How do you pick the optimal difference frequency? I see that 1kHz has a nice numerical property where you can read the frequency directly off the counter, you just need to mentally prepend the first 4 digits. With computers it's not that important, the difference can easily be a strange number if it optimizes performance. I'm wondering what difference frequency optimizes the performance of the mixer thing or if it really matters?
Do you worry about the phase-noise contribution of the 10.001MHz source? As I do the math, it seems that the phase noise of the mixing signal is subtracted out after the mixing, so it shouldn't mater that the 10.001Mhz source comes from a frac-N synthesizer and has a few random spurs. You say this isn't state of the art. Why not? Can't you run the timing collection for longer runs and get higher resolution results? jeff Pete wrote: > This topic has been addressed earlier; though with some > debate. I have proposed a simple heterodyne scheme for > beating 2 stable sources against each other & observing > a 1KHz difference frequency to resolve 1uHz deltas. > This is NOT a "state-of-the-art" scheme, but it will > provide better than 1E-12 resolution in less than 10s. > > This scheme does require some non-standard items. > > 1. You need a stable synthesizer with external clock > capability to yield 10.001 Mhz, phase locked to > one of your sources. The HP3336C or a PTS040 > work fine. > > 2. You need a 1KHz zero crossing detector to drive > your counter input with low jitter. The ZCD requires > 2 opamps & a few passive parts, including 2 > inductors you'll need to wind by hand. > > 3. You need a level 7 double balanced mixer to > heterodyne the second source & the 10.001Mhz > signal. Mixers optimized as phase detectors, like > the mini-circuits SYPD-1 work well for this. > You also need a diplexer on the mixer output > to separate the 1KHz beat signal from the other > mixer products. The diplexer is 6 passive parts. > > The results are stable & provide counter readings of > 9 significant digits down to 1uHz with the leading 4 > digits of frequency assumed from the mixing process. > The counter gate time setting provides useful & often > necessary averaging of the readings; so a variable > gate time counter is handy. I've used a H-P 5335A, > & don't know much about the Racal 199x series, > but I suspect they would do just fine. > > 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. > _______________________________________________ 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.