Correction from Dana: I meant "... without accumulation of phase error during normal times ...".
Sorry about that. Dana On Sun, Jun 23, 2019 at 11:45 AM Dana Whitlow <k8yumdoo...@gmail.com> wrote: > Leo, > > Are you saying that you want to abandon phase lock altogether in favor of > freq > lock? Or just during the reacquisition following loss of and restoration > of the > reference? > > By me definition of pure freq lock, there will generally be some permanent > (but varying) > frequency error, so that phase error could accumulate without limit; > clearly an undesirable > thing in most applications. > > My interest lies in having a stable LO for receiving, without accumulating > phase error (at least > during times of missing reference). When the reference goes away, I'll > accept some phase > error accumulation. So for me, I think the best approach is phase lock > under normal > circumstances, but switch to freq lock during reacquisition of phase lock. > > Dana K8YUM > > On Sun, Jun 23, 2019 at 9:01 AM Leo Bodnar <l...@leobodnar.com> wrote: > >> I have to draw your attention to practical aspects of why some designs >> use FLL rather than PLL. >> >> Consider a GPS locked OCXO outputting GPS synced 10MHz signal. >> >> Properly designed control loop will not produce much (if any) difference >> when the reference (GPS signal) is present. In the end, integral of zero >> is zero. >> >> When reference (GPS lock) is lost the things are very similar too, >> holdover is just flying blind in the rough direction you were facing last. >> Accumulating frequency and phase offset on the way. >> >> However, when reference is restored the things are much different. >> After regaining the reference (which in case of GPS signal has >> unambiguous absolute time embedded into its phase) *proper* PLL loop will >> try to correct for slipped phase at the highest slew rate. This can be >> huge. If phase has drifted 1ms apart the loop will have to slew the phase >> all the way until it gets those 10,000 cycles out of the way. This usually >> looks ugly in frequency domain and is very disrupting if you are using the >> device as frequency reference rather than an absolute time reference. >> >> Proper FLL loop will just gently (and reasonably quickly) get your >> frequency back and forget about all the lost phase. Which is what a lot of >> users want. >> >> Initially, I have used PLL mode on GPS clocks that I am making, but >> switched over to FLL during the last few years. >> >> Cheers >> Leo >> _______________________________________________ >> 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.