Hi all, If I have an NTP server with a refclock that is assumed very accurate (~few nanoseconds/day drift at most, with << 1 ns jitter), how well should NTP be able to keep the system clock "on time"?
I'm using a White Rabbit (WR) [1] SPEC PCI-card that receives accurate time over WR, and I wrote a shared-memory refclock driver [2] for it. I added it to ntp.conf with server 127.127.28.0 minpoll 4 maxpoll 4 and I get the following graph. The error is system time compared to WR-time, where we can assume that WR-time is very accurate. http://www.anderswallin.net/wp-content/uploads/2013/09/ntp_graph.png Are the minpoll and maxpoll parameters the only way to tune how NTP locks onto a refclock? To me the trace (blue in the graph) looks quite noisy and I would guess it could be improved by "PID-tuning" inside NTP? As a comparison the red trace shows another computer using standard NTP without a refclock and the trace is much smoother. If it matters I measured the free running system clock on this computer to have an error of about 40 ppm [3]. thanks, Anders [1] http://en.wikipedia.org/wiki/White_Rabbit [2] https://github.com/aewallin/ptp2ntpd [3] http://www.anderswallin.net/wp-content/uploads/2013/07/freerunning_vs_ntp_2013jul26.png _______________________________________________ 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.