piclist 1998\10\26\164318a >
Thread: Crystal Tutorial
www.piclist.com/techref/index.htm?key=crystal+tutorial
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face BY : Scott Dattalo email (remove spam text)



On Mon, 26 Oct 1998, Peter L. Peres wrote:

> 1) Temperature stabilization. The two obvious ways are crystal oven and
> thermistor controlled varactor. The oven is often ruled out due to power
> constraints, but the one-thermistor method could be used. Commercial
> expensive thermistors are ruled out. So I'd like to see how a transistor's
> 2.2 mV/K could be used for this while being itself the varicap (Ccb ?).
> Other ways ?


Mike Keitz posted a PIC-PLL that varied the pic's oscillation frequency
based on this principle (i.e. using the variable capacitance of a reverse
biased diode in a tuned circuit to change the resonant frequency). If
you're striving for oscillator accuracy it might be more desirable to use
this phase locking technique along with another closed-loop highly
accurate analog system. For example, if you had a means by which it was
possible to accurately measure frequency then you could use this circuit
to vary the diode's capacitance to compensate for errors. One such system
that comes to mind is a V/F converter. Of course, this isn't as elegant as
a 2N3904... but then again, if temperature is the only source of error
then it would be trivial to measure the temperature and compensate for
it's affect using the variable capacitance of the diode.

BTW, if you subscribe to Electronic Design you may want to check out last
week's 'Ideas for Design' section. They feature a 'Variable-Capacitance
Diode Phase Modulator' - a name I'm sure I heard on a Star Trek episode.
The diode (or bank of them I should say) is a good ol' 1N4003.


Scott

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