[time-nuts] Method for comparing oscillators

Steve Rooke sar10538 at gmail.com
Fri Aug 7 04:45:04 UTC 2009


2009/8/7 Magnus Danielson <magnus at rubidium.dyndns.org>:
> The Hadamard processing (Dev, ModDev or Tot) gives another B term, which
> considering that B is fairly small gives a drift-gain. As the series
> progresses, the drift derivate dies away faster (1/t^2 rather than 1/t) than
> for Allan processing so the longer time sequence used, the better
> suppression of the drift mechanism (which is true for Allan processing too).
>
> So, in this context Hadamard is better... but it still does not nail it.
> It may be sufficient however. Estimating A and B and remove the trend from
> the data isn't too hard.

To a certain extent it depends a lot on the oscillator that is being
measured. Allen Deviation is considered as being the best tool for
this type of work but it does suffer when the subject under test has
significant drift. For the quality of xtals and sources that some
time-nuts are testing, this is unlikely to be a huge problem but for
lesser sources this is a real factor hence my suggestion for the use
of Hadamard Deviation. For practical purposes though, the xtals we use
are generally embedded as part of a GPSDO which will compensate for
the drift in the oscillator but cannot practically compensate for a
noisy xtal and HDEV would make comparing one source with another easy.

Cheers,
Steve

> It is an interesting exercise to estimate A and B, produce the drift only
> time-series and see what Allan and Hadamard time-series for that looks like
> and then compare them to the Allan and Hadamard time-series of the raw data.
> Cranking out a compensated time-series and produce the Allan and Hadamard
> time-series for that isn't that hard either.
>
> Do plot the frequency or phase plot of the estimated curve along with the
> actual data, along with the difference to make sure you have a good match.
> This can give you a good hint that your drift estimate is either bad or has
> the wrong model.
>
> That way you can really estimate if you where drift limited or not.
>
> Cheers,
> Magnus
>
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-- 
Steve Rooke - ZL3TUV & G8KVD
A man with one clock knows what time it is;
A man with two clocks is never quite sure.




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