Study of Voltage Controlled Ocsillator (VCO)


To familiarise with the voltage controlled oscillator circuit.


A common type of VCO available in IC form is Signetics NE/SE 566. The pin configuration and the block diagram is shown in fig.1 and fig.2 respectively. Referring to the fig. 2, a timing capacitor linearly charged or discharged by a constant current source/ link. The amount of current controlled by changing the modulating voltage applied at the modulating input (pin 5) or by changing the timing resistor external to the IC chip. The voltage at pin 6 is held at the same voltage as pin 5. Thus if modulating voltage at pin 5 is increased, the voltage at pin 6  also increases, resulting in less voltage across and thereby decreasing the charging current.

                                       The voltage across the capacitor is applied to the inverting input terminal of the Schmitt trigger via buffer amplifier. The output voltage swing of the Schmitt trigger is designed to 0.5. If  = in the positive feedback loop, the voltage at the non-inverting input terminal of swings from 0.5 to 0.25. In fig. 3, when the voltage on the capacitor exceeds 0.5 during charging, the output of the Schmitt trigger goes LOW ( 0.5). The capacitor now discharges and when it is at 0.25, the output of the Schmitt trigger goes HIGH (). Since the source and sink currents are equal, capacitor charges and discharges for the same amount of time. This gives a triangular voltage waveform across which is also available at pin 4. The square wave output of the Schmitt trigger is inverted by the inverter and is available at pin 3. The inverter is basically a current amplifier used to drive the load. The output waveforms are shown in fig. 3.

Circuit Diagram:-


Output Waveform:

Apparatus table:


  1. Build the circuit according to the circuit diagram shown in fig.4.
  2. At first, don’t give any input voltage at pin 5.
  3. Change the values of CT and R First keep the capacitor constant and then change the resistors. Then keep the resistor constant, and change the capacitors. For each case measure the time period, frequency and duty cycle of the square waveforms.
  4. The output signal having 50% duty cycle is an ideal output. Take the value of CT and RT for that signal and give an input dc voltage vc on pin 5. The range of vc will be (2/3) VCC to VCC.
  5. Now measure the time period, frequency and duty cycle of the square and triangular waveforms.
  6. Plot the graph of modulating input voltage vs. output frequency.

Experimental result:

Table1: Without modulating the input voltage

Sl. noRT (KΩ)CT(µF)Time period (µs)Frequency (KHz)TON (µs)TOFF (µs)waveform





Table 2: with modulating input ( Here RT=15K, CT=0.1µF)

Sl. noVoltage (volts)Frequency(KHz)





Find out the free running frequency of the output waveforms and check whether it is closer to the theoretical value or not.


It has been seen that if we take the value of CT and RT 0.1µF and 10KΩ respectively, we will get a perfect waveform. Taking these values of resistor and capacitor we can do the experiment with modulating input voltage.