[time-nuts] New precision watch

[Dan Rae]

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