[time-nuts] Determining Allan Deviation From Interpolated Peak Frequency Readings

[Attila Kinali]

Hey Mark

On Wed, 6 Dec 2017 15:43:49 -0700
Mark Goldberg <marklgoldberg at gmail.com> wrote:

> https://sites.google.com/site/perseusmods/
> and
> https://sites.google.com/site/spectrumlabtesting/
> 
> using wide FFT bins and Spectrum Lab's peak frequency interpolation
> function. I would appreciate comments as to the effectiveness of this
> approach. I have a thick skin, so any criticism is welcome if it improves
> the process.

The approach using FFT works, but just using the peak frequency, you throw
away half of the data (the phase) and also limit yourself in precision
to the bin width. It's not 100% clear that estimating the frequency
using an FFT is unbiased in this case, thus you might get worse (or better)
results than what the oscillator actually does.

What you are trying to do is spectral estimation from a limited number of
samples. You want to have some kind of continuity, that might allow you to
track minute changes from block you are processing to the next block.
The easiest way to do this would be to downconvert the signal on the PC
to zero Hz and take the phase information (simplest way: use a NCO as a 
reference, then pass the reference and signal into a CORDIC to get the phase
difference). Recording this phase difference should give you a lower floor
for *DEV than your FFT method. It will also alow you to track small phase
changes (aka small frequency fluctuations) that happen over long periods.
Sherman and Jördens[1] describe the approach in more detail.

Other than that, the general approach looks ok.


			Attila Kinali


[1] "Oscillator metrology with software defined radio",
by Sherman and Jördens, 2016
https://arxiv.org/abs/1605.03505
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