tq
Hi
Looking at the data, I’m guessing the slower sample rate was done last. If so,
you may not be letting the GPSDO’s “warm up” long enough. Most designs
take days (if not weeks) to get to their ultimate stability.
Bob
Post by Mark Goldberg
I broke down and drilled a couple holes in the wall and moved the
antennas
Post by Mark Goldberg
for both GPSDOs outside with a better view of the sky, but still somewhat
shielded by the house and a metal RV cover. The number of sats is way up
and the TruePosition is in normal mode most of the time. The ADEV is not
improved and actually measured worse. I took data for twice as long at
two
Post by Mark Goldberg
sample rates, 0.6816 sec and 5.453 sec. The ADEV from the slower sample
rates is lower, possibly due to the measurement method, but they all are
in
Post by Mark Goldberg
the same order of magnitude. I've named the Heather screen captures for
the
Post by Mark Goldberg
inside and outside locations and shown one combined Timelab capture.
Regards,
Mark
<TruePos_Outside_Ant_Heather.png><TruePos_Inside_Outside_Compare_Ant_Allan.png><TruePos_Inside_Ant_Heather.png>_______________________________________________
Post by Mark Goldberg
To unsubscribe, go to
http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com
Post by Mark Goldberg
and follow the instructions there.
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Mark Goldberg
2018-12-03 16:21:19 UTC
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I have looked at it multiple times and so far cannot see a discrepancy. The
ration of the ADEVs for the two sample rates is very close to 2 for a wide
range of Tau in the 10-200 sec range. That can't be a coincidence. It's
between 2 and 3 for higher Tau but that may be due to temperature changes.

Basically, the Perseus SDR, clocked by my Bodnar GPSDO is sampling the 10
MHz from my TruePosition and using the DDC to down convert to 48 kHz. I
then do either a 32768 or 262144 point FFT, resulting in window times of
0.6816 or 5.453 sec and use a built in function in Spectrum Lab that as I
understand it interpolates the peak frequency from the highest two bins.
That calculated peak frequency is saved every 0.6816 or 5.453 seconds. That
data is fed to Timelab.

I could do the same size FFT for both and decimate the input by 1 and 8
respectively, which should result in the same FFT window time and check
that.

I'm scratching my head here.

Regards,

Mark
Post by Bob kb8tq
Hi
ADEV is pretty well documented and the results should *not* be dependent on the
sample rate. The proper approach to dropping the sample rate is decimation of the
data. I think you may want to look at the way you are doing ADEV.
Bob
Post by Mark Goldberg
I ran them again today and the results are almost identical. The slower
sample rate still shows a lower ADEV. I am not sure what the algorithm to
determine the peak frequency is but it is described as a linear
interpolation between the two highest peaks on the FFT. I could see how
that could be subject to math errors. For the slower sample rate, the
frequency bins are smaller. I have not researched how all this could
affect
Post by Mark Goldberg
the peak frequency and possibly introduce noise (or by truncation remove
small amounts of noise).
I will run it a few times in the next few days and will also try to move
the antennas to a location with lower multipath.
Thanks for the suggestions,
Mark
Post by Bob kb8tq
Hi
Looking at the data, I’m guessing the slower sample rate was done last.
If
Post by Mark Goldberg
Post by Bob kb8tq
so,
you may not be letting the GPSDO’s “warm up” long enough. Most designs
take days (if not weeks) to get to their ultimate stability.
Bob
Post by Mark Goldberg
I broke down and drilled a couple holes in the wall and moved the
antennas
Post by Mark Goldberg
for both GPSDOs outside with a better view of the sky, but still
somewhat
Post by Mark Goldberg
Post by Bob kb8tq
Post by Mark Goldberg
shielded by the house and a metal RV cover. The number of sats is way
up
Post by Mark Goldberg
Post by Bob kb8tq
Post by Mark Goldberg
and the TruePosition is in normal mode most of the time. The ADEV is
not
Post by Mark Goldberg
Post by Bob kb8tq
Post by Mark Goldberg
improved and actually measured worse. I took data for twice as long at
two
Post by Mark Goldberg
sample rates, 0.6816 sec and 5.453 sec. The ADEV from the slower sample
rates is lower, possibly due to the measurement method, but they all
are
Post by Mark Goldberg
Post by Bob kb8tq
in
Post by Mark Goldberg
the same order of magnitude. I've named the Heather screen captures for
the
Post by Mark Goldberg
inside and outside locations and shown one combined Timelab capture.
Regards,
Mark
<TruePos_Outside_Ant_Heather.png><TruePos_Inside_Outside_Compare_Ant_Allan.png><TruePos_Inside_Ant_Heather.png>_______________________________________________
Post by Mark Goldberg
Post by Bob kb8tq
Post by Mark Goldberg
To unsubscribe, go to
http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com
Post by Mark Goldberg
and follow the instructions there.
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Post by Mark Goldberg
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Bob kb8tq
2018-12-04 00:08:46 UTC
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Permalink
Hi

