Voltage Controlled Oscillator – Working Principle

What is an oscillator circuit?

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

Voltage Controlled Oscillator Waveform
Voltage Controlled Oscillator Waveform

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.

Voltage Controlled Oscillator
Voltage Controlled Oscillator

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

IC 566
IC 566

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:

Voltage Controlled Oscillator
Voltage Controlled Oscillator

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:

Voltage Controlled Oscillator
Voltage Controlled Oscillator

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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