[time-nuts] Changing ADEV, (was Phase, One edge or two?)
Tom Van Baak
tvb at LeapSecond.com
Fri Oct 24 21:31:42 UTC 2014
>>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.
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