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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