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

Magnus Danielson magnus at rubidium.dyndns.org
Sun Jul 7 22:30:37 UTC 2013


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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