Re: [time-nuts] GPSDO Sigma Tau at large observation times

[Tom Van Baak]

>> See also the examples here: http://leapsecond.com/pages/gpsdo/
> 
> Hi Tom, quick question: I've seen these plots before and they are very
> useful to know what to aim at for GPSDO performance. Am I right in
> thinking these were measured against a master - the page says " a very
> stable 10 MHz reference. " without more details (unless I missed it)

Hi Tim,

Yes, I think so. I can check, but if I say something like "a very stable 
reference" it usually means I picked something sufficiently accurate for the 
job. As a rule-of-thumb you want both your time/frequency comparator instrument 
and your time/frequency reference to be 10x better than the device you are 
testing. So to measure a typical GPSDO you want at least a rubidium, if not 
cesium or H-maser. But the key point is to always be aware of the limitations 
of your own test setup as they may affect the look of the ADEV plot. Using 
multiple comparators and references with N-corner hat techniques is often a 
useful trick.

If you want to get picky there are other factors hidden behind every ADEV plot. 
Often unstated is the bandwidth of the measurement and also the type of Allan 
statistic being used (ADEV, MDEV, HDEV, etc.). Temperature and airflow too: an 
ADEV plot may look different if it was created from data in a +/-0.1C lab vs. a 
+/-10 C lab. And with GPSDO, an ADEV plot can also be affected by the quality 
of your antenna, the quality of your sky-view, and especially, your latitude. 
So all these factors go into what the ADEV plot looks like.

BTW, one of my greatest surprises was the effect of still air on a plain TCXO. 
We call it "still", but it is nothing but stationary! The plots and two photos 
shows the massive difference a single sheet of TP makes: 
http://leapsecond.com/pages/LTE-Lite/


But back to your question. There's a good chance I used a TSC 5120 analyzer and 
a CH1-76 passive H-maser for all those GPSDO plots. This means all the data 
beyond about tau 1 s is fully trustable. The data nearer tau 0.1 s is likely 
distorted by a known short-term noise issue in that particular surplus maser.


And this brings up another point. Both Allan deviation (ADEV) and phase noise 
(PN) plots tend to cover a wide dynamic range. So in general no one reference 
is perfect both short-term and long-term, both close-in and far-out. For 
example, the typical cesium standard is good for long-term but not so good for 
short-term (a free-running high-quality quartz can be better). And my best ULN 
(ultra low noise) quartz standards have superb phase noise, but lousy ADEV. A 
good quartz will blow away GPS short- and mid-term; but GPS wins in the 
long-term. For short-term a free-running GPSDO is always better than a locked 
GPSDO. And so on.

All this is just a reminder that you constantly have to double check your 
assumptions. An ADEV or PN plot is merely the sum of all the noise in the 
"system": the DUT, the instrument, the REF noise, but also the room, weather, 
cables, wife/kids/pets, power lines, FM stations, etc. And that sum changes as 
you slide from the left to the right of the plots.

/tvb

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Re: [time-nuts] GPSDO Sigma Tau at large observation times

[Tim Lister]

On Wed, Oct 5, 2016 at 11:01 AM, Tom Van Baak  wrote:
> Hello again Estanislao,
>
> Nice set of questions. The short answer is that for a GPSDO,
> - The left side of the ADEV plot will show the difference between the quality 
> of the local oscillator, in your case TCXO vs. OCXO vs. Rb.
> - The middle of the ADEV plot will show the difference in the choice of 
> time-constant or the environmental conditions during the test.
> - The right side of the ADEV will show the quality of the GPS timing receiver.
>
> Also,
> - The left side of the ADEV plot may expose limitations in your measurement 
> equipment or the local low-noise reference that you're using.
> - The right side of the ADEV plot may expose limitations in your local 
> frequency reference. A cesium or H-maser helps here. If you use one GPSDO to 
> test another GPSDO the very right side of the ADEV plot is misleading.
>
> See also the examples here: http://leapsecond.com/pages/gpsdo/

Hi Tom, quick question: I've seen these plots before and they are very
useful to know what to aim at for GPSDO performance. Am I right in
thinking these were measured against a master - the page says " a very
stable 10 MHz reference. " without more details (unless I missed it)

>
> /tvb
>

Cheers,
Tim
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Re: [time-nuts] GPSDO Sigma Tau at large observation times

[Attila Kinali]

On Wed, 5 Oct 2016 11:01:36 -0700
"Tom Van Baak"  wrote:

> Nice set of questions. The short answer is that for a GPSDO,
> - The left side of the ADEV plot will show the difference between the quality 
> of the local oscillator, in your case TCXO vs. OCXO vs. Rb. 
> - The middle of the ADEV plot will show the difference in the choice of 
> time-constant or the environmental conditions during the test.
> - The right side of the ADEV will show the quality of the GPS timing receiver.
> 
> Also,
> - The left side of the ADEV plot may expose limitations in your measurement 
> equipment or the local low-noise reference that you're using.
> - The right side of the ADEV plot may expose limitations in your local 
> frequency reference. A cesium or H-maser helps here. If you use one GPSDO to 
> test another GPSDO the very right side of the ADEV plot is misleading.

Additional to this, I recommend having a look at Enrico's Chart[1].
It lables all the slopes in the different forms we measure stability
and what noise type they represent, and also their relationship.

Attila Kinali


[1] http://rubiola.org/pdf-static/Enrico's-chart-EFTS.pdf


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Re: [time-nuts] GPSDO Sigma Tau at large observation times

[Tom Van Baak]

Hello again Estanislao,

Nice set of questions. The short answer is that for a GPSDO,
- The left side of the ADEV plot will show the difference between the quality 
of the local oscillator, in your case TCXO vs. OCXO vs. Rb. 
- The middle of the ADEV plot will show the difference in the choice of 
time-constant or the environmental conditions during the test.
- The right side of the ADEV will show the quality of the GPS timing receiver.

Also,
- The left side of the ADEV plot may expose limitations in your measurement 
equipment or the local low-noise reference that you're using.
- The right side of the ADEV plot may expose limitations in your local 
frequency reference. A cesium or H-maser helps here. If you use one GPSDO to 
test another GPSDO the very right side of the ADEV plot is misleading.

See also the examples here: http://leapsecond.com/pages/gpsdo/

Now, for your specific question:

> My question is, 
> when you are in the second regime, you are basically measuring the 
> ability of each clock to follow each other (is like there is just one 
> clock!, with the worst one setting the Adev value),

Right. For long tau, a GPSDO is not so much an independent clock as just a 
time-transfer system. As such, ADEV is not an ideal or fair statistic. For 
long-term performance consider using TDEV instead or just the 1- or 2-sigma RMS 
of the phase (time) error. This will better reveal how well the PLL is working.

> does then the 
> sigma-tau becomes meaningless for GPSDOs at time scales larger than the 
> TC of the oscillator being locked to GPS?

Yes and no.

Yes -- it is often meaningless because a GPSDO is really just a PLL. ADEV was 
sort of intended to compare clocks. In the long-term the phase error of a PLL 
is bounded by some rms value and so ever longer tau doesn't change how the 
GPSDO works, but ADEV will keep showing a line dropping like a rock at -1 
slope. So ADEV is kind of misleading at this point. Consider using TDEV instead 
of ADEV. Also make sure your frequency reference is better than your GPS 
receiver for the tau you plot. Use low-noise quartz for short-term; use Cesium 
or H-maser for long-term.

No -- it is still meaningful because the ADEV you see out at 10^4 or 10^5 or 
10^6 becomes very much a function of the quality of your GPS timing receiver 
(and antenna and environment, etc). If you were to expand your experiments to 
use half a dozen different GPS timing receivers you would observe a significant 
difference in rms timing noise, all of which translates to the actual ADEV 
value as it crosses, say, tau 10^5 and 10^6 s.

/tvb

- Original Message - 
From: "Estanislao Aguayo" <eagu...@thinksrs.com>
To: "Discussion of precise time and frequency measurement" <time-nuts@febo.com>
Sent: Wednesday, October 05, 2016 8:57 AM
Subject: [time-nuts] GPSDO Sigma Tau at large observation times


> Hello Time-Nuts,
> 
> My name is Tani Aguayo, I'm a newbie in this exciting precision timing 
> world, and so I have lots of questions. So here I go, first, let me 
> describe my experimental set-up for you.  I've been measuring the Adev 
> of several GPSDOs against each other, and once they are locked with a 
> suitable TC, the Adev plots show two regimes of operation (See 
> attached), the first one, at short time scales, has a lot to do with the 
> oscillator being disciplined characteristics, temperature, pressure.. 
> and there is a second one, where the Adev just drops linearly (in a 
> log-log scale). Each oscillator gets to the second regime earlier or 
> later, depending on the GPS PLL TC. In  the attached plot, the TCs of 
> the units compared, have been carefully selected depending on the 
> individual oscillator behavior. My thinking was that once in this second 
> regime, every unit is following the same clock, with the jitter of the 
> worst clock being compared, and since the sigma-tau makes no sense to me 
> for a single clock, I need some help with this concept. My question is, 
> when you are in the second regime, you are basically measuring the 
> ability of each clock to follow each other (is like there is just one 
> clock!, with the worst one setting the Adev value), does then the 
> sigma-tau becomes meaningless for GPSDOs at time scales larger than the 
> TC of the oscillator being locked to GPS?
> 
> I just wanted to thank the active members of this list, I feel like I'm 
> learning a lot with the different discussions and I wanted to say thank you.
> 
> - Tani Aguayo
> 
> 
> 
>





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