[time-nuts] Clock quality: alternatives to ADEV

Richard (Rick) Karlquist richard at karlquist.com
Thu Apr 10 13:24:36 EDT 2014


The trouble with ADEV is that if you average
a long time it papers over anomalous events
like crystal jumps.  An alternative measure
might be to, instead of averaging, simply
keep track of the worse case change in frequency
during 1 sample period.  Sort of like peak jitter
versus rms jitter.

Rick

On 4/9/2014 3:38 PM, Hal Murray wrote:
>
> I've been watching the discussions and graphs for a while.  ADEV seems
> appropriate for cases where the noise pattern is "nice".  How does ADEV work
> if the noise isn't nice?  Are there alternatives?  What's the mathematical
> term for the type of noise that works well with ADEV?
>
> I can think of 3 examples:
>    Crystal jumps
>    GPSDOs going into holdover.
>    Power lines make all sorts of interesting not-quite jumps.
>
> Is there a way of characterizing that sort of event?  How do I turn a pile of
> data into a useful graph or chart?
>
> What does an ADEV graph look like if the data has crystal jumps?
>
> I'd expect that something like crystal jumps would follow some sort of power
> law: the bigger jumps would be less frequent.  But it wouldn't surprise me if
> the GPS or power lines had an underlying mechanism that turned into a
> different pattern.
>
>



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