[time-nuts] Allan variance by sine-wave fitting

[Tim Shoppa]

I wonder how much a fitting approach is affected by distortion (especially
harmonic content) in the waveform.

Of course we can always filter the waveform to make it more sinusoidal but
then we are adding L's and C's and their tempcos to the measurement for
sure destroying any femtosecond claims.

Tim N3QE

On Wed, Nov 22, 2017 at 5:57 PM, Ralph Devoe <rgdevoe at gmail.com> wrote:

> Hi,
>        The fitting routine only takes up 40 uS of the 1 sec interval
> between measurements, as shown in Fig. 1 of the paper. This is less than
> 10(-4) of the measurement interval. It just determines the phase difference
> at the start of every second. I don't think the filtering effect is very
> large in this case.
>         The interesting thing is that good results are achievable with such
> a short fitting interval. One way to think of it is to treat the fitting
> routine as a statistically optimized averaging process. Fitting 40 uS, that
> is 4096 points at 10 ns/point,  should reduce the noise by a factor of 64
> (roughly). The single shot timing resolution of the ADC is about 10 pS (see
> Fig. 4), so dividing this by 64 brings you down into the 100's of fs range,
> which is what you see.
>
> Ralph
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