On 12/19/2011 5:34 AM, Chuck Harris wrote:
It is not clear to me that a 32KHz xtal is any less stable
than a 262KHz xtal, though. I would think there would be a lot
more to be gained by using a microprocessor/thermistor to
measure the temperature within the watch, and provide an
adjustment to compensate for the xtal's natural temperature
sensitivity.
Chuck, I have recently made a couple of Real Time Clocks for my homebrew
radios using the Maxim DS3232 IC which has a built in TCXO at 32+ kHz
doing what you describe, the temperature is checked at intervals and the
corrections applied. The performance is excellent, once set, getting to
what I term "Harrison Level", i.e. less than a second a week error.
The downside is that the standby battery demand is pretty high and my
first builds using a Lithium button cell ran for only around a month
max, so I had to go over to NiMH rechargeables. I suspect using this
technology in a watch one would have the same problem.
The best crystals for room temperature use are around 4 MHz with
temperature inflections around 20C, and this is what was used in an
early Braun alarm clock I had which also had this kind of performance.
Long gone, alas.
Dan
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