Terje Mathisen <terje.mathi...@tmsw.no> wrote: > The 500 ppm limit is not at all arbitrary! > > In fact, it was originally just 100 ppm, but when too many systems > turned up with a system clock which was a bit too far out, Prof Mills > redid the control loop to allow a 500 ppm range. > > It could have been a lot more, but the ultimate stability of the control > loop is supposed to be better this way.
I think it is hogwash. The static drift cannot have any influence on the stability of the control loop. A static drift is just a frequency error that is determined once and can then be forgotten about. What is important is the short-time changes in the drift, the unstability of the frequency. THAT can influence the control loopt. But that is not at all what NTP restricts. I can fully understand that ntpd cannot control a clock that is -500ppm now and +500ppm the next hour, but it should not have any problem controlling a clock that has a +5000ppm error that stays the same all the time. In fact, it is mostly the job of the kernel to do that. I remember that in the past sometimes a specific adjtime command was used to pre-configure a static error so that ntpd would not see it. No idea if this still works. Note that that "carefully designed control loop" does not even handle the derivative of the drift. When everything is stable the offset neatly wobbles around zero, but when there is a linear change in temperature that results in a linear change in drift, the control loop maintains a constant error (determined by the gain of the loop), it does not realize that it is handling a changing drift and pre-compensate for it. _______________________________________________ questions mailing list questions@lists.ntp.org http://lists.ntp.org/listinfo/questions
