Voltage Controlled Oscillators and Phase-Locked Loops

A voltage-controlled oscillator (VCO) is an electronic oscillator whose output frequency is set by an applied control voltage. Raising or lowering this voltage tunes the frequency over a range, which makes the VCO a building block for circuits that must generate or adjust frequencies electronically. On its own a VCO is not very stable, because its frequency drifts with temperature and supply variations, so it is usually combined with a feedback system that holds it on target.

That feedback system is the phase-locked loop (PLL). A PLL compares the phase of the VCO's output with that of a stable reference signal using a phase detector. The detector produces an error signal that, after filtering, adjusts the VCO's control voltage so the output stays locked in step with the reference. Once locked, the VCO inherits the accuracy and stability of the reference, which is often a quartz crystal oscillator.

Adding a frequency divider in the feedback path turns this arrangement into a frequency synthesizer. The loop then forces the VCO to run at a frequency that is the reference multiplied by the divider setting, so changing the divider selects different output frequencies in precise steps. This is how radios, transmitters and many other devices tune to exact channels from a single crystal reference. Phase-locked loops are also used for clock generation, demodulation and recovering timing from data streams.

Frequently asked questions

What does a voltage-controlled oscillator do?

It produces an output signal whose frequency depends on an applied control voltage, so the frequency can be tuned electronically by changing that voltage.

How does a phase-locked loop stabilise a VCO?

It compares the VCO output's phase with a stable reference and adjusts the VCO's control voltage to keep them in step, so the output gains the reference's stability.

How is a frequency synthesizer built from a PLL?

A divider is placed in the feedback path so the loop locks the VCO to a multiple of the reference. Changing the divider selects different precise output frequencies.