Hi Well, situation one:
You have two perfect sources. Your measuring device is noiseless If your devices are in perfect sync, you get a series of zeros Your ADEV is zero Situation two: Same sources Noisy measuring device You get the standard deviation of the difference in measurements Your ADEV is simply a measure of the noise of the measuring device Situation three: Your sources are much worse than 1x10^-9 at 1 second Your ADEV is the proper number for your sources (or close to it) Situation four: The real world, you have a bit of each and you really don’t know what is what. Lots of possibilities and no single answer. Bob > On Oct 29, 2016, at 7:38 PM, Stewart Cobb <[email protected]> wrote: > > What's the expected value of ADEV at tau = 1 s for time-interval > measurements quantized at 1 ns? > > This question can probably be answered from pure theory (by someone more > mathematical than me), but it arises from a very practical situation. I > have several HP5334B counters comparing PPS pulses from various devices. > The HP5334B readout is quantized at 1 ns, and the spec sheet (IIRC) also > gives the instrument accuracy as 1 ns. > > The devices under test are relatively stable. Their PPS pulses are all > within a few microseconds of each other but uncorrelated. They are stable > enough that the dominant error source on the ADEV plot out to several > hundred seconds is the 1 ns quantization of the counter. The plots all > start near 1 ns and follow a -1 slope down to the point where the > individual device characteristics start to dominate the counter > quantization error. > > One might expect that the actual ADEV value in this situation would be > exactly 1 ns at tau = 1 second. Values of 0.5 ns or sqrt(2)/2 ns might not > be surprising. My actual measured value is about 0.65 ns, which does not > seem to have an obvious explanation. This brings to mind various questions: > > What is the theoretical ADEV value of a perfect time-interval measurement > quantized at 1 ns? What's the effect of an imperfect measurement > (instrument errors)? Can one use this technique in reverse to sort > instruments by their error contributions, or to tune up an instrument > calibration? > > I'd be grateful for answers to any of these questions. > > BTW, thanks to whichever time-nuts recommended the HP5334B, back in the > archives; they're perfect for what I'm doing. And thanks to fellow time-nut > Rick Karlquist for his part in designing them. > > Cheers! > --Stu > _______________________________________________ > 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.
