>> So what is optimum... from control theory we learn, that with an even >> better model of your system, you can push performance to the edge! But >> you always loose robustness in doing that.
One thing to remember: control theory is akin to a taxinomy which has only "elephants" and "non elephants", in that sense that non-linear control theory is not widely taught and generally considered an evil to stay away from at all cost. Traditional PLL methods are inherently linear and consequently limited in what they can do, but you can achive suprisingly good results by combining a classical PLL with non-linear higher modes or mode switches. You can of course, in the absense of solid theory for what you are attempting, also get it horribly wrong, as in the NTP PLL :-) But the machine-control and robotics community has finally realized that to get machines to "have the moves" they have to abandon classical control theory, drop the poles and zeros of the plane and instead build physical predictive models. The first area to pick up on this big time were disk drive arm actuators, which are nowhere near a simple differential equation any more. And timekeeping lends itself well to experiment with this, not the least because getting it wrong doesn't smash anything valuable :-) For instance, one thing to think about in the context of GPSDO's is that in addition to the PPS signal, we also have all the other information. For intance it would make sense to loosen the PLL a bit when satellites enter and leave the solution, because that often moves the GPS signal a few nanoseconds abrubtly, which is enough to throw most PLLs into thinking you had a phase jump. There is also the complex 12h signal in most GPS receivers PPS, should that be notched out of the PLL so that it will not react to offsets that have a 12h period ? (obviously only in stationary applications.) So many things to try, so little time... Poul-Henning -- Poul-Henning Kamp | UNIX since Zilog Zeus 3.20 [email protected] | TCP/IP since RFC 956 FreeBSD committer | BSD since 4.3-tahoe Never attribute to malice what can adequately be explained by incompetence. _______________________________________________ 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.
