[ntp:questions] Red Hat vote for chrony

Charles Swiger cswiger at mac.com
Tue Dec 9 21:32:47 UTC 2014


On Dec 9, 2014, at 11:38 AM, William Unruh <unruh at invalid.ca> wrote:
>> 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).

For a specific example, Beagle documents that in their specs, ie:

  http://www.beaglesoft.com/clcaspecs.htm

"The precision of the ClockCard is entirely dependent on the quality of the oscillator circuit. There are three sources of error in the oscillator:  (1) calibration error, (2) temperature stability, and (3) aging.  Understanding these will allow you to estimate the precision of the ClockCard in your application.

Calibration error:  The ClockCard oscillator is calibrated at the factory to within 1 PPM part per million) of its specified frequency at room temperature (23° C or 73° F).

Temperature Stability:  The frequency of oscillation of crystal oscillators is highly dependent on temperature. The oscillator used in the ClockCard has an extremely low temperature dependency of 5 PPM from 0° C to 50° C (32° F to 122° F).  Since the oscillator is calibrated to 1 PPM at room temperature (23 °C), it will only exhibit 1 PPM error if its environment is held to this temperature. The worst case condition is if the temperature of the ClockCard is held at one of the extremes, 0 or 50 °C. At these points, there will be an error of 5 PPM. If the temperature variation covers a smaller span, less error will be exhibited.

Aging:  All crystal oscillators have an aging characteristic. The crystal used in the ClockCard uses the coldweld manufacturing technique, which exhibits the lowest aging characteristic of 1 PPM per year.  In practice, this aging rate improves significantly with time, but for practical purposes the value of 1 PPM is adequate."

...and that's for a good crystal used as a TCXO.  Cheaper crystals are usually going to be worse.

If you meant more in general, Wikipedia has collected decent data here:

  http://en.wikipedia.org/wiki/Crystal_oscillator

...but if you wanted published reference data, perhaps the NIST docs here:

   http://tf.nist.gov/general/pdf/906.pdf
   http://tf.nist.gov/general/pdf/581.pdf

Regards,
-- 
-Chuck



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