); SAEximRunCond expanded to false Errors-To: [EMAIL PROTECTED] RETRY In a message dated 9/2/2007 13:16:32 Pacific Daylight Time, [EMAIL PROTECTED] writes:
>But for many frequency (e.g., transmitters) or time interval >applications (e.g., frequency counters with finite gate times), >I'd like to understand, in detail, what the difference between >a PLL- and FLL-based GPSDO really is. Hi Tom, an FLL has some distinct advantages: * Sometimes quartz crystals exhibit well-documented random jumps in frequency/phase. Causes for this are speculative, but the effect on some crystals can be rather significant. In an FLL, the recovery is only 1/2 as long since only the original frequency has to be attained. In fact if it is just a phase jump, then there won't be any correction needed in an FLL, but the PLL will have to fully recover by pulling the frequency off it's optimum. In a PLL, the original frequency, plus an additional negative frequency offset has to be attained to push-back the phase, so the frequency is "off" for much longer. * An FLL can exhibit much less frequency error than a PLL. The amount is related to the temperature sensitivity of the OCXO. A PLL has to modify the frequency much more to attain an overall zero phase difference. This is more pronounced the more sensitive the OCXO is to temperature changes. * An FLL can have very low deviation on the crystal and still work correctly, say a maximum of +-1E-014 change per second, as long as the total available frequency change is greater than the total expected frequency error due to temp, aging, motion etc. Thus an FLL will exhibit higher frequency accuracy over time than a PLL. A PLL will have to be much more aggressive on it's control response to maintain phase lock since it has to correct the accrued phase error over time, while the FLL just has to correct for instantaneous error, not integrated error. Of course most of the instruments found in a lab are more sensitive to frequency errors than phase errors, such as a frequency counter (used in frequency mode), Spectrum analyzer, Jitter analyzers, RF signal generators etc so a PLL can actually degrade their performance versus an FLL. Lastly, the phase-error in an FLL is related to the sensitivity (gain) of the loop, and is smaller the better the OCXO is. An FLL can be simply seen as a PID controller where the I and D terms are set to zero. The literature has lot's of mathematical information about why the noise is much less in a proportional-only loop then when adding the integral (phase) part to it. This brings up another question: how good are true PID loops that also make use of the differential term (e.g. correcting for rate of change of the phase)? The literature talks about the differential term being hardly used in the industry because it can add noise and instability to a system... bye, Said ************************************** Get a sneak peek of the all-new AOL at http://discover.aol.com/memed/aolcom30tour _______________________________________________ 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.