Pierce oscillator

Virtually all digital IC clock oscillators are of Pierce type, as the circuit can be implemented using a minimum of components: a single digital inverter, one resistor, two capacitors, and the quartz crystal, which acts as a highly selective filter element.

The low manufacturing cost of this circuit and the frequency stability of quartz crystals make it advantageous for many consumer electronics applications.

Normal crystals are lossless enough to make this a reasonable approximation: the amplifier does not drive the resonant circuit, but merely stays in sync with it, providing enough power to match losses.

Recently, MEMS (Micro-Electro-Mechanical-System) resonators fabricated by surface micromachining have enabled ultra-low power stable pierce oscillators.

The tiny form factor of MEMS resonators greatly reduced the power consumption of the oscillator while keeping the good stability thanks to their very-high Q.

The series resistor Rs reduces the chance of overtone oscillation and can improve start-up time.

To assure operation at the correct frequency, one must make sure the capacitances in the circuit match this value specified on the crystal's data sheet.