What is an oscillator circuit?
Assume an electrical circuit produces the following waveform output (voltage or current output).
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.
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:
- 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.
- 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.
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:
frequency fo = (2/(R1C1))*((Vcc-Vc)/Vcc)
- No connection
- Square wave output
- Triangular wave output
- Modulating/Control voltage VC
- Timing resistor R1 (connected between pin 6 to supply voltage VCC)
- Timing capacitor C1 (connected between pin 7 to ground)
- Supply voltage VCC
A rough internal circuit is shown below:
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:
Applications of VCO
- Tone generators
- Frequency modulation
- Function generator
- Phase Locked Loop
This article is written by Sayantan Roychowdhury.
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