You guys are starting to get way off target here. Removing the caps
from your circuit doesn't make it series resonant. The resistor
across the amplifier terminals is selected so that it has minimal
effect on the crystal parameters while still biasing the amp. A
ceramic resonator works in the same way as a crystal, has parallel &
series resonant points, but all parameters are much looser than a
quartz crystal.

Back to this parallel series thing. At the series resonant point, the
L & C components of the crystal cancel, leaving just the R. This is
the same as a series RLC circuit you build out of an R, L and C
(hence the name). The remaining resistive component has no phase
shift (just like a resistor). So at the series resonant frequency,
what comes out of the crystal is the same phase as what went in.

At parallel resonance the inductive component of the crystal forms a
parallel resonant circuit with its own Cp (the plating on the
electrodes) in parallel with the two caps on the outside and the
amplifier (effective) capacitance. At this point what comes out of
the crystal (circuit) is 180 degrees phase shifted from what goes in.

In order to make an oscillator you need a total of 360
degrees phase shift. The inverting amp gives you 180 degrees and a
parallel resonant crystal gives you another 180 degrees.

To use a crystal at the series resonant point you must provide an
amplifier with the 360 degree phase shift needed. The faster you go,
the harder this gets because any time taken to get around the circuit
equates to a phase shift.

The crystal has inductive properties of 100's of Henries so
if you just consider that as an L and add the two capacitors, it
isn't a far leap to the frequency selection section of a Colpitts
oscillator.

If you don't fit the external capacitors you are still left with the
internal capacitance plus the amplifier capacitance. ie a parallel
resonant circuit but at a different load capacitance.

Steve.

> I was the guy who was talking about a parallel resistor, instead of the
> two caps, and of relatively low value. In this case, its role is to
> destroy the Q of the crystal when parallel resonating such that the series
> mode will be dominant.
>
> I don't think that ceramic resonators apply for this, because they use a
> different principle of operation, and do not have a series mode at all (or
> rather, they better not have one). Also I think that the parallel resistor
> is not needed at all for this, especially with a PIC, that lready has
> such an apparatus internally.
>
> BTW I've also used the series method with parallel R on other chips, such
> as NS16550A at 8 MHz. It always seems to work for me ;)

======================================================
Steve Baldwin                Electronic Product Design
TLA Microsystems Ltd         Microcontroller Specialists
PO Box 15-680, New Lynn      http://www.tla.co.nz
Auckland, New Zealand        ph  +64 9 820-2221
email: steveb@tla.co.nz      fax +64 9 820-1929
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