Crystal Oscillators

A crystal oscillator is an electronic circuit that uses a piece of quartz to generate a signal at a very stable and precise frequency. Quartz is piezoelectric, meaning it produces a small voltage when mechanically stressed and deforms when a voltage is applied. When cut and shaped correctly, a quartz crystal has a sharp mechanical resonance, and this resonance behaves electrically like a circuit with an extremely high quality factor (Q), far higher than can be achieved with ordinary inductors and capacitors.

That high Q is the reason crystal oscillators are so accurate. In an oscillator the crystal acts as the frequency-determining element, forcing the circuit to oscillate at or very near the crystal's resonant frequency and resisting drift caused by temperature, supply voltage or component tolerances. As a result crystals provide the timing reference for clocks, microprocessors, radio transmitters and communication equipment, where frequency stability matters far more than it does in a simple LC oscillator.

Several circuit topologies put a crystal to work. The Pierce oscillator uses a single amplifying device with the crystal in the feedback path and is widely used for microprocessor clocks because of its simplicity and stability. Colpitts-style crystal oscillators use a capacitive divider to provide feedback. Crystals can be operated at their fundamental frequency or, for higher frequencies, on an overtone. Compared with LC oscillators such as the Hartley or Colpitts that rely on inductors and capacitors alone, crystal oscillators trade tunability for greatly superior frequency precision.

Frequently asked questions

Why are crystal oscillators more accurate than LC oscillators?

A quartz crystal has an extremely high quality factor and a sharp, stable mechanical resonance, so it holds the frequency far more precisely than the inductors and capacitors of an LC circuit.

What makes quartz suitable for oscillators?

Quartz is piezoelectric, so mechanical vibration and electrical signals are coupled. A properly cut crystal resonates at a very stable frequency that can control an electronic circuit.

Where are crystal oscillators commonly used?

They provide timing references in clocks, watches, microprocessors and radio equipment, wherever a precise and stable frequency is required.