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

Magnus Danielson magnus at rubidium.dyndns.org
Sun Jul 7 17:00:49 EDT 2013


Bob,

On 07/07/2013 09:27 PM, Robert Darby wrote:
> This is a question that has probably been addressed on the list and in
> various texts but I've been unable to find an answer that I can fathom
> so here's a request for some info about the behavior of TimeLab (and
> probably all other similar programs).
>
> I have been trying to find the source of some periodic noise that
> appears when using a 5370B to measure an FTS 1050B against a 5065B. The
> noise manifests itself varying from 12s to 20s in a repetitive fashion.
> I asked C. Dawson about this and one of his suggestions was to try a
> longer sampling period. Down the rabbit hole I went!
>
> I ran four trials in succession at sample intervals of .07s, .25s, .5s,
> and 1s. The result is as if the adev ,modified, and Hadamard curves have
> been slid down and to the right along the noise floor of the 5370.
>
> 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?

TimeLab will use the sampling interval you gave it or it learned during 
monitoring and scale results accordingly if properly given.

If you have systematic noise, try loading it into a FFT rather than 
doing an ADEV. The trouble is that a sine modulation will show up as 
multiple bumps on the ADEV but a single spike on FFT. If you have 
multiple signals, it becomes easier to identify in the FFT.

Cheers,
Magnus


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