On 2015-02-12, Charles Swiger <[email protected]> wrote: > On Feb 12, 2015, at 1:56 AM, Rob <[email protected]> wrote: >> Charles Swiger <[email protected]> wrote: >>> On Feb 11, 2015, at 7:23 AM, Rob <[email protected]> wrote:
> >> However, what I observe is that the plots of the offset show the derivative >> of the environment temperature, which unfortunately cannot be controlled >> any better. I am considering to locate the crystal that is responsible >> for the timing and see if it could be ovenized or replaced by a more >> temperature-stable oscillator. However, one can argue that it could be >> fixed in software as well. ntpd could sense a changing drift and extrapolate >> it, if necessary helped by input from a temperature sensor. > > You're describing a TCXO; using a temperature sensor to compensate for thermal > drift would gain perhaps a factor of 5 accuracy. No, that is a hardware solution. There are software solutions-- a termistor to meaure the temperature of the crystal ( or somethign nearby) which feeds that measurement to the OS. the revised ntp then reads the temperature, and corrects the drift rate as a function of that temperature. This means that the change in the ntpd drift rate does not only depend on the offset meaured but also on that temperature. Since it takes a while for a temperature to be reflected in the offset, this makes ntpd track the correct rate of the clock much more closely. Yes, factors of 5 are easy. Actually, I suspect that oneof the reasons that chrony does so much better than ntpd does in disciplining the clock ( 2-20 times better) is because it reacts to such temperature changes much more rapidly. It can do so because it keeps a memory of the drifts and offsets and can see changes much more quickly. It also does not throw away 85% of the measurements to correct round trip errors, so can also react faster because of that. This is all without controlling the temperature of the oscillator (TCXO) but rather measuring that temperature-- much cheaper. > > Regards, _______________________________________________ questions mailing list [email protected] http://lists.ntp.org/listinfo/questions
