David Andersen wrote:
Local stable crystal: Actually, you could make it more than stable
enough, but it would exceed your power requirements, because you'd
probably fall back to an oven controlled oscillator. There goes your
battery. But why did you try your initial experiments with 32.768Khz
watch crystals? You're much more likely to find a good, solid 10Mhz
reference with an SC cut TCXO. For instance, that maxim IC you
mentioned has +- 2ppm, which is really quite awful by instrumentation
standards. Compare to this one:
http://www.bdelectronic.com/frequency/oscillatorTCXO.html
.3ppm tempco, +- 1ppm/year. They don't show their overall allen
deviation curves, but you get the idea - it'll be within 1ppm by the
end of the year, and since that aging will probably happen over time,
I'd guess it would probably get you something like 10 seconds within a
year. Or something like:
http://www.vectron.com/products/tcxo/tc140.pdf
(... which is probably expensive, but which you can get in 0.2 ppm
accuracy vs. temperature and <2ppm/10 years).
-Dave
Dave,
The requirement that you seem to have missed is the 18 month battery
lifetime. A 10 MHz oscillator is a couple milliapmeres, so it won't do
the trick. The watch crystal needs only about 10 microamperes to
oscillate.
Mike,
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.
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.
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.
You should be able to evaluate the feasibility of using a compensated
crystal based on the above.
_______________________________________________
time-nuts mailing list
time-nuts@febo.com
https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts