Re: Query about quartz 32 kHz crystals

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,
--
Nicabod =+= Waltham, Mass.
.

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