Voltage Controlled Oscillator - Working Principle

What is an oscillator circuit?

Assume an electrical circuit produces the following waveform output (voltage or current output).

[caption id="attachment_722" align="aligncenter" width="548"]Voltage Controlled Oscillator Waveform Voltage Controlled Oscillator Waveform[/caption]

This output is a square wave. It can be considered to be a sequence of repeating the following wave at an interval of time period 4.

[caption id="attachment_723" align="aligncenter" width="538"]Voltage Controlled Oscillator Voltage Controlled Oscillator[/caption]

This circuit which is producing a waveform by repeating a wave after a specific time interval is an oscillator circuit. Another example can be of a circuit producing continuous sine wave by repeating one cycle of a sine wave.

What is voltage controlled oscillator (VCO)?

The produced continuous waveform produced by the oscillator circuit has a frequency. A circuit in which the frequency of the produced output can be varied by the magnitude of a separately applied external voltage (other than the main supply voltage VCC) is known as voltage controlled oscillator.

Types of VCO:

  1. Linear or harmonic oscillator: This type of oscillator produces a sine wave. It consists of an LC tank circuit or crystal oscillator. The frequency of a tank circuit can be varied by changing the value of the capacitor. Now, a varactor diode’s capacitance can be varied by varying the applied voltage across it. So a varactor diode if used in an LC circuit converts it to a VCO.

  2. Relaxation oscillator: The output signal is a saw tooth or triangularwaveform. This circuit employs the charging and discharging of a capacitor through a resistance. The output frequency depends on the time of charging and discharging of the capacitor.If it is desired to produce a square wave, a triangular wave can be differentiated to produce so. Also a periodic waveform can be passed through a Schmitt trigger to produce a square wave.

IC 566

The IC 566 (or LM566) is an integrated circuit that produces a triangular wave and a square wave output from two different output pins. It is an 8 pin IC shown below:

Pin Configuration

[caption id="attachment_724" align="aligncenter" width="282"]IC 566 IC 566[/caption]

frequency fo = (2/(R1C1))*((Vcc-Vc)/Vcc)

  1. Ground

  2. No connection

  3. Square wave output

  4. Triangular wave output

  5. Modulating/Control voltage VC

  6. Timing resistor R1 (connected between pin 6 to supply voltage VCC)

  7. Timing capacitor C1 (connected between pin 7 to ground)

  8. Supply voltage VCC

A rough internal circuit is shown below:

[caption id="attachment_725" align="aligncenter" width="599"]Voltage Controlled Oscillator Voltage Controlled Oscillator[/caption]

Basically, the principle of operation is as such:

The Schmitt trigger switches the current source from charging and discharging the capacitor.

The IC charges and discharges the external capacitor C1 through the resistor R1. A triangular waveform is obtained by passing the voltage waveform across the capacitor C1 through Buffer Amplifier 2 and obtained as output through pin 4.

The voltage waveform across the capacitor when passed through a Schmitt trigger, produces a square wave which is passed through the Buffer Amplifier 1 and obtained as output through pin 4.

Modulating voltage VC should be in the range of  (3/4)Vcc < Vc < Vcc where VCC is the supply voltage.

VCC should be within 10 to 24 Volts.

The frequency modulation (by applying a varying modulating voltage VC) can be done in 10:1 ratio.

The frequency of the output waveform is f0 = (2/R1C1)*((Vcc - Vc)/Vcc) .

An example circuit is shown below:

[caption id="attachment_726" align="aligncenter" width="606"]Voltage Controlled Oscillator Voltage Controlled Oscillator[/caption]

Applications of VCO

  1. Tone generators

  2. Frequency modulation

  3. Function generator

  4. Phase Locked Loop

This article is written by Sayantan Roychowdhury.

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