Hi In a GPSDO, an FLL can be done with no “cycle slips” between readings. In that case, the I term will indeed correct for long term errors. The net result will be effectively the same as a PLL for long term error. That is by no means to say that *all* FLL’s are done this way. Only that it is one possible implementation.
Bob > On Jun 24, 2019, at 9:16 PM, Glen English VK1XX > <[email protected]> wrote: > > Leo is right > > Depends on the application. Phase lock for 1pps to trigger say, simultaneous > capture of many radio telescopes around the globe is a good need for phase > lock to a source. Frequency lock might suit many . change of phase between > two sources might indicate frequency change, or duty cycle change. > > For all my digital PLL and digital FLL implementations, the method of > capturing the data is identical... just the transfer function of the filter > is different. So they are nearly indistinguishable. > > A choice might depend on the control loop bandwidth, and whether the initial > error (acquisition) is beyond the BW. In those cases depending on the type of > comparison, it might make sense to measure the frequency error, and then > move the (controlled) source within phase lock without cycle slip range in > one step. Useful for very long time constants and large initial errors. > > ...and then what do you do when including relativistic effects ... > > glen. > > > On 25/06/2019 3:42 AM, Leo Bodnar wrote: >> Hi Dana, >> >> I am just saying that, properly implemented, PLL and FLL are >> indistinguishable as long as output signal is concerned while lock is >> present and that the phase slew at regaining lock in PLL loop is >> counterproductive for one but necessary evil for others. I have a feeling >> that FLL is looked down upon by general public ever since PLL became a >> household term. >> >> In a well designed PID loop "I" term makes sure that you don't have >> "permanent but varying error." >> >> All my messing about with loops, holdovers and recovery was pretty much with >> your application in mind. >> >> Cheers! >> 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 >> _______________________________________________ >> 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. _______________________________________________ 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.
