Sorry this is a bit long-ish, but I figure I'm saving time putting in all the details up front.
The canonical time-nut way to set up a MVAR measurement is to feed two sources to a HP5370 and measure the time interval between their zero crossings often enough to resolve any phase ambiguities caused by frequency differences. The computer unfolds the phase wrap-arounds, and calculates the MVAR using the measurement rate, typically 100, 10 or 1 Hz, as the minimum Tau. However, the HP5370 has noise-floor in the low picoseconds, which creates the well known diagonal left bound on what we can measure this way. So it is tempting to do this instead: Every measurement period, we let the HP5370 do a burst of 100 measurements[*] and feed the average to MVAR, and push the diagonal line an order of magnitude (sqrt(100)) further down. At its specified rate, the HP5370 will take 1/30th of a second to do a 100 sample average measurement. If we are measuring once each second, that's only 3% of the Tau. No measurement is ever instantaneous, simply because the two zero crossings are not happening right at the mesurement epoch. If I measure two 10MHz signals the canonical way, the first zero crossing could come as late as 100(+epsilon) nanoseconds after the epoch, and the second as much as 100(+epsilon) nanoseconds later. An actual point of the measurement doesn't even exist, but picking with the midpoint we get an average delay of 75ns, worst case 150ns. That works out to one part in 13 million which is a lot less than 3%, but certainly not zero as the MVAR formula pressume. Eyeballing it, 3% is well below the reproducibility I see on MVAR measurements, and I have therefore waved the method and result through, without a formal proof. However, I have very carefully made sure to never show anybody any of these plots because of the lack of proof. Thanks to Johns Turbo-5370 we can do burst measurements at much higher rates than 3000/s, and thus potentially push the diagonal limit more than a decade to the left, while still doing minimum violence to the mathematical assumptions under MVAR. [*] The footnote is this: The HP5370 firwmare does not make triggered bust averages an easy measurement, but we can change that, in particular with Johns Turbo-5370. But before I attempt to do that, I would appreciate if a couple of the more math-savy time-nuts could ponder the soundness of the concept. Apart from the delayed measurement point, I have not been able to identify any issues. The frequency spectrum filtered out by the averaging is waaaay to the left of our minimum Tau. Phase wrap-around inside bursts can be detected and unfolded in the processing. Am I overlooking anything ? -- Poul-Henning Kamp | UNIX since Zilog Zeus 3.20 p...@freebsd.org | TCP/IP since RFC 956 FreeBSD committer | BSD since 4.3-tahoe Never attribute to malice what can adequately be explained by incompetence. _______________________________________________ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.