[time-nuts] Crystal Ageing
Luis Miguel Brugarolas
lmbruga at ieee.org
Wed Oct 10 14:26:50 EDT 2007
On Wed, 10 Oct 2007 07:34:30 +0200, Bernd Neubig wrote
>There is much more to say, but I will stop here, hoping that this
>contribution gave some time nuts a better understanding - and may kill
>their firm believe into aging predictions ;-)
I have been working hard on adaptative algorithms to predict OCXO ageing, and
I completely agree with Bernd.
Besides the fact that ageing is actually random (althoug it can be linearly
modeled for time intervals of days, may be a week), there is an effect that
should be added to ageing: temperature variations. Depending on many factors,
temperature effect can be orders of magniture more significant than ageing.
Moreover, temperature change (altough small in a good oscillator) produce non
linear effects: change temperature and go back to original one: frequency
will not be the original. It is beautiful to perform an experiment: measure
temperature and oscillator frequency and change temperature smoothly (e.g.
normal room day/night variations). Revove ageing from frequency measurements
and make a temperature/frequency X-Y plot. Do not expect a line, but
something simmilar to a cloud. It slongly depends on oscillator and the
amount of temperature variation.
My experience is that nice prediction ('nice' definition: time drift lower
than, say 1 us) is difficult to maintain for more than few days, but this
*strongly* depends on temperature variations and oscillator characteristics.
Please notice that 1 us in one day is a really good figure: these all
discussions depend on how good matching you want to obtain.
I have run all my work with free running OCXOs. I have no direct experience on
disciplined oscillators, but I suspect that non linear effects may be even
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