Hi > On Jan 27, 2020, at 1:32 PM, Charles Steinmetz <[email protected]> wrote: > > Bob wrote: > >> Digital pots have *lots* of issues. A high quality wire wound pot likely >> will be significantly >> more stable and lower noise than your typical digital unit. In addition the >> 10 or 20 turn wire >> wound will have far more “steps” than a digital pot. > > Digipots come in two flavors -- resistive ladders, and multiplying DACs > ("MDACs"). Each has lots of issues, some in common and some different. > > Even with the MDAC variety (which can have as many as 16 bits worth of > steps), I can't imagine ending up with sufficient resolution to give > satisfactory step sizes for time nuts purposes, unless you cascade at least > two of them in a "coarse and fine" arrangement. Look back through the > archives at the many discussions of suitable DACs for homebrew GPSDOs, for > discussions of how many bits of resolution you need and what the tradeoffs > are [e.g., lack of range]. Resistive ladder digipots are just hopeless, at > around 10 bits of resolution maximum. By the time you had sufficiently small > increments to be useful to time nuts, you would no longer have enough range > to compensate for oscillator drift over a usefully long period. > > Also, look at the temperature coefficients on the datasheets. You see values > in the high hundreds to thousands of ppm per degree C. Not ppb, mind you, > ppm. This, by itself, is very likely a fatal flaw when trimming measured in > ppb is at issue. > >> If you apply a 1 ns rise time step to the EFC of an oscillator it will not >> change frequency in >> a nanosecond. The tune port has a bandwidth. On an OCXO that bandwidth might >> be in >> the 10’s of Hz range. >> >> If you have bypass caps all over the place (and some large resistance here >> and there) on your >> EFC then indeed the caps can have various issues (leakage changes, >> dielectric absorption …) >> that can take a while to settle out. Is that minutes or days? It very much >> depends on just what >> you have wired up. > > And fundamentally, the quartz takes time to "relax into" its new frequency. > This can be hundreds of ppb or more, and can take anywhere from a few days to > a few months to settle within ppb. Just one more reminder that there is no > hurrying precision oscillators.
If your oscillator has “hundreds of ppb” issues settling after an EFC adjustment, that part has major flaws in it’s design. Indeed, aging and warmup will take a while. They should be unrelated to a tuning adjustment. Bob > > Best regards, > > Charles > > > > _______________________________________________ > time-nuts mailing list -- [email protected] > To unsubscribe, go to > http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com > and follow the instructions there. _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com and follow the instructions there.
