On 07/07/2013 11:38 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.
I had a measurement with a sine being overlaid, and just for fun I wrote
a small pre-processing program that put a pair of zeros close to the
unity circle and about the right frequency. The end result was very
clean and the unwanted artifact was removed. Care in ensuring unity gain
was needed, but once that was done it worked like a charm.
This "trick" is a bit dirty, but keeping the Q high on the zeros makes
sure that the other noise is not affected gravely, and the ripples of
the sine was cleaned out.
The benefit of doing an equalizer to notch it out compared to trying to
measure the amplitude and phase of a sine and then subtract the
estimated sine is what the notching method will be relatively
insensitive to amplitude, phase and frequency errors that will limit the
usefulness of the perfect matching. Any slow shifts will also be fairly
ignored. I used a very rough period estimation to tune it.
Cheers,
Magnus
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