Hi, On 2021-03-10 14:23, Bob kb8tq wrote: > Hi > >> On Mar 10, 2021, at 6:04 AM, Hal Murray <hmur...@megapathdsl.net> wrote: >> >> >> kb...@n1k.org said: >>> The gotcha here is that if you want accurate *time*, you are better off >>> using >>> the sawtooth corrected output from a (good) GPS module rather than a GPSDO. >> Why is that? > The controller gets in the way. If you want good frequency stability you have > long > time constants. The local reference is free to (and does) wander inside that > time > constant.
Indeed. It is different to optimize for ADEV or TDEV performance. However, even when phase/time is interesting, a frequency optimization has benefits when it comes to hold-over performance, i.e. how stable it will be after loosing tracking. You very quickly come to a situation where frequency error dominates the phase error, and only later does drift and environment start to kick in. > >> I would have guessed that a GPSDO would average over many GPS pulses thus >> reducing the noise. > It does and it does. However that does not help the accuracy of your 1 second > time tick. A good GPS module *with* sawtooth. can get you down to a fraction > of a nanosecond (ADEV) on your 1 PPS. There is no need to average that noise > down any further. Actually, you go below that if you average long enough, but the sawtooth for sure helps. There is enough of noise and systematics to make things move around. It's still not very good thought, so you end up with a fuzzy border about there. As always, it depends. :) > >> Is it something like GPSDOs are normally designed for good frequency rather >> than good time, so when they find the time is off, they use a small >> frequency >> offset for a long time to correct rather than a big frequency offset for a >> short time? >> >> Are there any GPSDOs designed for good time? Or any with parameters that >> can >> be tweaked to provide good time? > There are a few. They steer the 1 PPS so it follows the PPS output of the > module. > If all you want is time, just use the module PPS and save on the electric > bill. Indeed, unless you need hold-over capability, that's when an OCXO or Telecom Rubidium may kick in and be relevant. As time-nuts, we do it regardless, because it is fun, but that is a different account. :) Cheers, Magnus _______________________________________________ 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.
