[ntp:questions] Red Hat vote for chrony

David Lord snews at lordynet.org
Tue Dec 9 21:02:50 UTC 2014

William Unruh wrote:
> 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).

My xtal book gives either parabolic or lazy-s curves but the
real problem with cheap crystals is that the turning point or
flat sections can be way off the ambient temperature.


>> Regards,

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