[time-nuts] Crystal Ageing

[Luis Miguel Brugarolas]

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 
worst.

Best regards
Luis Miguel