[time-nuts] Digital Pots, wiper noise and fine tweaking oscillators

[Bob kb8tq]

Hi

> On Jan 27, 2020, at 1:32 PM, Charles Steinmetz <csteinmetz at yandex.com> 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
> 
> 
> 
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