In message <20120506021212.ec21a800...@ip-64-139-1-69.sjc.megapath.net>, Hal Murray writes: > >p...@phk.freebsd.dk said: >> If you cannot apply the negative sawtooth, you will get better results by >> disciplining almost any random quartz xtal, ovenized or not to the GPS, >> divide it down to PPS and then discipline the PRS10 to that. > >I don't understand that. What am I missing?
You are missing that the average of the 1PPS pulse only can be trusted to be zero over a timescale of many hours. This is an error-source distinct from GPS reception, caused by the picking a preexisting flank nearest to the epoch, with no attempt to keep the average of the resulting error zero. Imagine that the GPS receivers clock happens to run on perfect frequency for a while: That means that the flank used to generate the PPS will have a fixed location relative to the epoch, for instance always 12 nanoseconds early or late. I belive that some GPS receivers have deliberately de-tuned Xtals for this very reason, but unfortunately that is only a partial fix, as the problem is a modulus-issue, so not only is perfect frequency bad, but "perfect +/- n Hz" is equally bad. The hanging bridges Tom has plots of on leapsecond.com, arises when the frequency of the GPS xtal changes. At one point I tried putting a 1W resister close to the xtal and feed it with a very slow sine-wave to see if "jittering" it would get me an average of zero of shorter timespans. My experiment was inconclusive, but the idea is not unsound. -- Poul-Henning Kamp | UNIX since Zilog Zeus 3.20 p...@freebsd.org | 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 -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.