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

William Unruh unruh at invalid.ca
Thu Feb 12 17:11:35 UTC 2015


On 2015-02-12, 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:
> Yes I would prefer that, but chrony does not support local references
> so it is useless to me.

Yes, it does and has for about 3 or 4 years now. 


>
>>> In practice a changing drift is often caused by changing temperature,
>>> and it would be better to take the first derivative into account as well.
>>
>> Certainly it is true that changing temperature will cause a change in crystal
>> frequency, on the order of ppm's to tens of ppm's per 10 C temperature change.
>>
>> But if you're willing to tolerate over 500ppm systemic error, why worry about
>> a second-order effect in the 10s of ppm's?
>
> Those two things are not related.  I have some systems that I need to
> keep within a few microseconds (with PPS) and those do not have that
> 500ppm error (none of my systems do), they usually within about 30ppm.
>
> 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.

ntp is a very simple feedback loop. It does not have a memory except for
current drift. It never extrapolates. 
Note that versions of ntpd with temperature input have been created and
posted over the years, and they do result in a much tighter control of
the time. 
I do not have a reference to them but googling or waiting for others on
the list will supply the references quickly.



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