A series crystal is to be used with a series capacitor as specified. Many
PLLs (f.ex. TSA5xxx from Philips) use series crystals. S.c. have an
advantage ofer parallel circuiits in that they are MUCH harder to pull or
influence by stray circuit impedances. Also many PAL/NTSC color decoders
and TV jungle circuits use parallel crystals at Fsc or 2*Fsc or 4*Fsc. RFI
radiation is kept to a minimum if the layout is not too bad. The most
often used oscillator scheme with s.c. is the Lambda diode (usually
integrated) oscillator (negative resistance). The disadvantage is smaller
signal (voltage wise). This is not a problem if you know how to use it.

Peter

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