|The Capacitance external
to the crystal contained within the feedback loop of the oscillator circuit.
For "parallel" resonant crystals (see below for discussion on parallel vs.
series resonant crystals), the value of load capacitance needs to be specified
by the customer to insure initial Frequency Tolerance. For "series" resonant
crystals, load capacitance is not used. Load capacitance can be measured
as the amount of capacitance across the crystal terminals on the PCB. A
parallel resonant mode crystal needs to have a load capacitance (CL) specified
when ordering (this was already said above). The approximate crystal loading
for a given circuit can be determined from the formula (SEE below for more
CS is the stray capacitance of the circuit and the input/output capacitance of the inverter or microprocessor chip at the Crystal 1 (C1) and Crystal 2 (C2) pins, plus any parasitic capacitances. CS may be assumed to equal 5 pF. Most crystal manufacturers will specify standard parallel resonant load capacitances of either 18 or 20 pF. These values have been found to provide for good Frequency Tolerance in most circuits. Changes in load capacitance will result in changes in the output frequency. .
Series vs. Parallel:
Parallel resonant crystals are intended for circuits which contain reactive components (capacitors) in the oscillator feedback loop. These circuits depend on the reactive components and the crystal to achieve the phase shift needed to start and maintain ocsillation at a specified frequency. Series resonant crystals are intended for circuits that contain no reactive components in the oscillator feedback loop. Please see Figure A for Series and Figure B for Parallel resonant crystals: