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Hi Dan:

It turns out the the aging of a quartz crystal is caused by molecules
either attaching to are being knocked off the surface of the crystal.
So the larger a crystal is mechanically the less it ages.  That's why
the 32,768 Hz. crystals have the lowest aging rate of any crystal I know of.

You can get around aging drift by running the crystal for a long time
and characterizing the aging rate.  Then some electronics can
dynamically change the division ratio used to get to a second as the
crystal ages.  This may in fact be done in some high end watches.
Increasing the crystal frequency has an advantage in that the aging
adjustment can be done in finer steps.

Temp compensation is also very important for good timekeeping.

Have Fun,

Brooke Clarke, N6GCE
http://www.precisionclock.com
PS I've been working on setting the rate of a Cesium standard for the
past 4 months. But the closer to perfect you get the longer it takes to
know how well you're doing. Started out watching for hours, then days,
not weeks. This is caused by only being able to see tens of nanoseconds
of change.


Dan Allen wrote:

> On Mar 17, 2005, at 8:23 PM, Pierre Brial wrote:
>
>> Other high-end quartz which claim an accuracy of less than 20 sec a
>> year are Seiko "Grand Seiko", Longines VHP, Breitling Colt and Rolex
>> Oysterquartz. All of them are battery powered.
>
>
> The Seiko SLL033 I have is a perpetual chronometer (automatic day and
> leap year) that I measured to be very accurate, about 1.5 ppm on
> average over 6 years, which is about 47 seconds a year.
>
> An Omega 1552.30 I have is even better at about 0.5 ppm or 15 seconds a
> year, also on average over 6 years.  I do not know if they still make
> it.  It is about 8 times more expensive than the Seiko.
>
> Both of these watches run their quartz crystals at 4-6X the usual 32768
> Hz rate.  I bought the watches in 1999.
>
> Thermally compensated twin-quartz watches used to be made by Seiko and
> Omega in the 1980s and they may have even been more accurate.  Recent
> designs have neither used multiple quartz oscillators nor have they
> been thermally compensated, which are both big wins for the best
> accuracy.
>
> Dan
>
>

   

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