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Load Capacitance
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 useful equations):

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:

Figure A - Series Figure B - Parallel

Useful Equations:

Equations Definitions
fS = (Series) frequency = C0 = Static Capacitance in farads
CL = Load capacitance = C1 = Motional capacitance in farads
Co = Shunt capacitance = CL = Load capacitance in farads
C1 = Motional capacitance = f = Nominal frequency in Hz
L1 = Motional inductance = fL = Anti-resonant frequency in Hz
R1 = Series resistance = fS = Series resonant frequency in Hz
Q = Quality factor = L = Inductance into Henrys
fL - fS = f = PL = Pullability (ppm/pF)
PL = Pullability = Q = Quality factor
R1 = Series resistance in ohms | Site Map