Can someone point me to a detailed explanation of this, though? I know we
have been through this before, but I didn't find it very satisfying
(especially now that I have looked into it a little further).

When you put the xtal in this configuration, with capacitance to gnd at
either end, it really is not running at either resonance (series or
parallel), but has to look inductive in order to cause a 180 deg phase
shift from a resistive source feeding the xtal/cap network to a resistive
load on the other end. It looks more like a pi network than an xtal with an
effective parallel capacitance, as far as I can tell.

I did an analysis of this with the help of spice once and, IIRC, I found
that depending on several factors (perhaps the load and driving
resistances, and the difference between the capacitance on either side) you
could cause a net 180 deg phase shift through the network over a range of
frequencies (just like a variable xtal oscillator does), so that actually,
if the frequency printed on the can is intended to be the exact operating
point, they must have made some assumptions about the external circuit in
addition to just specifying one (net) capacitance value.

Sean


At 06:51 PM 2/24/2003 -0500, you wrote:
>At 06:42 PM 2/24/2003 -0500, you wrote:
>> > Note quite. It's actually the net capacitance that's expected to be in
>> > parallel with the crystal. This includes the series combination of your
>> > external capacitors, plus the distributed capacitance of the PCB traces
>> > and the chip pins.
>>
>>Are you really sure, Dave?  I thought (although can't find reference right
>>now) that the capacitance rating was the load capacitance required on the
>>output of the crystal to produce the proper phase shift at the rated
>>frequency.  I further thought that the capacitor on the input side of the
>>crystal was only there to soften the edges and reduce the harmonics from
>>the driver.
>
>Yes, it IS correct, and I think we've been through this before.
>
>Best regards,
>
>Spehro Pefhany --"it's the network..."            "The Journey is the reward"
>speff@interlog.com             Info for manufacturers: http://www.trexon.com
>Embedded software/hardware/analog  Info for designers:  http://www.speff.com
>
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