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 < [email protected]> 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 > _______________________________________________ > time-nuts mailing list -- [email protected] > To unsubscribe, go to > https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > and follow the instructions there. > _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
