Jeff, There's nothing magic about 1KHz as the heterodyne frequency. I picked it to avoid power line pickup & permit a tuned circuit to limit the noise bandwidth. But, any tuned circuit in the signal path will have temperature driven phase variations & possibly signal level phase variations, as well.
The opamps contribute some noise & they also limit the output slew rate; yielding approx. 400ps rms jitter. Using rather stable L & C components limits the total impact of these effects to less than +/-1E-9 @ 1KHz (equivalent to +/- 1E-13 @ 10MHz). These issues speak against extending this scheme to much longer times. The synthesizer PN does contribute to the counter measurements. But, even with 2s gate time you are averaging 1000 samples & things quiet down nicely. Some synthesizers are better used at 9.999MHz as opposed to 10.001MHz, but I've no solid data on this. Either frequency is OK in this scheme. Pete Rawson _______________________________________________ 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.
