VCOs

A voltage-controlled oscillator (VCO) is an electronic oscillator whose output frequency is determined by a control voltage applied to its input. The control voltage modulates a parameter of the oscillator, such as the reactance of a resonant tank or the bias of a nonlinear element, causing the oscillation frequency to vary accordingly. VCOs are widely used in radio frequency hardware for frequency synthesis and modulation, and in audio equipment, particularly analog synthesizers.

RF and microwave VCOs are typically built around a resonant circuit, such as an LC tank or a ring oscillator. A common tuning method is varactor-based tuning, where a diode’s capacitance changes with control voltage, shifting the tank frequency. In integrated circuits, VCO cores may also rely on transistor transconductance or current steering to alter frequency. Most VCOs provide multiple waveform outputs, including sine, square, and triangle waves, and some include buffers or divide-by-n stages for versatility.

In music synthesis, VCOs are often controlled by a keyboard or other control sources and are designed to track musical pitch. This is frequently implemented using a voltage-to-pitch convention such as 1 volt per octave. In communications and signal processing, VCOs are core components of phase-locked loops and frequency synthesizers, where a reference signal is compared to the VCO output and the resulting error is fed back to adjust the control voltage to maintain a desired frequency and phase relationship.

Key specifications include tuning sensitivity (Hz/V or MHz/V), linearity, phase noise, output amplitude, and temperature stability.