[time-nuts] Allen deviation? Yes indeed that subject again

paul swed paulswedb at gmail.com
Sat Jan 9 09:40:51 EST 2016


Magnus
Thanks what you are saying and what I see in the NIST paper 2028 agree.
Thats why I decided to ask.
I am not using timelab as I mentioned. Instead a HP3575 Gain and phase
meter with a windaq charting a/d and program. The reference a HP5071 in
very good standing.

I guess in the case of the cleaning lady at NIST, it really was a random
walk.
Regards
Paul
WB8TSL

On Sat, Jan 9, 2016 at 12:11 AM, Magnus Danielson <
magnus at rubidium.dyndns.org> wrote:

> Paul,
>
> On 01/09/2016 03:05 AM, paul swed wrote:
>
>> I have been looking at the subject of Allen deviation and this has been
>> discussed numbers of time on Time-nuts.
>> But my question is this.
>>
>> If an oscillator is stable in frequency but shifts phase 90 degrees and
>> then comes back in a short time. From my reading I don't think that will
>> show up in a typical Allen deviation plot that runs 1000s of readings at 1
>> second intervals. Typical HP5370 test setup. I think this capture approach
>> will miss the issue.
>>
>> How often? Correct phase .5-3 hours, shift 5-10 minutes it seems random
>> actually and the duration varies.
>> Am I looking at this correctly?
>>
>
> No.
>
> OK, I might be a bit more specific. :)
>
> We assume that it does it's phase-shift dance in perfect symmetry, then it
> is not as obvious that it will be observable, as if it didn't get exactly
> back we would naturally be able to observe that difference.
>
> The condition for you not to observe it is really that the occurrence of
> these evens is synchronous to your measuring rate, and that these evens
> occur phase-wise in-between the two phase samples (as then we don't have to
> assume much more about the signal). For this case, it is obvious that you
> will always miss it. However, here we have the synchronization condition,
> which is obscure.
>
> If we don't have the sampling rate and occurrence being synchronous,
> it will be visible... but hard to notice. The power averaging would shift
> only so slightly that it would be hard to detect and it would disappear in
> the noise. If random as mechanism, it would only appear as the increase of
> the noise level. This is the case where ADEV isn't is necessarily your
> preferred tool, and is not intended to be your tool. It *might* appear as
> random-walk frequency noise, but it takes a little more analysis to
> conclude that.
>
> Story-time: At NIST they saw how one of their cesium standards started to
> show unexpected random-walk frequency noise. This is exceptional as there
> is no real random-walk frequency noise source in clocks. They discovered
> this because they actually looked at their data. Turned out that the
> cleaning lady had to move the standard over the floor whenever she came in
> to clean up, and then she moved it back. The vibration from dragging it
> across the floor caused the modulation. Some re-arrangement in the lab she
> didn't have to move things around and the random-walk noise got back to
> normal.
>
> So, look at the random walk noise, but do look at the phase-plot instead,
> especially the linear or quadratic fit residue plot of the phase. The
> normal frequency offset and even slow drift might obscure these deviations
> from being visible in the plot, only due to dynamic range. Remove the
> systematic shifts and you can see the fine-grain details. For TimeLab,
> press r for viewing the linear residue while viewing phase or frequency
> plot.
>
> So, this is why I say you are not looking at it the right way.
>
> Cheers,
> Magnus
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