A voltage controlled oscillator(VCO) is an electronic circuit that produces signal whose frequency is controlled by an input voltage. Thus a voltage controlled oscillator is also called voltage to frequency converter. A voltage controlled oscillator circuit are used in frequency modulation(FM) circuits, as function or signal generators, in frequency shift keying(FSK) as modulator, tone generation etc. The LM566 is a voltage controlled oscillator(VCO) Integrated Circuit(IC) which can be used to generate signal with frequency upto 1MHz controlled with voltage input. The LM566 is also available as NE566 or SE566 IC.
LM566 Voltage Controlled Oscillator(VCO)
The following shows LM566 VCO IC pinout.
The following shows LM566 block diagram.
The modulating signal is applied at the pin 5 and the output is taken at the pin 3 and pin 4. At pin 3 square wave is generated and at pin 4 triangle wave is generated. As can be seen from the block diagram above both the output are buffered and the output impedance is 50Ohm.
The following shows typical NE566 VCO circuit implementation.
In the above circuit the capacitor C2 of 1nF is used between 5 and 6 pins to reduce parasitic oscillation. The frequency of the LM566/NE566/SE566 is set by the capacitor C1, resistor R1, the supply voltage Vcc and the voltage Vcn at the junction of the voltage divider circuit formed by resistors R2 and R3. The frequency according the the LM566 datasheet is given,
\( f_o = \frac{2.4(V_{cc}-V_{cn})}{R_1C_1V_{cc}} \) ------->(1)
where R1 should be in the range 2KOhm < R1 < 20KOhm.
The voltage Vcn is given by,
\( V_{cn}=\frac{R_3}{R_2+R_3} V_{cc}\) -------------->(2)
The following shows circuit diagram of LM566 VCO that outputs 50KHz square wave and the triangle wave.
The VCO square wave output is from 5.1V to 11V so the square wave has 5.9V.
The triangle wave generated by the VCO has amplitude from 3.75V to 6.1V, that is 6.1V-3.75=2.35Vpp.
The following shows the frequency spectrum of VCO outputs.
The following shows the square wave and triangle wave generated by the NE566 VCO without any modulation signal input.
FM with LM566 VCO
The LM566 VCO can be used to generate FM wave by applying modulating signal(Vcrtl) to the pin 5 via a coupling capacitor C3. The following shows circuit diagram of generating FM(frequency modulation) signal with the LM566 voltage controlled oscillator by applying sine wave of frequency 1KHz and amplitude of 1.5Vpp to the pin 5 using a function generator.
The following shows the FM square wave(Vsq) and FM triangle wave(Vtri) waveform along with modulating signal input(Vctrl) and the DC biased modulating signal(Vcn) input waveform.
Video demonstration
The following video demonstrates how the LM566/NE566 voltage controlled oscillator(VCO) works including FM wave generation.
Conclusion
Overall, it was explained how the LM566 voltage controlled oscillator(VCO) works with circuit diagrams and important equations. The application of LM566 VCO was explained with an example of generating FM wave by applying a sinusoidal modulating signal.
References:
[1] LM358 based Voltage Controlled Oscillator(VCO)
