[time-nuts] Any guesses as to how Citizen is claiming ±1 second/year with using this AT-cut 8.4MHz XTAL?

[tnuts at joshreply.com]

That comes out to about 30ppb, and this is a pocket watch so they don’t seem
to depend on the temp stabilization of being attached to a human wrist. 

 

https://www.ablogtowatch.com/wp-content/uploads/2018/03/Citizen-Cal-0100-Eco
-Drive-Movement-04.jpg

 

I’ve been reading about the new watch that contains this crystal for about a
month, but just saw some more detail today


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AT-CUT QUARTZ CRYSTAL OSCILLATOR

While AT-cut quartz crystals have indeed been in production and use since as
early as 1934, the technology is more common in larger applications and not
necessarily wristwatches. To address the needs of individuals seeking only
the most accurate performance in a wristwatch, Citizen sought to apply and
optimize this available technology in a way that could serve watch consumers
on a more direct and personal level. When working to reach the accuracy of
the Cal.0100, Citizen opted for an AT-cut quartz oscillator instead of a
more traditional tuning fork shape (XY cut). Perhaps most notably, AT-cut
variations allow for greater temperature tolerances, specifically in the
range of -40°C to +125°C. Additionally, this configuration allows for
reduced deviations caused by wearer orientation, which can cause significant
changes in accuracy that aren't negligible when attempting this kind of
performance. As a result, wearers will not have to worry about errors caused
by spatial orientation and positioning becomes less of a concern. The same
can be said about durability, which Citizen also improved upon in
conjunction with the AT-cut oscillator. After all, shock experienced in
day-to-day situations could easily prove detrimental even for quartz
movements. And when the goal is an annual accuracy of ±1 second, that just
isn't acceptable.

 

https://www.ablogtowatch.com/citizen-cal-0100-eco-drive-watch-movement/

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Is this possible with an MXCO running across this wide temp range? How are
they compensating for aging at this level of precision?

 

Thanks!