[time-nuts] Watch Crystal Aging Rates

[Magnus Danielson]

From: Dr Bruce Griffiths <bruce.griffiths at xtra.co.nz>
Subject: Re: [time-nuts] Watch Crystal Aging Rates
Date: Sat, 21 Apr 2007 11:17:05 +1200
Message-ID: <462949F1.4020201 at xtra.co.nz>

> 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

Bruce,

> 32kHz watch crystals are actually very low mass items as they use a 
> different vibrational mode to common high frequency crystals.

Thanks for pointing this out, it was anoying me. The clock crystals at 32,768
kHz is either of XY or NT cutting, which has second-degree frequency responce
with a peak at about 25 degrees C. Compare with the typical AT cut which has a
third-degree curve with a sadle point usually set at 85 degrees.

Also notice that depending on resonance mode, the way that physical dimensions
interact to give frequency vary alot. Length, thickness and width (and for AT
cut, overtone number) come together in different forms. 

Cheers,
Magnus