On Sun, 23 Mar 2014 21:33:07 -0400, [email protected] wrote: > Message: 1 > Date: Mon, 24 Mar 2014 00:48:01 +0100 > From: Magnus Danielson <[email protected]> > To: [email protected] > Subject: Re: [time-nuts] GPSDO simulation tool > > Jim, > > On 23/03/14 16:00, Jim Miller wrote: >>> To handle higher tau performance I think we want a higher degree loop. >> >>> Cheers, >>> Magnus >> >> Is a higher degree loop possible while maintaining stability? Commanding >> frequency while measuring phase is one pole, integrating the result of the >> phase comparison is a second pole and closing the loop will result in >> oscillation unless a zero is inserted (the P in PID). >> >> How would stability be maintained? > > A third degree PLL is stable if the pole-pair is suitably located. The > second degree PLL is kind of hard to bring into self-oscillation while > that is much easier with the third degree, so more care needs to go into it. > > When you think about it, putting an exponential averager just after the > phase-detector (AVG1 in gpsim1) is in fact making the PLL a third > degree, but since the time-constant needs to be constrained to be well > below the loop time-constant "for stability reasons" it is in fact to > avoid having the poles of this third-degree loop from deviating away > into instability. > > I did a temporary hack on the PID code to convert the D-term into I^2 > term, by integrating the integrator output. First attempt was indeed > quite resonant just to show that I was in the unsafe region. Backing > down on the strength of the component sure did remove much of the > resonance, but I did not see any appreciable improvement in filtering > performance, so quick and dirty hacking isn't sufficient, darn.
I recall from an analysis of third-order PLLs read a few years ago that stability also depended on the amplitude of the signal to which the PLL was locking. This sensitivity made 3rd-order PLLs difficult to use as radio receivers, but for use as a timing-chain component one can always arrange to have adequate amplitude (and have a level detector to warn if amplitude isn't adequate). I always suspected that the old Symmetricom 1050A disciplined oscillator was a 3rd-order loop, but was never able to find anybody who knew. Joe Gwinn _______________________________________________ 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.
