[time-nuts] Watch Crystal Aging Rates

Dr Bruce Griffiths bruce.griffiths at xtra.co.nz
Fri Apr 20 19:17:05 EDT 2007


Brooke Clarke wrote:
> Hi:
>
> I've heard that the aging rate of crystals gets better as the frequency gets 
> lower.  The idea is that the more mass in the crystal the less an atom here or 
> there will be missed.  So the 32768 Hz watch crystals should be very good 
> compared to a 10 Mhz crystal.
>
> Does anyone have any data on watch crystal aging?
>
> If there was a known aging rate then it could be backed out in the countdown 
> circuit resulting in much better accuracy.
>
> Have Fun,
>
> Brooke Clarke
> w/Java http://www.PRC68.com
> w/o Java http://www.pacificsites.com/~brooke/PRC68COM.shtml
> http://www.precisionclock.com
>
>
>
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>   
Brooke

32kHz watch crystals are actually very low mass items as they use a 
different vibrational mode to common high frequency crystals.
As such there aging rate isnt spectacularly low. Also their temperature 
coefficient is comparatively high.
However they are usually designed so that there is a stationary point in 
the frequency vs temperature curve at the temperature experienced by a 
wrist watch when worn on the wrist. Thus instead of an oven they make 
use of the wearers intrinsic thermal regulation.

Bruce




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