>>What I don't understand is that these are parallel cut crystals,
>>the type I use all the time with micros (which require parallel
>>cut), yet the series circuit seems to work so much better

>I'm no expert on crystal oscillators; so I could be wrong here.  But
>AFAIK there isn't anything structurally different between "parallel"
>resonant and "series" resonant crystals; all crystals have parallel and
>series resonant frequencies, as I understand it, and a particular unit
>is defined as parallel or series simply because that's the resonant mode
>that'll give you the frequency that's marked on the can.

Correct. There may be slight differences in the processing to change the ESR
of a series mode specification, versus the parallel resonant frequency at a
certain parallel capacitance. In parallel resonant mode the processing will
most likely include minimising the close (and harmonic) spurs that the
crystal may try and oscillate at. If you have a look at the picture in the
Fairchild App note and expanded that to a wider frequency, you would see
more of the same reactance's (although of different amplitudes) quite close
to the nominal resonant frequency. It is always worth having a look at a
crystal using a spectrum analyser with tracking oscillator - it makes you
wonder how the thing ever finds the correct frequency. Marconi used to have
an App Note that showed how to do this with the 2370 Spectrum Analyser that
they built back in the 1970's.

>Either mode will "work" but only the mode specified will give the right
>frequency.  The series mode resonance and parallel mode resonant
>frequencies may be very close together.  I think.

Correct again. It may also be worth remembering that the old oscillator IC
for the 8080 series micros had a tank circuit to ensure the crystal started
in the correct mode.

Typically to have a crystal work in series mode will require a series tank
circuit (both C and L) to force it into series mode - especially running in
overtone mode.

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