Pierce Oscillator - Load Capacitance

Load Capacitance

The total capacitance seen from the crystal looking into the rest of the circuit is called the "load capacitance". When a manufacturer makes a "parallel" crystal, a technician adjusts a Pierce oscillator with a variable capacitor (often 18 or 20 pF) to trim the crystal to oscillate at exactly the frequency written on its package.

To assure operation at the correct frequency, one must make sure the capacitances in the circuit match the value specified on the crystal's data sheet. Load capacitance C_L can be calculated from the series combination of C₁ and C₂, taking into account C_i and C_o, the input and output capacitance of the inverter, and C_s, the stray capacitances from the oscillator, PCB layout, and crystal case (typically 3-9 pF):

When a "series" crystal is used in a Pierce oscillator, the Pierce oscillator (as always) drives the crystal at nearly its parallel resonance frequency. But that frequency is few kilohertz higher than the series resonant frequency printed on the package of a "series" crystal. Increasing the "load capacitance" slightly decreases the frequency generated by a Pierce oscillator, but never enough to reduce it all the way down to the series resonant frequency.