Quartz Crystal Design Notes

"Series" resonant crystals are intended for use in circuits which contain no reactive components in the oscillator feedback loop. "Parallel" resonant crystals are intended for use in circuits which contain reactive components (usually capacitors) in the oscillator feedback loop.

Oscillator Theory of Operation

Crystal controlled oscillators may be considered as consisting of an amplifier and a feedback network that selects a part of the amplifier output and returns it to the amplifier input.

Quartz Crystal Theory Of Operation

Quartz crystal units serve as the controlling element of oscillator circuits by conversion of mechanical vibrations to electrical current at a specific frequency