One for the Experts - What are quartz timing crystals shaped like?

Isthmus said:

I was reading with interest the thread about Bulova's and tunning for watches, and i started thinking about how the timing mechanism on a quartz watch differed from a tunning fork. More specifically I remembered that under specified currents, a quartz crystal could be made to oscillate at very specific frequencies. What I don't remember reading was how exactly this was achieved and what shape did these quartz crystals have. I wonder, are we talking about quartz crystals shaped like a tuning fork or something else?

Does anyone know? If anyone has any pictures of one that would be wonderful.

Thanks in advance.

Click to expand...

Hi Gabe,
Interesting question. I've got an early-mid 70's Seiko Quartz Information Booklet, which was intended to educate salespersons about this new-fangled quartz thing. Here is a good picture from it, showing crystals in various stages of manufacturing. They are from a Seiko 38xx, which has a humongous quartz canister (and crystal inside) compared to current production canisters nowadays (which are almost entirely tuning fork shaped as well. I'm sure they refined the methods of production since then, so the appearance is most likely a little different, too):



It is interesting that the 38xx series quartz crystals vibrated at 16, 384hz, compared to the now more common 32, 768hz. Seiko made available for after-sales servicing a replacement circuit starting around December 1978 for the 38 series that had a much smaller quartz canister and vibrated at the higher frequency. I've yet to see one of these service replacements, but I'm sure they are out there:
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I've also got some pictures of a current curiosity to me. Most quartz crystals are vacuum sealed inside a cylindrical, metal canister. Not so with the 5855 I have from the late 70's. It has a ceramic-looking rectangular compartment for the crystal, sealed on one side with a see-through medium (like epoxied-glass). I have no idea why this is, and the tech guide I have for it is entirely in Japanese, so I can't understand what it says about the circuit/crystal. If there are any technically-minded translators out there, I've got a few pdf pages that might shed some light on this:
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And up close:
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I've never wanted to destroy a quartz canister just to look at the quartz crystal, so it was a real treat to see this when servicing the above 5855. The 58xx series also has a twin quartz version (maybe 9256?), so maybe this is part of the answer, but I've yet to see where the second quartz crystal is in that caliber- I can't recall being able to find movement pics online of it.


If the idea of introducing electricity to a mineral and counting it's vibrations in order to keep time in a wrist watch does not boggle your mind, what does? (Rhetorical question)

Isthmus said:

I was reading with interest the thread about Bulova's and tunning for watches, and i started thinking about how the timing mechanism on a quartz watch differed from a tunning fork. More specifically I remembered that under specified currents, a quartz crystal could be made to oscillate at very specific frequencies. What I don't remember reading was how exactly this was achieved and what shape did these quartz crystals have. I wonder, are we talking about quartz crystals shaped like a tuning fork or something else?

Does anyone know? If anyone has any pictures of one that would be wonderful.

Thanks in advance.

Click to expand...

This is a picture of an early crystal used in timing for a computer, it oscillated(not any more though ) at 32768Hz just like most quartz watch crystals do.
There are numerous crystal shapes in use and one of the most common is the 'tuning fork' type pictured in Noah's reply...


1. This is the actual crystal(usually silicon)
2. This is one of the pads through which an electric current is passed to an identical pad on the back side of the crystal
3. These are the electrical terminals that connect the crystal to outside circuitry

The shape of the crystal only has a small affect on it's resonating frequency; it's the thickness that is most important. The piezo affect happens when the crystal is bent or deformed then released. When it is released, it produces an electric current. Conversely, if it has an electric current applied to it, it will bend or deform. The thinner the crystal; the faster it can bend and unbend.

Modern crystals are built to several levels of accuracy, measured in 'ppm'. The smaller the 'ppm' the more accurate it is. This accuracy is usually achieved by laser trimming the crystal until it can oscillate at it's specified frequency.