Alan Melia wrote: > This is an interesting thread again.....it may be similar to ones that have > been discussed, but one or two furthur questions occur to me. I have a > Montronics sytem that does comparisons by the multiply and mix process, and > I find (also common to more modern Kethly systems) that the limitation is > around a part in 10^10 where the noise on the phase output makes it not > really usable (without a lot of averaging) being around or in excess of 90 > degreees even with a couple of very good OXCOs. How does the 10G comparision > avoid this problem with standard multipliers? I doubt you can go all that > way with low-noise multipliers and have any useful signal left, or have I > missed something. At present I use a phase meter (lock in amps can be quite > good) at the MHz range and datalog the phase drift for several hours. I have > determined that setting "on the nose" is not necessary (for my > applications). It is more useful to know how far a source is "off". > How does the mix down compare with the seemingly more popular "mix down and > timestamp" I understand from previous threads that this has more potential > but might it also be as good even using simpler circuits that the NIST > system. > > Thanks for all your efforts inthe background John..... great reading > material ! > > Alan G3NYK > > Alan
The principal limitation with dual mixer systems is the relatively large phase shift tempco of the mixers (~ 10ps/C for 10MHz inputs). If one uses a high end sound card (or equivalent ADC) to timestamp the beat frequency zero crossings a resolution of better than 1E-12/Tau is possible when the thermal environment permits. However a stable low noise offset source frequency is required. It is also essential to ensure there is sufficient isolation between the sources being compared to avoid injection locking. A low end sound card can also be used together with a Collin's style bandpass limiter/slope amplifier to achieve similar performance if the slope gain is large enough. 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.
