[time-nuts] Changing ADEV, (was Phase, One edge or two?)
Bob Camp
kb8tq at n1k.org
Fri Oct 24 22:33:11 UTC 2014
Hi
Grab an OCXO that has been powered off for a long time.
Turn it on and start plotting ADEV. Do it from about 10 minutes after turn on. Run 15 to 30 minute tests every so often for the first few hours.
Come back the next day and run the same series for a few hours. Repeat a week later, and a month later.
Curve fit out the drift and run the ADEV numbers out to < 100 seconds tau. That’s true even if you compare the best of each batch. It really is getting better.
Do that on enough oscillators and you will indeed find many that do get better (like 2X better for some, 10 or 20% for others) on ADEV after they have been on a while.
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Run an OCXO and watch the ADEV on the Time Pod. Look at enough of them and you will find some that drop ADEV for a while (say 10 minutes or so) and then climb by a bit (say 1.5:1). Hmmm, what’s going on? Look at the phase plot and there’s an abrupt shift in phase over some period ( which depends on the cause, there’s more than one possibility). Let’s say it’s 10 seconds. The whole ADEV plot climbs, not just the part for > 10 second tau. Why - there’s energy there both at short and long tau.
——————————————
Look at a GPSDO / disciplined oscillator / temperature compensated Rb. Let it run for a good long time. If it’s got a loop that steps out to *long* time constants, it may only bump the frequency once every 15 minutes or longer. Plot the ADEV over the time segment when it steps and compare it to the time period it does not step. Short tau ADEV is worse at the step.
————————————————
Look at a very normal OCXO on your TimePod. After 100 seconds, the 1 second ADEV *should* be pretty well determined. After 1000 seconds it should be *very* well determined. Flip on the error bars if you want an idea of how good it should be.
Watch for a while, Does it move outside the error bars? Hmmmm ….. It’s not the error bars that are the problem. The math is correct. The statistics are what is the issue. The ADEV hast changed for the worse as the run has gone on. It’s a very common thing.
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Those are just the first few off the top of my head.
Bob
> On Oct 24, 2014, at 5:31 PM, Tom Van Baak <tvb at LeapSecond.com> wrote:
>
>>> ADEV most certainly does change with time, even for short tau's.
>>
>> Can you elaborate?
>> Such as when, why, what kind of change, how much change,
>> at how short of tau's, over how long of time,
>> and using what type Oscillators?
>> Do you know what in the freq or Phase plot is causing the ADEV to change?
>
> I'm happy to let Bob answer his own claim here. I'm curious as well. Unless he's talking about thermal noise, in which case I now believe him 100%.
>
> OTOH, for time intervals of minutes to hours or days, the plotted ADEV can often vary. When in doubt, enable error bars in your ADEV calculations or use DAVAR in Stable32, or use "Trace History" of TmeLab to expose how little or much the computed ADEV depends on tau and N.
>
> In general, never do an ADEV calculation without visually checking the phase or frequency time series first.
>
>> Of the many OCXO type Oscillators that I've tested (HP10811 & MV89),
>> seldom have I seen any significant change (say greater than 10%),
>> in the short tau (0.01 sec to 1 sec) ADEV values, after the systematic
>> type errors are removed. (even when starting soon after turn on)
>
> This is not my experience at all. Let's figure out what's happening to you.
>
> If all your standards look sort the same from tau 0.01 to tau 0.1 to tau 1 then either you need more oscillators to play with or maybe you have a measurement problem. This is especially true if you are doing post-comparator averaging. Averaging, by definition, tends to remove noise, to smooth things out. If your goal is to measure noise, the last thing you want to do is create any electronics or use any analog or digital or numerical filtering that removes or reduces the very thing you're trying to measure.
>
> I remind you of this page http://leapsecond.com/pages/adev-avg/ of the perils of averaging data.
>
> For most of the world, there's signal and noise. Signal good. Noise bad. But for us, measuring precision clocks, the noise is the signal. So don't do anything that removes or reduces noise.
>
>> ADEV is used to measure random types of noise so there are of
>> course the statistical uncertainty variations that are a function of
>> the number of valid data points. I find that using a minimum of
>> a thousand points at each tau gives good consistent results.
>
> Are you crazy? The minimum is just 3 or 4 or 5 data points. Not 1000! You should not see much difference at 10 or 100 or 1000 points. If so, something is wrong with your measurement model. If ADEV(tau) is *that* dependent on tau, check the frequency time-series. Consider removing drift or using HDEV instead of ADEV. We need to talk. If your logic was true, we'd all have to wait 3 years before we could compute the ADEV of a GPSDO at tau 1 day.
>
> /tvb
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