[ntp:questions] NTP offset doesn't change.

David Lord snews at lordynet.org
Fri Feb 13 01:20:03 UTC 2015


William Unruh wrote:
> On 2015-02-12, Charles Swiger <cswiger at mac.com> wrote:
>> On Feb 12, 2015, at 1:56 AM, Rob <nomail at example.com> wrote:
>>> Charles Swiger <cswiger at mac.com> wrote:
>>>> On Feb 11, 2015, at 7:23 AM, Rob <nomail at example.com> 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.

Solutions that measure the temperature require calibration
for the individual crystal as with the cheap crystals used
the drift per deg C can be either positive or negative and
also depending on "cut" of the crystal can follow a
parabolic or "lazy S" curve.

The alternative of fitting a simple heater with temperature
control to the crystal seemed to be more effective and with
pps ntp source the offset was < 300n.

There might still be examples on the web where this has been
done but the references I used have long since been taken
down.


David



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