Magnus Danielson wrote: > Stephan Sandenbergh wrote: >> Hi, >> Thanks for the replies. I agree. One is interested in the timing of beat >> notes. >> >> But, I'm slightly confused now. It might be a language problem on my >> side. >> >> Quoting Howe, Allan and Barnes, 1981, "...adjust the phase so that >> the two >> beat frequencies are nominally in phase; this adjustment sets up a nice >> condition that the noise of the common oscillator tends to cancel..." >> >> I can see a few issues here: >> >> 1) if the beat frequencies are in phase, there will be a very small time >> interval between their zero crossings. This might be difficult to >> measure >> with accuracy. > > No, it is not a particular issue, unless you have problem with > isolation. The method to workaround isolation problems that I > described is just one possible approach out of many, including > designing it out of being an issue. > Most time interval counters have problems with crosstalk if the START and STOP edges are too close together. However one can always insert a suitable length of coax (> 10ns or so delay between the START and STOP inputs should work well with the 5370A/B time interval counters) to delay the STOP signal sufficiently to avoid this problem whilst maintaining near coincidence at the DMTD system outputs. > What they are after is that as coincidence in time makes signals from > a DTMD perspective correlate better in the regard as supressing the > transfer oscillator phase stability. If you keep the coincidence time > to say within 100 us much of the low-frequency noise of the transfer > osicllator beyond 100 us correlate quite well, so cross-correlation > will perform well to supress it where as instabilities with shorter > times will not correlate and thus polute the measurement. As > time-separation increases, more integrate time-noise of the transfer > oscillator will be exposed and induced into the measurement results, > for which only time-averaging will help. > JPL papers on their latest DMTD mixer system use a beat frequency of around 100Hz or so to allow some compensation for the phase noise of the offset oscillator to be made when the 2 beat frequency zero crossings aren't coincident. >> 2) I agree that reference oscillator noise will cancel to some extend >> because the measurements are done closer to the same time, which >> makes the >> reference oscillator noise better correlated between the start and stop >> edges. > > Correct. > >> 3) Also, one would like to compare both clkA and clkB at the same >> time. Not >> one at t=0 and the other at t=1day to exaggerate a little. I'm not >> planning >> to measure atomic standards, or the best OCXO available, so I doubt this >> will bother me. > > The closer together they are, the better cross-correlation gain can be > achieved. On the other hand, tbe closer the edges is and thus > cross-correlation losses may also be found. > >> Other than the above, I agree: it is better to have a greater phase >> offset >> between the beat notes. >> >> Does my thinking my sense at all? > > Sure thing. > > Cheers, > M'agnus > > _______________________________________________ > 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. >
Bruce _______________________________________________ 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.
