VCO Basics: Understanding Voltage Controlled Oscillators

This page describes the basics and function of a VCO (Voltage Controlled Oscillator).

Oscillators whose frequencies are controlled using an external input DC voltage are referred to as VCOs. The full form of VCO, as mentioned, is Voltage Controlled Oscillator.

A voltage control crystal oscillator is known as VXO. They are used in applications where voltage to frequency conversion is needed. They are also used in FM modulators and RF synthesizer designs.

Refer to RF Synthesizer Design which uses a VCO in its applications.

There are many ways VCOs can be constructed:

• Implemented using discrete transistor and varactor diode circuits in high-frequency applications.
• Implemented using IC with RC multi-vibrator. Here, frequency is controlled using a wide range of AC/DC input voltages. These low-frequency VCOs have a frequency range of operation from 1 Hz to about 1 MHz. Normally, IC NE566 is used for such applications.

The figure above shows IC NE-566 used as an FM modulator application. The center frequency of the circuit is set using R1 and C1. RF carrier frequencies up to 1 MHz can be used with this VCO circuit.

VCO is also referred to as SCO. The full form of SCO is SubCarrier Oscillator.