Hi If you need that sort of isolation, it certainly can be done. NIST has papers on very simple / DIY compatible cascode amps that will do the trick. ( chain of common base stages driven by a common emitter). Some folks on the list have gone a lot further in terms of complexity than NIST did.
Device wise, the cascode amps seem to work pretty well with some very humble transistors ( 2N3904 etc ). There likely are fancier parts out there, but some of the really old stuff appears to be “good enough”. Why cascode ( or just common base) ? You can manage the gain pretty well. The amps likely are going to be quite low gain along with low noise. Thats not a great combination. There are a lot of OCXO’s out there that don’t need anything as crazy as the multi stage NIST amps. If you are looking at a system (as opposed to an oscillator) isolation already has been addressed in the instrument. Part of the “look at the beat note” process is to observe that it looks like a proper clipped triangle wave. The slopes on the rising and falling edges should be similar and of a reasonable slope. If you have injection locking ( and with a typical low frequency note) the rising and falling edges will begin to distort. The “lock” process will start to occur as the note crosses zero. This is yet another good reason to use a low frequency note. It also is good to look at the slope of *both* zero crossings. Bob > On Jul 5, 2022, at 12:16 AM, Leon Pavlovic via time-nuts > <[email protected]> wrote: > > I've tried to do a simple and low-cost sound-card PN system too back in the > days. It's fun, when you see the broadband thermal resistor noise in your > FFT plots :) But when doing experiments with the crystal oscillators, I've > found out that one of the *MOST* important things to worry about is > injection locking, especially if you do OCXOs and very low PN oscillators. > > Before feeding the RF and LO ports of the mixer, you should have isolation > amps on both ports with about or more than 100dB S12 isolation, not to get > into trouble. Without them, you'll be measuring unreal PN noise... > > Cheers, > Leon > _______________________________________________ > time-nuts mailing list -- [email protected] > To unsubscribe send an email to [email protected] _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe send an email to [email protected]