Let’s back up a bit:

The input to an ADEV calculation is a record of phase (time between wide spaced
edges). Often this is from a PPS output on a device. It could equally well be from
10 MHz edges spaced a count of 10,000,000 edges apart.

The first part of the formula takes the difference between adjacent samples.

The next step puts that difference into a standard deviation calculation.

If you are starting with something else, then you likely are not calculating ADEV.

Bob
Post by Mark Goldberg
I have looked at it multiple times and so far cannot see a discrepancy. The
ration of the ADEVs for the two sample rates is very close to 2 for a wide
range of Tau in the 10-200 sec range. That can't be a coincidence. It's
between 2 and 3 for higher Tau but that may be due to temperature changes.
Basically, the Perseus SDR, clocked by my Bodnar GPSDO is sampling the 10
MHz from my TruePosition and using the DDC to down convert to 48 kHz. I
then do either a 32768 or 262144 point FFT, resulting in window times of
0.6816 or 5.453 sec and use a built in function in Spectrum Lab that as I
understand it interpolates the peak frequency from the highest two bins.
That calculated peak frequency is saved every 0.6816 or 5.453 seconds. That
data is fed to Timelab.
I could do the same size FFT for both and decimate the input by 1 and 8
respectively, which should result in the same FFT window time and check
that.
I'm scratching my head here.
Regards,
Mark
Post by Bob kb8tq
Hi
ADEV is pretty well documented and the results should *not* be dependent on the
sample rate. The proper approach to dropping the sample rate is decimation of the
data. I think you may want to look at the way you are doing ADEV.
Bob
Post by Mark Goldberg
I ran them again today and the results are almost identical. The slower
sample rate still shows a lower ADEV. I am not sure what the algorithm to
determine the peak frequency is but it is described as a linear
interpolation between the two highest peaks on the FFT. I could see how
that could be subject to math errors. For the slower sample rate, the
frequency bins are smaller. I have not researched how all this could
affect
Post by Mark Goldberg
the peak frequency and possibly introduce noise (or by truncation remove
small amounts of noise).
I will run it a few times in the next few days and will also try to move
the antennas to a location with lower multipath.
Thanks for the suggestions,
Mark
Post by Bob kb8tq
Hi
Looking at the data, I’m guessing the slower sample rate was done last.
If
Post by Mark Goldberg
Post by Bob kb8tq
so,
you may not be letting the GPSDO’s “warm up” long enough. Most designs
take days (if not weeks) to get to their ultimate stability.
Bob
Post by Mark Goldberg
I broke down and drilled a couple holes in the wall and moved the
antennas
Post by Mark Goldberg
for both GPSDOs outside with a better view of the sky, but still
somewhat
Post by Mark Goldberg
Post by Bob kb8tq
Post by Mark Goldberg
shielded by the house and a metal RV cover. The number of sats is way
up
Post by Mark Goldberg
Post by Bob kb8tq
Post by Mark Goldberg
and the TruePosition is in normal mode most of the time. The ADEV is
not
Post by Mark Goldberg
Post by Bob kb8tq
Post by Mark Goldberg
improved and actually measured worse. I took data for twice as long at
two
Post by Mark Goldberg
sample rates, 0.6816 sec and 5.453 sec. The ADEV from the slower sample
rates is lower, possibly due to the measurement method, but they all
are
Post by Mark Goldberg
Post by Bob kb8tq
in
Post by Mark Goldberg
the same order of magnitude. I've named the Heather screen captures for
the
Post by Mark Goldberg
inside and outside locations and shown one combined Timelab capture.
Regards,
Mark
<TruePos_Outside_Ant_Heather.png><TruePos_Inside_Outside_Compare_Ant_Allan.png><TruePos_Inside_Ant_Heather.png>_______________________________________________
Post by Mark Goldberg
Post by Bob kb8tq
Post by Mark Goldberg
To unsubscribe, go to
http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com
Post by Mark Goldberg
and follow the instructions there.
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Post by Mark Goldberg
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Mark Goldberg
2018-12-04 01:58:58 UTC
Reply
Permalink
I am starting with what should be a frequency measurement at the sample
rate. Timelab can import that and should be able to determine ADEV from
that. I agree that something is not right though. I unfortunately am trying
to measure with what I already have. I don't have a TIC.

