Re: Query about quartz 32 kHz crystals

In article <d_-dnRTFzf65t67XnZ2dnUVZ_uSdnZ2d@xxxxxxxxxxxx>, Nicholas
Bodley <n_bod_ley@xxxxxxxxxxxxx> wrote:

On Thu, 11 Jun 2009 05:32:50 -0400, zeno333 wrote:

In the mid 70s, some of the cheap quartz watches just used a slab of
quartz that was in the open air and not even in a metal container....i
rememebr i got a 20 dollar Litronix Avatar LED watch around 1975 that
was made this way. Needless to say, changes in outside temperature
effected this watch big time.

Very interesting! Pre-WW II crystal holders had screws to disassemble
them, and radio hams sometimes would grind the crystal slab to raise (?)
its frequency (or lower it, by grinding differently?). I suspect that
hermetically sealed enclosures came into wide use after WW II.

I have a hunch that Litronix didn't have a lot of capable engineers.
Whatever the case, they must not have known how to make quartz crystal
[resonators] so that temperature changes don't affect them much. I once
read that watch crystals have a parabolic curve of temp. vs. frequency,
but that seems odd. The "flat" part of the parabola was set to be at
typical operating temperature.

Given a single crystal of quartz, there are numerous different ways to
cut a slab (or fork) to obtain various behaviors. They have letter
designations.

Hewlett-Packard made a lab. instrument some decades ago with a specially-
cut crystal that made its frequency be temperature-dependent in a well-
defined and stable way. The crystal was housed in a probe with a cable.

Another temp.-stable quartz oscillator inside the instrument itself
provided a frequency reference, and the difference between the two
frequencies, as I recall, was simply the probe's temperature, multiplied
by a conversion factor.

To keep a quartz crystal's frequency more or less independent of
temperature, one way is to put it into a little oven with a temperature
control; idea is that in any expected operating condition, the oven will
be hotter than its surroundings. That's the traditional way; radio
broadcast transmitters used them.

Some modern oscillators have a temperature sensor, and circuitry to tune
the crystal to keep it much closer to desired frequency than it would be
"barefoot". These are referred to as TCXO's, for "temp.-compensated..."
They use far less power than an oven, and are a lot smaller and less
costly.

Yes, crystals can be tuned, although not by much.

Regards,

I suspect that Litronix wanted to save some money on the watch I
got....it was the first ever all stainless steel case LED wtach that
sold for 20 dollars, so they probably cut corners on the quartz crystal
by having it in the open air like it was.
.

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