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 < [email protected]> 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 -- [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.