Regards,

Mark
Post by Bob kb8tq
Hi
The input to an ADEV calculation is a record of phase (time between wide spaced
edges). Often this is from a PPS output on a device. It could equally well be from
10 MHz edges spaced a count of 10,000,000 edges apart.
The first part of the formula takes the difference between adjacent samples.
The next step puts that difference into a standard deviation calculation.
If you are starting with something else, then you likely are not calculating ADEV.
Bob
Post by Mark Goldberg
I have looked at it multiple times and so far cannot see a discrepancy.
The
Post by Mark Goldberg
ration of the ADEVs for the two sample rates is very close to 2 for a
wide
Post by Mark Goldberg
range of Tau in the 10-200 sec range. That can't be a coincidence. It's
between 2 and 3 for higher Tau but that may be due to temperature
changes.
Post by Mark Goldberg
Basically, the Perseus SDR, clocked by my Bodnar GPSDO is sampling the 10
MHz from my TruePosition and using the DDC to down convert to 48 kHz. I
then do either a 32768 or 262144 point FFT, resulting in window times of
0.6816 or 5.453 sec and use a built in function in Spectrum Lab that as I
understand it interpolates the peak frequency from the highest two bins.
That calculated peak frequency is saved every 0.6816 or 5.453 seconds.
That
Post by Mark Goldberg
data is fed to Timelab.
I could do the same size FFT for both and decimate the input by 1 and 8
respectively, which should result in the same FFT window time and check
that.
I'm scratching my head here.
Regards,
Mark
Post by Bob kb8tq
Hi
ADEV is pretty well documented and the results should *not* be dependent on the
sample rate. The proper approach to dropping the sample rate is
decimation
Post by Mark Goldberg
Post by Bob kb8tq
of the
data. I think you may want to look at the way you are doing ADEV.
Bob
Post by Mark Goldberg
I ran them again today and the results are almost identical. The slower
sample rate still shows a lower ADEV. I am not sure what the algorithm
to
Post by Mark Goldberg
Post by Bob kb8tq
Post by Mark Goldberg
determine the peak frequency is but it is described as a linear
interpolation between the two highest peaks on the FFT. I could see
how
Post by Mark Goldberg
Post by Bob kb8tq
Post by Mark Goldberg
that could be subject to math errors. For the slower sample rate, the
frequency bins are smaller. I have not researched how all this could
affect
Post by Mark Goldberg
the peak frequency and possibly introduce noise (or by truncation
remove
Post by Mark Goldberg
Post by Bob kb8tq
Post by Mark Goldberg
small amounts of noise).
I will run it a few times in the next few days and will also try to
move
Post by Mark Goldberg
Post by Bob kb8tq
Post by Mark Goldberg
the antennas to a location with lower multipath.
Thanks for the suggestions,
Mark
Post by Bob kb8tq
Hi
Looking at the data, I’m guessing the slower sample rate was done
last.
Post by Mark Goldberg
Post by Bob kb8tq
If
Post by Mark Goldberg
Post by Bob kb8tq
so,
you may not be letting the GPSDO’s “warm up” long enough. Most designs
take days (if not weeks) to get to their ultimate stability.
Bob
Post by Mark Goldberg
I broke down and drilled a couple holes in the wall and moved the
antennas
Post by Mark Goldberg
for both GPSDOs outside with a better view of the sky, but still
somewhat
Post by Mark Goldberg
Post by Bob kb8tq
Post by Mark Goldberg
shielded by the house and a metal RV cover. The number of sats is way
up
Post by Mark Goldberg
Post by Bob kb8tq
Post by Mark Goldberg
and the TruePosition is in normal mode most of the time. The ADEV is
not
Post by Mark Goldberg
Post by Bob kb8tq
Post by Mark Goldberg
improved and actually measured worse. I took data for twice as long
at
Post by Mark Goldberg
Post by Bob kb8tq
Post by Mark Goldberg
Post by Bob kb8tq
two
Post by Mark Goldberg
sample rates, 0.6816 sec and 5.453 sec. The ADEV from the slower
sample
Post by Mark Goldberg
Post by Bob kb8tq
Post by Mark Goldberg
Post by Bob kb8tq
Post by Mark Goldberg
rates is lower, possibly due to the measurement method, but they all
are
Post by Mark Goldberg
Post by Bob kb8tq
in
Post by Mark Goldberg
the same order of magnitude. I've named the Heather screen captures
for
Post by Mark Goldberg
Post by Bob kb8tq
Post by Mark Goldberg
Post by Bob kb8tq
the
Post by Mark Goldberg
inside and outside locations and shown one combined Timelab capture.
Regards,
Mark
<TruePos_Outside_Ant_Heather.png><TruePos_Inside_Outside_Compare_Ant_Allan.png><TruePos_Inside_Ant_Heather.png>_______________________________________________
Post by Mark Goldberg
Post by Bob kb8tq
Post by Mark Goldberg
Post by Bob kb8tq
Post by Mark Goldberg
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Bob kb8tq
2018-12-04 02:20:12 UTC
Reply
Permalink
Hi

