HI In some cases, the difference can be your definition of "time accuracy". If short term GPS time is what you are worried about, then indeed that's a different beast than a 30 day plot against your direct line to USNO.
Bob On Sep 16, 2012, at 4:12 PM, Dennis Ferguson <dennis.c.fergu...@gmail.com> wrote: > > On 16 Sep, 2012, at 00:40 , Tom Van Baak wrote: >> I worry in your example about the long cross-over time. This may be ideal >> for frequency stability, but probably is not good for time accuracy. If one >> is using the GPSDO as a timing reference, I would think a shorter time >> constant will keep the rms time error down. Has anyone on the list done work >> optimizing the timing accuracy rather than the frequency stability? > > I'm not sure there could be a difference between the goals of > frequency accuracy and time accuracy that would effect that time > constant. The time error is the time integral of the frequency > error, so anything which manages to minimize the frequency error > of the oscillator (both the magnitude of the error and its > duration) will also minimize the time error. The time constant > is selected to be the minimum value which makes it probable that > the frequency or time error you have measured (for a PLL the data > are time errors) is in fact an error that the oscillator has > made rather than an artifact of the noise in the measurement > system. > > There might be a difference in the best control action to take to > optimally achieve each of those goals. In particular if your goal > is frequency accuracy the best control action in response to the > measurement of a frequency error might be to correct that error, > i.e. to minimize the frequency error once you know you have one. > If your goal is time accuracy, however, then the response to a > measured frequency error is going to be to intentionally make a > frequency error in the other direction for a while to correct the > accumulated time error. In this case, though, it seems to me > that by selecting a PLL as the control discipline (rather than, say, > a FLL) you've already made the decision to take control actions > which ensure time accuracy. > > Dennis Ferguson > _______________________________________________ > time-nuts mailing list -- time-nuts@febo.com > To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > and follow the instructions there. _______________________________________________ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
