[time-nuts] Low cost synchronization

[Mike Ciholas]

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 
software.

> 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 
don't know.

Here is an example datasheet:

http://www.ecsxtal.com/pdf/ecs-3x8.pdf

> 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
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