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Thread: Crystal Tutorial
www.piclist.com/techref/index.htm?key=crystal+tutorial
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face BY : John Payson email (remove spam text)



>>
Okay, I think I'm starting to understand this.  Basically the idea
is that you'd like what the inverter is trying to do to its end of
the crystal to match as closely as possible what the crystal wants
to do itself.  If there's a resistor between the inverter's output
and the crystal (and there's always inherent resistance if nothing
else) a properly-tuned crystal circuit should have essentially
identical waveforms on both sides of it.  If the circuit is tuned
too fast or slow, this may be seen by the inverter's output leading
or lagging the crystal's signal.  The crystal will resist being
pulled off-frequency, but the poor tuning will increase power con-
sumption, reduce frequency accuracy, and lead to increased wear on
the crystal.
<<

Sorry to respond to my own post, but I'd like to know
if I've finally figured out how selection of caps should
work, etc.

The PIC will try to operate as a fairly linear amplifying
inverter.  As the amplitude increases, it's necessary that
the PIC's gain decrease (since if the net gain around the
loop were greater than one, the oscillations would grow
exponentially); this is the cause of the slight "flat top"
observed on the PIC's output.

Because the PIC's output will try to match the input (but
inverted) the two signals should be precisely 180 degrees
out of phase.  If the phase difference is greater or less,
then the R in the circuit (real or inherent) will have to
absorb the difference between what the PIC's output is try-
ing to do and what that end of the crystal is actually do-
ing.

Using a scope, it should be possible to see if the caps are
selected appropriately: place a 1M resistor in series with
each probe, and then look at the two sides of the crystal.
The resistor will cause phase shifting on the scope input
as well as a significant loss of amplitude, but both of the
scope inputs will be affected equally.  If the caps have
been chosen correctly, the two signals should appear 180
degrees out of phase.  If this is achieved, circuit power
consumption will be minimized, and so will EMI emissions
(since the two caps will be trying to feed roughly equal
and opposite currents through their ground returns).

The biggest question I'd still have, then, would be how to
select the best value for the caps from among all those that
produce the perfect 180 degree phase shift.  Would the ideal
value be one where the ratio of the caps roughly matched the
ratio of the signal strengths (so as to best balance out the
ground currents)?


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