Well, what you are coming up with as a result simply is not correct. Properly
calculated ADEV does not do what you show it doing. TimeLab has been around
long enough to be considered “correct” in this regard. The normal input to an
ADEV calculation is phase rather than frequency.

Bob
Post by Mark Goldberg
I am starting with what should be a frequency measurement at the sample
rate. Timelab can import that and should be able to determine ADEV from
that. I agree that something is not right though. I unfortunately am trying
to measure with what I already have. I don't have a TIC.
Regards,
Mark
Post by Bob kb8tq
Hi
The input to an ADEV calculation is a record of phase (time between wide spaced
edges). Often this is from a PPS output on a device. It could equally well be from
10 MHz edges spaced a count of 10,000,000 edges apart.
The first part of the formula takes the difference between adjacent samples.
The next step puts that difference into a standard deviation calculation.
If you are starting with something else, then you likely are not calculating ADEV.
Bob
Post by Mark Goldberg
I have looked at it multiple times and so far cannot see a discrepancy.
The
Post by Mark Goldberg
ration of the ADEVs for the two sample rates is very close to 2 for a
wide
Post by Mark Goldberg
range of Tau in the 10-200 sec range. That can't be a coincidence. It's
between 2 and 3 for higher Tau but that may be due to temperature
changes.
Post by Mark Goldberg
Basically, the Perseus SDR, clocked by my Bodnar GPSDO is sampling the 10
MHz from my TruePosition and using the DDC to down convert to 48 kHz. I
then do either a 32768 or 262144 point FFT, resulting in window times of
0.6816 or 5.453 sec and use a built in function in Spectrum Lab that as I
understand it interpolates the peak frequency from the highest two bins.
That calculated peak frequency is saved every 0.6816 or 5.453 seconds.
That
Post by Mark Goldberg
data is fed to Timelab.
I could do the same size FFT for both and decimate the input by 1 and 8
respectively, which should result in the same FFT window time and check
that.
I'm scratching my head here.
Regards,
Mark
Post by Bob kb8tq
Hi
ADEV is pretty well documented and the results should *not* be dependent on the
sample rate. The proper approach to dropping the sample rate is
decimation
Post by Mark Goldberg
Post by Bob kb8tq
of the
data. I think you may want to look at the way you are doing ADEV.
Bob
Post by Mark Goldberg
I ran them again today and the results are almost identical. The slower
sample rate still shows a lower ADEV. I am not sure what the algorithm
to
Post by Mark Goldberg
Post by Bob kb8tq
Post by Mark Goldberg
determine the peak frequency is but it is described as a linear
interpolation between the two highest peaks on the FFT. I could see
how
Post by Mark Goldberg
Post by Bob kb8tq
Post by Mark Goldberg
that could be subject to math errors. For the slower sample rate, the
frequency bins are smaller. I have not researched how all this could
affect
Post by Mark Goldberg
the peak frequency and possibly introduce noise (or by truncation
remove
Post by Mark Goldberg
Post by Bob kb8tq
Post by Mark Goldberg
small amounts of noise).
I will run it a few times in the next few days and will also try to
move
Post by Mark Goldberg
Post by Bob kb8tq
Post by Mark Goldberg
the antennas to a location with lower multipath.
Thanks for the suggestions,
Mark
Post by Bob kb8tq
Hi
Looking at the data, I’m guessing the slower sample rate was done
last.
Post by Mark Goldberg
Post by Bob kb8tq
If
Post by Mark Goldberg
Post by Bob kb8tq
so,
you may not be letting the GPSDO’s “warm up” long enough. Most designs
take days (if not weeks) to get to their ultimate stability.
Bob
Post by Mark Goldberg
I broke down and drilled a couple holes in the wall and moved the
antennas
Post by Mark Goldberg
for both GPSDOs outside with a better view of the sky, but still
somewhat
Post by Mark Goldberg
Post by Bob kb8tq
Post by Mark Goldberg
shielded by the house and a metal RV cover. The number of sats is way
up
Post by Mark Goldberg
Post by Bob kb8tq
Post by Mark Goldberg
and the TruePosition is in normal mode most of the time. The ADEV is
not
Post by Mark Goldberg
Post by Bob kb8tq
Post by Mark Goldberg
improved and actually measured worse. I took data for twice as long
at
Post by Mark Goldberg
Post by Bob kb8tq
Post by Mark Goldberg
Post by Bob kb8tq
two
Post by Mark Goldberg
sample rates, 0.6816 sec and 5.453 sec. The ADEV from the slower
sample
Post by Mark Goldberg
Post by Bob kb8tq
Post by Mark Goldberg
Post by Bob kb8tq
Post by Mark Goldberg
rates is lower, possibly due to the measurement method, but they all
are
Post by Mark Goldberg
Post by Bob kb8tq
in
Post by Mark Goldberg
the same order of magnitude. I've named the Heather screen captures
for
Post by Mark Goldberg
Post by Bob kb8tq
Post by Mark Goldberg
Post by Bob kb8tq
the
Post by Mark Goldberg
inside and outside locations and shown one combined Timelab capture.
Regards,
Mark
<TruePos_Outside_Ant_Heather.png><TruePos_Inside_Outside_Compare_Ant_Allan.png><TruePos_Inside_Ant_Heather.png>_______________________________________________
Post by Mark Goldberg
Post by Bob kb8tq
Post by Mark Goldberg
Post by Bob kb8tq
Post by Mark Goldberg
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Post by Mark Goldberg
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MLewis
2018-12-03 14:02:43 UTC
Reply
Permalink
... and will also try to move
the antennas to a location with lower multipath.
Thanks for the suggestions,
Mark
Mark,

