Oscillator circuits

An oscillator is a circuit that generates a repeating, periodic signal on its own, without needing an input waveform to copy. Starting only from a steady direct-current supply, it produces an output that swings back and forth at a regular rate, which may be a smooth sine wave, a sharp square wave, a triangle wave or another shape. Oscillators are fundamental building blocks: they provide the clock pulses that time digital systems, the carriers used in radio, the tones used in audio, and the reference signals used in test equipment.

Oscillation arises when a circuit feeds part of its output back to its input in a way that reinforces a particular frequency. The frequency is usually set by components that store and exchange energy, such as a capacitor charging and discharging through a resistor, or a coil and capacitor resonating together. Crystal oscillators use the precise mechanical resonance of a quartz crystal to fix the frequency very accurately, which is why they pace clocks and microprocessors. The waveform shape depends on the circuit type and any additional shaping stages.

Practical oscillator circuits cover a wide range of uses. Audio oscillators generate test tones and low-frequency sine waves; function generators produce switchable triangle, square and sine outputs for testing; and radio-frequency oscillators form the heart of transmitters and receivers. Important design goals include a stable, predictable frequency, a clean waveform with little distortion, and reliable starting. Collections of oscillator circuits include sine, square and triangle generators along with crystal-controlled designs for accurate timing.

Frequently asked questions

What does an oscillator do?

It generates a repeating periodic signal, such as a sine, square or triangle wave, from a steady direct-current supply.

What sets an oscillator's frequency?

Energy-storing components such as a resistor and capacitor, or a coil and capacitor; crystals fix the frequency very accurately.

Why are crystal oscillators used for clocks?

The precise mechanical resonance of a quartz crystal gives a very stable, accurate frequency suitable for timekeeping and processors.