[time-nuts] Zero dead time and average frequency estimation
Bruce Griffiths
bruce.griffiths at xtra.co.nz
Mon Feb 1 12:16:49 UTC 2010
Magnus
Magnus Danielson wrote:
> Tom,
>
> Tom Van Baak wrote:
>> Magnus,
>>
>> Correct, all the terms cancel between the end points. Note
>> that this is exactly equivalent to the way a traditional gated
>> frequency counter works -- you open the gate, wait some
>> sample period (maybe 1, 10, or 100 seconds) and then
>> close the gate. In this scenario it's clear that all the phase
>> information during the interval is ignored; the only points
>> that matter are the start and the stop.
>
> There is a technical merit to take samples in between even if they
> cancel... you avoid counter overflow, but you can do better, much better.
>
>> Modern high-resolution frequency counters don't do this;
>> and instead they use a form of "continuous counting" and
>> take a massive number of short phase samples and create
>> a more precise average frequency out of that.
>
> Yes, and the main point for creating this little thread is to make
> people aware that how you process your data do make a difference. It
> can make a huge difference in fact. The effective two-point frequency
> calculation only use two sample point to estimate the frequency and
> thus also use the systematic value and noise of one sample to cancel
> the noise of the first sample. For 1/f power noises this is an effect
> that can even becomes larger (for 1/f^3 noise) with time as it is
> non-convergent, so by looking at it briefly you don't realize it is a
> noisy number.
>
>> There are some excellent papers on the subject; start with
>> the one by Rubiola:
>>
>> <
>> http://www.femto-st.fr/~rubiola/pdf-articles/journal/2005rsi-hi-res-freq-counters.pdf
>> >
>>
>> There are additional papers (perhaps Bruce can locate them).
>
> In particular, there is one paper that corrects some mistakes of
> Rubiola, Australian if I remember correctly.
Yes, the paper by Dawkins, McFerran and Luiten:
http://ieeexplore.ieee.org/Xplore/login.jsp?url=http%3A%2F%2Fieeexplore.ieee.org%2Fiel5%2F4318993%2F4318994%2F04319178.pdf%3Farnumber%3D4319178&authDecision=-203
<http://ieeexplore.ieee.org/Xplore/login.jsp?url=http%3A%2F%2Fieeexplore.ieee.org%2Fiel5%2F4318993%2F4318994%2F04319178.pdf%3Farnumber%3D4319178&authDecision=-203>
Bruce
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