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

[Bob kb8tq]

Hi

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. 

One advantage that a digital pot has is small size. If you want to ovenize a pot to improve
it’s temperature performance, that’s a good thing. Indeed a lot of modern oscillators have
digital pots in them to set this or that during the production process. 

====

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. 

So as usual, no simple answer, only a lot more questions.

Bob

> On Jan 27, 2020, at 1:53 AM, Perry Sandeen via time-nuts <time-nuts at lists.febo.com> wrote:
> 
> Yo Bubba Dudes!,
> Previous posts mentioned wiper noise and stability of a mechanical pots after tweaking.
> My questions are:
> Do digital pots after setting have wiper noise?
> When making fine tuning tweaks to the EFC of an OCXO, can one move it to its *dead on* setting right away or is there some lag that must be considered requiring to do it in steps?
> Regards,
> Perrier
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