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 [EMAIL PROTECTED] Evansville, IN 47713 http://www.ciholas.com _______________________________________________ time-nuts mailing list [email protected] https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
