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

[jimlux]

On 1/27/20 10:32 AM, Charles Steinmetz 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.


Why, "all you gotta do" is just put the digipot in an oven that controls 
the temperature to 0.001 degrees.  :)

yes, this whole "adjusting by billionths" thing is hard.
That's why as much as I can, I'm going to systems that use "knowledge" 
not "control"