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

John Miles john at miles.io
Sun Jul 7 17:38:09 EDT 2013


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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