); SAEximRunCond expanded to false Errors-To: [EMAIL PROTECTED] RETRY From: "Tom Van Baak" <[EMAIL PROTECTED]> Subject: Re: [time-nuts] GPSDO Question Date: Sun, 2 Sep 2007 13:13:39 -0700 Message-ID: <[EMAIL PROTECTED]>
> ); SAEximRunCond expanded to false > Errors-To: [EMAIL PROTECTED] RETRY > > > Jerry > > > > It is amusing/distressing to see that the myth that using an FLL to lock > > an oscillator to the PPS output of a GPS receiver is a good approach > > still persists. > > The optimum solution is a phase lock loop. > > Whilst building an FLL is instructive/educational, if you want the best > > GPSDO performance you should really use a PLL. > > > > Bruce > > It would seem for timekeeping applications, a PLL-based > GPSDO will inherit the long-term accuracy of GPS with > great fidelity. > > 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. > > Can someone point me to real data or even simulations > with plots that show rms or adev differences between the > two camps? I'm sure there is alot of it if you only look carefull enougth. There is alot in the GPS tracking camp. As always, FLLs are great ways to get started in tracking in quickly (they certainly beat PLLs) but for stability the FLLs residue errors, it is basically a derivate measure of the incomming phase and errors will result in zig-zagging around the goal. This is a D or PD-regulator, where as a PFD based PLL becomes a PID-regulator. However, onces lock has been acheived the D-term (frequency) can be removed as the PI-regulator is usually supperiour unless you expect very big phase-derivations on the input to cause the the PI-regulator out of lock. This is the classical books. In addition, it is worth mentioning that several PFD detectors have problems with the how internal gate times cause excess push-pull operation in the charge-pump and results in excess pumping in frequency. Works well for some applications, but not for others, especially low-jitter high step-up frequency multiplication. It is clear that a well done PLL is better and FLL. PLLs is however being outperformed by Kalman filters. This is all covered in literature, so I don't think we need to measure things, it is already established. There are FLLed GPSDOs out there. For many purposes they are sufficienly good, so it is not necessarilly a *BAD* thing. Good FLLs is limited by PPS resolution jitter anyway. Cheers, Magnus _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
