[time-nuts] Limitations of Allan Variance applied to frequencydivided signal?

Bob Camp lists at rtty.us
Fri May 13 16:05:14 UTC 2011


Hi

For AVAR you want a time record not a frequency measure. Your time stamps
are a direct phase estimate. They are what you would use directly for the
AVAR calculation. If they are faster than your shortest tau, all is well.
Divide, mix down, what ever, just stamp faster than the shortest tau. 

Bob

-----Original Message-----
From: time-nuts-bounces at febo.com [mailto:time-nuts-bounces at febo.com] On
Behalf Of Tijd Dingen
Sent: Friday, May 13, 2011 11:28 AM
To: time-nuts at febo.com
Subject: [time-nuts] Limitations of Allan Variance applied to
frequencydivided signal?

In trying to put together a way to calculate Allan variance based on a
series of timestamps of every Nth cycle, I ran into the following...

Suppose you have an input signal, but it's a bit on the high side. So you
use a prescaler to divide it down to a manageable frequency range. And now
you want to use that signal to be able to say something useful about the
original high frequency signal.

Now taking a look at the part about "Non-overlapped variable tau estimators"
in the wikipedia article here:

http://en.wikipedia.org/wiki/Allan_variance#Non-overlapped_variable_.CF.84_e
stimators

It seems to me that "divide by 4 and then measure all cycles back-to-back"
is essentially the same as "measure all cycles of the undivided high
frequency signal back-to-back" and decimate. Or "skipping past n - 1
samples" as the wiki article puts it. And that is disregarding /extra/
jitter due to the divider, purely for the sake of simplicity.

Plus, I strongly suspect that all these commercial counters that can handle
6 Ghz and such are not timestamping every single cycle back-to-back either.
Especially the models that have a few versions in the series. One cheaper
one that can handle 300 MHz for example, and a more expensive one that can
handle 6 GHz. That reads like: "All models share the same basic data
processing core and the same time interpolators. For the more expensive
model we just slapped on an high bandwidth input + a prescaler."

Anyways, any drawbacks to calculating Allan Variance of a divided signal
that I am overlooking here?

regards,
Fred


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