[ntp:questions] Red Hat vote for chrony

William Unruh unruh at invalid.ca
Tue Dec 9 19:38:06 UTC 2014


On 2014-12-09, Charles Swiger <cswiger at mac.com> wrote:
> On Dec 9, 2014, at 2:41 AM, Terje Mathisen <terje.mathisen at tmsw.no> wrote:
> [ ... ]
>>>> Yes; you're describing calibrating a temperature-compensated XO, or TCXO.
>>> 
>>> There are also versions of ntp which have a temp
>>> compensation/measurement system compiled in to apply to the clocks. It
>>> does tend to give much better control of the clock than regular ntpd
>>> apparently.
>> 
>> It does help:
>> 
>> On motherboards with a temperature sensor close to the master crystal, you can get somewhere in the 2-10x range improvement in the size of temperature excursions.
>
> I'd agree with this, although the best case is probably not quite an
> order of magnitude, more like a factor of 5x.  Or perhaps I shouldn't
> be too optimistic about how bad a really cheap crystal can be.  :-)
>
>> The correct solution is of course to not depend on $0.10 crystals as the time base for dedicated NTP servers. :-)
>
> Well, yes.  You can get a PCI(e) card with a TCXO or OCXO and an
> optional GPS module like the Beagle ClockCard or a SpectraCom TSync
> for a few hundred bucks.
>
> That's quite a bit more than a $40 GPS puck, but these will also
> freewheel for a lot longer before losing or gaining a second in
> error: ~2 seconds/month if kept stable at 23C, I believe one said.

I suspect even the cheap ones can do that if kept stable at 23C. 
(that is about 1PPM) And if you could put a fast thermal probe onto the
crystal, you could probably do as well even in a flutuating environment
with an addition to ntpd/chrony to use the temp data to compensate the
clock rate. Then it would be really useful to keep a long string of data
on the offsets and the temp to get a better set of coeficients for the
temperature dependence of the rate. 
Does anyone know in general what fraction of the variablility of those
cheap crystals is due to temp, and how much is due to other
sources(crystal defect motion for example, or capacitor aging drift).

>
> Regards,



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