Voltage Controlled Oscillators (VCO)

Voltage Controlled Oscillators (VCO)
Voltage Controlled Oscillators (VCO)

The amplitude of the input (control) signal controls the output frequency of a voltage-controlled oscillator. Communications and phase-locked loop circuits benefit from this building piece. A circuit that generates a continuous, repeated, alternating waveform without a stimulus is an oscillator. A VCO is distinctive because its output frequency can be adjusted by voltage without changing hardware features. The output frequency is proportional to the input voltage amplitude, which can be changed to change it. Voltage controls output frequency, hence the name Voltage Controlled Oscillator.

Voltage-controlled oscillators: basics

LC, RC, Hartley, Colpitts, Clapp, and relaxation oscillators are common. Variable capacitors or inductors are used to tune these oscillators to a specific frequency range. Voltage Controlled Oscillators (VCO) generate frequency based on applied voltage, unlike regular oscillators.

Wireless signal transmission is the goal of cellular communication and base stations. Voltage-controlled oscillators are essential to signal processing. Simple gadgets can adjust light, sound, and test applications. VCOs change radio frequencies with high-frequency pulses.

You’re an electronics enthusiast or maker ready to learn about voltage controlled oscillators (VCOs), but where to start? Everyone’s been there! VCOs may seem intimidating, but this guide will explain what a VCO is, how it works, the key components and parameters, how to set one up from scratch, and how to tweak a VCO circuit to create sweet analog waveforms for your next project. With a few parts, basic circuits, and this guide, you’ll learn VCOs quickly. Continue reading to explore VCOs’ versatility and creativity!

VCOs are voltage-controlled oscillators.

A voltage controlled oscillator (VCO) generates a sine wave or square wave. The VCO voltage determines signal frequency. As input voltage changes, frequency does too.

Radio transmitters, wireless phones, and instruments use VCOs. Use them wherever a tunable frequency source is needed. Common VCOs include:

An inductor and capacitor generate oscillation in LC VCOs. Less customizable but more stable.
VCO ring oscillators use odd numbers of inverters or logic gates in loops. Simple, noisy, unstable.
Relaxation VCOs—Charge and discharge a capacitor for a sawtooth wave. Cost-effective but complicated.
VCOs are tuned by adjusting input voltage. The oscillator speeds up or slows to fit the required frequency. Many VCOs have a tuning sensitivity of MHz/Volt, showing how much frequency changes per volt of input. A greater tuning range with higher sensitivity can reduce stability.

Frequency synthesizers employ VCOs to create several frequencies from a single reference. Voltage-controlled filters, analog synthesizers, and phase-locked loops use them. A VCO, comparator, and low-pass filter lock the VCO frequency to a reference signal in phase-locked loops (PLLs). The output matches the input phase and frequency.

Based on input voltage, VCOs oscillate over a variety of frequencies. They’re used to generate tunable frequencies in radios, wireless systems, synthesizers, etc. Knowing how VCOs function will help you comprehend numerous electronic devices and applications.

Voltage-controlled oscillators work how?

An applied voltage controls the frequency of a voltage-controlled oscillator (VCO). Telecommunications, radio transmitters, and synthesizers employ VCOs.

A VCO works by changing the capacitance of varactors and diodes with voltage. Changing the voltage given to varactor diodes in an oscillator circuit controls the frequency.

As voltage rises, varactor diodes lose capacitance. This raises the oscillator’s frequency. As voltage drops, varactor diodes increase capacitance, lowering oscillator frequency.

In their oscillator circuit, VCOs use active devices like BJTs or FETs. The varactor diodes are connected to the oscillator’s inductor and capacitor-based resonating tank circuit. Changing the varactor diode voltage changes the tank circuit’s capacitance. This changes the tank circuit resonance frequency, which regulates the oscillator frequency.

Some VCOs control frequency with variable inductors instead of varactor diodes. An external control voltage adjusts a capacitive or inductive component in the resonant circuit.

Frequency synthesizers, phase-locked loops, FM broadcasters, and others use VCOs as tunable frequency sources. Understanding VCOs is essential to frequency control and generation, whether you’re designing radio equipment or learning about oscillators.

VCOs: LC, RC, Crystal, etc.

Each form of electronic VCO has strengths and cons. The most frequent are:

LC VCOs calculate oscillation frequency using an inductor (L) and capacitor (C). These are basic but unstable and non-linear. They perform best at higher frequencies.


Resistor and capacitor make up RC VCOs. Their linearity and stability are limited to lower frequencies. RC VCOs are cheap and easy to make.

Crystal VCOs employ piezoelectric crystals to control frequency. They are steady and accurate but confined to crystal frequencies. Crystals cost extra and are untunable.


MEMS VCOs create signals using an integrated mechanical resonator. They are stable and accurate at a lesser cost than crystals. They are also fixed-frequency and harder to design and produce.

Ring Oscillators

Ring oscillators loop odd numbers of inverters or delay elements. Although simple, they are noisy and imprecise. Ring VCOs use high frequencies but are unstable.

Your application and needs determine the VCO you use. LC and RC VCOs are good for simple adjustable oscillators. Crystals and MEMS produce steady fixed-frequency signals. When precision isn’t important, ring oscillators can generate high-frequency clock signals. Understanding the possibilities lets you pick the proper VCO for your next project.

Synths, Phase-Locked Loops, Frequency Modulation VCO Applications

Many electronic circuits and applications require VCOs.


VCO oscillators provide musical pitches in synthesizers. Changing a VCO’s voltage affects its frequency, generating different tones. Harmonic and complex sounds are created by many VCOs. Many 70s and 80s analog synths used VCOs to create their distinctive sounds.

Locked-phase loops

VCOs create phase-matched output frequencies for PLLs. To match the reference, they compare the VCO’s frequency phase and modify its voltage. PLLs can multiply, divide, and synthesize frequencies. Radios, TVs, and telephones employ PLLs to generate steady high-frequency signals from a lower reference frequency.

FM modulation

FM synthesis encodes information by varying audio frequency. VCOs are ideal because minor voltage changes cause frequency variations. FM transmits radio audio. An FM radio converts this frequency-modulated signal into audible sounds.

VCOs allow frequency-generating, modulating, and locking circuits. Their voltage sensitivity and reactivity enable new sound creation, signal processing, and telecommunications applications. VCOs simplified frequency synthesis, allowing technology that powers our most vital tools.


You now understand voltage-controlled oscillators—they’re not so mysterious! Synthesizers’ sounds come from VCOs turning control voltage into frequency. With some basic knowledge, you may experiment with VCO circuits to make analog synthesizer tones. Hearing your personalized VCO sing for the first time is gratifying. Breadboard a simple VCO circuit to practice. Your VCO virtuosity will be on its way!

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