[time-nuts] How to measure Allan Deviation?
Didier Juges
didier at cox.net
Sun Oct 22 10:34:47 EDT 2006
Hi Warner,
Does it mean I should divide the 10 MHz down to 1 Hz output and use the
5370 to measure TI compared to it's internal timebase once per second,
and feed that to the computer, store it to a file and feed the output to
AlaVar? (obviously, the divider would have to use synchronous counters)
Or should I work directly on the 10 MHz output? The 5370 can average up
to 100k samples, so by averaging 100k samples and polling the GPIB
once/sec, would I be correct if I use the computer to average those
further down to 2/sec, 4/sec and so on as needed before feeding the data
to Alavar?
Now, let's assume I have a big hard drive (250 GB + 120 GB at the
moment, with probably close to a total of 200 GB available), other than
computing time (which may not be negligible), how can I determine the
best acquisition interval? (as the one that will give me meaningful data
in the shortest amount of time)
I understand AlaVar only works in batch mode (no real time capability),
so I have to collect a certain amount of data "blindly" before I can
find out if it is any good. If I am checking a GPS disciplined
oscillator, that will take several hours at a time and I am trying to
speed up the process, at least to make sure the procedures and
algorithms are OK.
See, you can't provide answers without getting more questions :-)
Thanks
Didier
M. Warner Losh wrote:
> Just a quick note on this topic. I'll not be able to answer all your
> questions, but here's a few.
>
> Allen Variance (or Allen Deviation) is usually used to measure clock
> stability over 1s or longer. As this is a way to judge the stability
> of an oscillator, very short term numbers tend to be less useful.
> This means that fast data collection rates end up having lots of
> redundant data that contributes little added benefit to the
> calculations, but has large storage requirements. If you are
> computing a Tau of 100s, for example, 10Hz vs 1Hz data will only give
> a factor of 3 better confidence interval (the standard deviation of
> the avarage at a Tau of 100s, which goes as the square root of the
> number of samples), but you'd have a 10x increase in storage.
>
> Allen Variance usually is measured at 1s, 2s, 4 (or 5s), 10s, 20s,
> etc up to some fraction of the period of time the data covers. The
> fraction is again determined by the type of deviation and the number
> of points used to calculate the number.
>
> Warner
>
>
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