Hi Bob, Precisely the kind of sanity check I was looking for. Thank you!
regards, Fred ----- Original Message ----- From: Bob Camp <[email protected]> To: 'Tijd Dingen' <[email protected]>; 'Discussion of precise time and frequency measurement' <[email protected]> Cc: Sent: Friday, May 13, 2011 6:05 PM Subject: RE: [time-nuts] Limitations of Allan Variance applied to frequencydivided signal? 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: [email protected] [mailto:[email protected]] On Behalf Of Tijd Dingen Sent: Friday, May 13, 2011 11:28 AM To: [email protected] 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 _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there. _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
