[time-nuts] Trueposition Antenna Location

Discussion:

Mark Goldberg

2018-11-30 04:22:44 UTC

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

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

Permalink

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

Permalink

Post by Attila Kinali
On Thu, 29 Nov 2018 21:22:44 -0700

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

Permalink

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

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
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Mark Goldberg

2018-12-03 05:30:51 UTC

Permalink

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

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

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Post by Mark Goldberg
and follow the instructions there.

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Bob kb8tq

2018-12-03 13:26:46 UTC

Permalink

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 Mark Goldberg
I broke down and drilled a couple holes in the wall and moved the

antennas

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
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

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Post by Mark Goldberg
and follow the instructions there.

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Mark Goldberg

2018-12-03 16:21:19 UTC

Permalink

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

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.