[time-nuts] Question about effect of sample interval on ADEV

[Robert Darby]

Thanks to all who responded.  I didn't phrase my question very well I'm 
afraid.  The periodic noise/beat note issue is what lead me to try a 
different sample interval. Previously I had always let TimeLab set the 
sample interval (about .07s for the5370B).

I was surprised by the difference in the adev traces when I varied the 
sample interval from .07s to .25s to .5s to 1.0s and that is what I 
hoped someone could explain.

When I edit a trace and change the sample interval there is not a 
substantial change to the Tau / Sigma (Tau) values yet when I actually 
run at the different sample intervals I get minimum values for each run 
of  400s 5.60e-13, 600s 1.70e-13, 1000s 9.55e-14, 3000s 4.15e-14.  The 
traces are totally different; same oscillators and counter, just 
different sample intervals.  That's what I hoping one of you could explain.

John, I'll explore the the beat note issue to see if that's the problem.

Tom, since I get a similar issue when I swap the FTS for an Austron 
(more battery chargers) I don't think the FTS is the culprit.

Thanks again.

Bob


On 7/7/2013 5:38 PM, John Miles wrote:
> This won't be a sampling-interval issue.  It sounds like a beat note.   To
> diagnose it, you can use the 5370B in frequency mode (with its internal
> timebase) to measure the frequency of the 5065B and the FTS 1050B.  Subtract
> the two readings, then see if the reciprocal of the frequency difference
> corresponds to the location and spacing of the periodic ADEV bumps.  If so,
> that's likely to be the explanation, and you can confirm it by tweaking the
> FTS 1050B's frequency and seeing if the beatnote moves accordingly.
>
> As far as getting rid of the artifact is concerned, it may help to use
> double-shielded cables, although I don't know if the isolation between the
> START and STOP inputs on the 5370B is good enough to eliminate the
> possibility of beatnotes in a TI measurement with HF signals on both jacks.
> If you are feeding the 5/10 MHz inputs to both START and STOP inputs, try
> using a 1-pps divider on the START source.
>
> You could also try using the 5370B in frequency-count mode, with the 5065A
> as an external reference and the FTS 1050B at the STOP input.   There will
> be a reduction in ADEV fidelity due to the dead time but it will probably be
> less objectionable than the beatnote ripple.
>
> -- john, KE5FX
> Miles Design LLC
>   
>>> My assumption, apparently incorrect, was that the software would take
>>> the sampling interval into account so that I would get essentially the
>>> same plot. When I edit a plot changing the sample interval, the trace
>>> remains essentially unchanged and this seems inconsistent with the
>>> results noted above. Can anyone explain in relatively simple terms what
>>> I'm missing?
>>>
>>> Thanks,
>>> Bob Darby
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