Antenna position can be odd.

Shortly after you first posted "Trueposition Antenna Location", as I'd
noticed that my skyview's 'transformer hole' had filled in after moving
the antenna 4" to the left on the window sill, I restarted LH to get a
fresh 'map' of my skyview raw signal level.

I've got a M8T-10 running with GPS, GLO & GAL. The antenna is on a ~nine
foot long window sill, left of centre on its wide centre pane. The
building, window & antenna are facing 30 degrees east of south.

- The upper image is LH from Nov 10, 2018. Antenna is ~six inches left
of centre on the wide centre pane.

The hole in reception centred 10 degrees north of east is due to the
small overhanging roof over the building entrance.
The hole centred 7 degrees west of south is a transformer on a pole.
The ~5 to ~10 degree gap at the horizon is due to buildings.
This skyview is typical of my location for my M8T-01 and M8T-10.
With GPS, GLO & GAL, the M8T-10 was averaging 4 satellites, max 9, min
0, regularly below 2, often 1, and a too high number of times it hit zero.
(The M8T-01 with GPS & GLO, would regularly hit zero a number of times a
day)

- The lower image is LH from Dec 3, 2018. Antenna was moved 4" left
after Nov 10th. For this skyview, it's ~ten inches left of centre on the
wide centre pane.

The signal strength in the best part of the skyview has increased.
The entrance roof hole remains.
The transformer on a pole now appears to be transparent...
The M8T-10 is averaging 7 satellites, max 13, min 0, rarely below 3, hit
1 a number of times, hit zero once.

For what it's worth,

Michael
Mark Sims
2018-11-30 18:39:35 UTC
Reply
Permalink
I have just completed a comparison test of 17 different GPSDOs (I hope to get the results posted soon). All the UCCM receivers (Trimble / Symmetricom / Samsung) that I tested were among the best performers. ADEVs at 10,000 seconds were in the low E-13 range. Maybe your Trimble has issues.

------------------
Post by Attila Kinali
Do not get one of the Trimble UCCM, they have quite horrible
frequency jumps.
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Mark Sims
2018-11-30 21:58:29 UTC
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Not all the "holdovers" on the Trueposition are due to signal issues. Many seem to be related to it tuning itself up after a power cycle and they improve or go away after a couple of days of running.

