[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 :-)



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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