[time-nuts] Homebrew frequency counter, need help

Tom Van Baak tvb at LeapSecond.com
Fri Dec 12 14:20:09 EST 2014


Li,

> Since I really want to reduce the noise, what is the best test set you suggest?
...
> How to analyze the ADEV plot to get information about noise?

Note these two statements are at odds. On the one hand we all want to reduce noise. But ADEV is all about measuring noise, not making it go away. Think frequency instability (not frequency stability). On a log-log ADEV plot the top has more instability and the bottom has less instability.

> The uncertainty of slope (the frequency ratio of ref and signal ) is contributed by these 2 measurements.
> With linear regression of all 9000 data within one second, the uncertainty will reduced to smaller one.

Correct, for frequency measurements, the finer you can measure the slope the better. That's why linear regression is good for a continuous rolling average frequency counter.

But for instability measurements, it's not the slope but the small deviations around the slope that matter. In this case linear regression is not helpful. Instead just take the raw phase data, noise and all, and the ADEV/MDEV calculation takes care of all the rest.

/tvb



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