But your antenna signal level map is rather awful... lots of red. It could be antenna, cable, or location of the antenna. My antenna is in a horrible location, but still works well.
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Mark Goldberg
2018-12-01 01:57:30 UTC
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Yes, it seems that it switches to holdover or waiting if a new sat comes
into view or one drops off. It did not work with the antenna that the
Bodnar worked fine with. I am using a Motorola PCTEL 8508851k66 antenna off
eBay. No idea how good or bad it is. It did work better facing west with a
clear view of the sky even with an extra 30 ft piece of 1/4 hardline and
some adapters added.

It also does not like it if I transmit with my HF radio.

I've attached an older image of the Lady Heather screen with the antenna in
a much better location on the west side of our house. It was in normal mode
most of the time. Lots more sats and much higher signal strength.

Any suggested antenna that might be better?

Regards,

Mark
Post by Mark Sims
Not all the "holdovers" on the Trueposition are due to signal issues.
Many seem to be related to it tuning itself up after a power cycle and
they improve or go away after a couple of days of running.
But your antenna signal level map is rather awful... lots of red. It
could be antenna, cable, or location of the antenna. My antenna is in a
horrible location, but still works well.
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Mark Sims
2018-12-03 15:56:13 UTC
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There is something seriously wrong with your antenna/cable... With an M8T and GPS/GLO/GAL I typically track 20-28 satellites. With a crappy indoor antenna I can track 10-15 sats.

------------------
Post by MLewis
With GPS, GLO & GAL, the M8T-10 was averaging 4 satellites, max 9, min
0, regularly below 2, often 1, and a too high number of times it hit zero.
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MLewis
2018-12-03 18:10:18 UTC
Reply
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Post by Mark Sims
There is something seriously wrong with your antenna/cable... With an M8T and GPS/GLO/GAL I typically track 20-28 satellites. With a crappy indoor antenna I can track 10-15 sats.
------------------
Post by MLewis
With GPS, GLO & GAL, the M8T-10 was averaging 4 satellites, max 9, min
0, regularly below 2, often 1, and a too high number of times it hit zero.
Mark,

My bad.

I forgot to say that the count I was reporting is the green sats in LH.
Those sats with a strong enough signal and the ublox tags them as
available-for (used in?) the 'solution'. I forget the name of the setting.

With that and my skyview, the difference in your count vs. my current
counts makes sense to me.

I have less than half of a sky view.
- Less than a 180 sweep of azimuth (building, window frame).
- Less than 90 of elevation (window frame, buildings up from horizon
across from the antenna).
- Plus the portion of that blocked by the roof over the entrance and its
supporting poles.
Plus tons of multipath from multiple buildings, electrical cables, metal
poles & cars. (the road in the attached is actually a parking lot, cars
nose in on each side with two lanes down the middle)

Three layers of congregated steel above me, one for each floor/story,
each held a layer's worth of concrete while it originally cured. Plus
steel mesh in the walls. The view between those layers to the 180
degrees to the 'North' are blocked by buildings at the same or different
height with the same construction; so the distant view out from between
those layers is largely blocked.

BUT, when the M8T was sitting on its board on the table, in 3D position
mode it was reporting a somewhat wide 3D range. The M8T-10 stayed within
200 m of me and largely on a "track" (while a few years ago the M8T-01
wandered as far as 2 km between S and SE). When the board was put in an
insulated aluminum bottle (fairly thick wall), the bulk of that
wandering ended, and its intersection with the building at the upper
left red tag is correct. In the bottle it did still go for a walk during
testing. See attached.
In or out of the bottle, the reported altitude varied from 29 m to 69 m;
actual is 80.032 m. This was before the 4" antenna
re-position/cable-jostling. I haven't checked it on positioning mode
since then.

Seeing how much it improved being moved 4" along the sill, from
Post by Mark Sims
averaging 4 satellites, max 9, min 0, regularly below 2, often 1, and
a too high number of times it hit zero.
to
Post by Mark Sims
averaging 7 satellites, max 13, min 0, rarely below 3, hit 1 a number
of times, hit zero once.
the difference could very well be the cable getting jostled. But that
now transparent transformer is a neat trick.

Right now it sees/tracks twelve sats (reports a signal strength). Six of
those are green. While writing this, the green have counted between four
and seven. In the past 12 hours, green available/used: max 10, min 3.

Does that make it add up better?

Michael
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