[time-nuts] Low cost synchronization
mikec at ciholas.com
Thu Aug 18 15:45:06 EDT 2005
On Thu, 18 Aug 2005, David Forbes wrote:
> The 32K crystal may be usable, but you'd have to put some
> effort into the design to get the temp compensation tuned to
> the particular crystal, and you'd have to grade the crystals
> for tempco in the mfg stage. That might be doable in quantity,
> if you come up with the right sort of computerized test fixture
> in an oven.
All of the temp compensation can be in software and "after the
fact". I don't need the crystal "tweaked", I just need to know
what numerical corrections I need to apply to the counter. Thus
it becomes zero electronics (besides some temp sensor) and only
> I have built a few nixie tube wristwatches using the cheap
> 32KHz crystals, so I have direct experience in this matter.
> (Has anyone else on this list built an electronic wristwatch?)
> Getting the crystal adjusted to 1ppm is not too hard. You'd
> have to temperature compensate it to get to 0.1 ppm, and that
> would be limited to perhaps 10C-30C temperature range.
You give me more hope than I had previously. I understand how to
capture the initial tolerance (operate the device at the factory
and record the variation in internal memory). I know how to
correct the temperature curve (the parabolic deviation away from
25C). But I am left with aging as a concern. Most of these
crystals claim aging to be +/- 3ppm the first year. That's +/-
1.5 minutes for a year which is unacceptable. Maybe crystals
from one batch all age the same, maybe they age based on
shock/vibration (which will vary from unit to unit). I just
Here is an example datasheet:
> It's a lot easier to compensate the crystal if it's worn on the
> wrist rather than sitting in a car, since a person's wrist is
> essentially an oven. The real world has ridiculous temperature
> extremes - don't even think about stabilizing a crystal used
> outdoors unless it's thermally connected to a human.
Just like someone who leaves their watch in the car on a sunny
day, I can't be sure the device will be in a temperature stable
environment. The best I can do is try to model the temperature
effect and correct for it. Hence the temp curve in the
datasheet. Tracking it to within +/- 0.1ppm seems tough.
> You should be able to evaluate the feasibility of using a
> compensated crystal based on the above.
Yes, I think it is going to be possible to achieve +/- 1.0ppm. I
think +/- 0.5ppm can be done with some hard work. I don't think
+/- 0.1ppm (+/- 3 seconds/year) is realistic.
Mike Ciholas (812) 476-2721 x101
CIHOLAS Enterprises (812) 476-2881 fax
255 S. Garvin St, Suite B mikec at ciholas.com
Evansville, IN 47713 http://www.ciholas.com
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