Perrier, I see that several others have beat me to the punch on the digipot issues. So I think I'll sit this one out and just try to learn.
Dana On Mon, Jan 27, 2020 at 7:50 AM Dana Whitlow <k8yumdoo...@gmail.com> wrote: > Perrier, > > In answer to your 2nd question, I am unaware of any oscillator technology > for which > tuning the actual oscillator frequency has either (much less both) of > instant full > response or absolutely zero effect on its frequency drift trend > afterwards. > > Instantaneous full response is generally not expected nor even desirable, > as long > as it's fast enough as not to interfere with closed loop bandwidth & > desired loop > stability goals. This is true even if the EFC input is being adjusted > manually- it is > frustrating to tune a slowly-responding oscillator by hand, for the > operator then has > to wait around for some time before it is possible to meaningfully assess > the effect > or each trial frequency adjustment. > > On the other hand, a truly instantaneous full response to EFC input is > possibly > incompatible with use of a high-Q resonator in the oscillator. Further, > it would > imply infinite tuning bandwidth, which would require extra care, possibly > *considerable* > extra care, to assure that high frequency noise in the tuning circuitry is > not passed > along to the VCO. If proper attention is not paid to this detail, one is > likely to find that > the phase noise sidebands of the oscillator's output are intolerably > strong. > > As to trend effects there are numerous mechanisms by which resetting the > frequency > of an oscillator will also alter its drift trend. The severity of this > effect varies widely with > circumstances, but in many cases leads to intolerable problems. Hence in > critical > applications is it often preferred to leave the primary oscillator alone > and deal with > drift in some other way. Celestial navigators of yore were well aware of > this issue and > adjusting the rate of a watch used for this application was a cardinal sin > (especially if > attempted at sea). Instead, they started a voyage armed with a record of > past drift rate, > updated this as circumstances permitted, and probably kept a healthy > degree of > skepticism regarding each new update. > > In the more modern context, the primary oscillator is left alone and the > desired final > output frequency is achieved by use of a "microstepper", which is a > effectively a special > purpose synthesizer. A good example of this is the well-regarded MHM-2010 > active > hydrogen maser, which comes equipped with a good integrated microstepper. > > Obviously an exception can (and must) be made in the case of a secondary > VCO which > is being phase locked to the primary oscillator, as a part of such a > microstepper or other > synthesizer, for example. Here, long term drift of the VCO is of less > immediate concern > until it grows so large that the phase locking loop is no longer able to > maintain lock. > > Regarding "digital pots", I'll write separately, probably as a private > note. > > Dana > > > > On Mon, Jan 27, 2020 at 12:54 AM Perry Sandeen via time-nuts < > time-nuts@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 >> _______________________________________________ >> time-nuts mailing list -- time-nuts@lists.febo.com >> To unsubscribe, go to >> http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com >> and follow the instructions there. >> > _______________________________________________ time-nuts mailing list -- time-nuts@lists.febo.com To unsubscribe, go to http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com and follow the instructions there.
