A couple of points here.
Yes, a 10MHz oscillator will severely blast the budget - power wise. But
you should remember that 10MHz is also not actually a good frequency for
temperature coefficient of crystals - except for SOME SC cut types.
Generally speaking the zero tempco rollover frequency for many SC's and
most AT's is between 4.5 and 5.5MHz. You can actually get very good
tempco's by running two oscillators either 500kHz or 1MHz apart and mixing
them. Mounted in a thermal mass but without an oven they will oven give
better than TCXO and moderate oven type performance.
Real time temperature compensation is actually quite common. In several
radio applications I have designed systems using two techniques.
1.. Have the manufacturer supply AT crystals with carefully controlled cut
angles - for a given cut angle the tempco will not vary much. Then use a
simple compensation table based on angle.
2.. Learn the crystal characteristics. In a couple of chamber runs measure
the characteristics of each crystal. You probably only need to do 20
temperatures to cover -10 to +60C. Work out the slope and using some nice
interpolation algorithm work out a correction table. Then voltage control
the Xtal osc to keep the frequency on target.
#2 works well enough that a pretty standard crystal will deliver TCXO or
better performance because very few TCXO's use active compensation.
But really, can we fit all of this ins key fob? Nope! I suspect in the best
research tradition the OP needs to look at WHY his application demands
synchronization at this sort of level. In one project where my client
initially asked for timing accuracy of this order, we found that we could
collect data with timestamps and temperatures, and when we looked at the
logger and the data we could measure the offset for a particular logger and
then back correct the timestamps on the data. In that case we used a
correction system like #1 above, but we had something like 150x50x10mm to
work in, not a key fob!
John
At 02:18 PM 8/18/2005, you wrote:
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.
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