Generation of FM signal using VCO(Voltage Controlled Oscillator) LM566 IC

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Frequency Modulation (FM) is a widely used modulation technique in communication systems due to its resilience to noise and distortion. FM is a type of modulation that varies the frequency of a carrier signal in proportion to the amplitude of a modulating signal. In order to generate an FM signal, a voltage-controlled oscillator (VCO) can be used. In this article, we will discuss the process of generating an FM signal using the LM566 VCO IC(Integrated Circuit).

The LM566 is a VCO IC that can generate an output signal with frequency ranging from minimum of 500KHz to typically frequency of 1MHz. The output signal frequency of the LM566 can be varied with an external varying voltage, making it ideal for building FM transmitter circuit. The LM566 can also be used to generate other types of signals, such as amplitude modulation (AM) and pulse width modulation (PWM). Application of the LM566 VCO IC includes FM modulation, signal generation, function generation, FSK(Frequency Shift Keying), Tone generation, building PLL(Phase Locked Loop) circuit etc.

The LM566 VCO IC includes a Schmitt trigger circuit, current source, current switch and buffer amplifier inside the integrated circuit. The current source and the differential amplifier circuit inside the IC is connected to the external biasing resistor and timing capacitor and resistor. The following shows the LM566 VCO IC with internal block diagram.

LM566 block circuit diagram
 

To generate an FM signal using the LM566 VCO IC, we can use the following circuit:

LM566 FM generation circuit diagram

 In the above circuit, the external dc current is applied at the modulation pin 5 via the external voltage divider circuit formed by the resistors R2 and R3. Without the external modulation signal this resistor voltage divider provides the necessary input current to the LM566 IC and square and triangle signal appears at the pin 3 and 4 respectively. The recommended bias voltage at the pin 5 using the resistors R2 and R3 is between 3/4Vcc and Vcc where Vcc is the DC power supply. 
 
The dc bias voltage at the modulation pin is given by,
 
\(V_{cn} = \frac{R_3}{R_2+R_3}V_{cc}\) 

With the value of R2=14.29KOhmand R3=100KOhm the dc bias voltage at pin 5 is Vcn=10.5V.
 
 When dc current is applied to the LM566 pin 5, it goes into the current source circuitry inside the chip.
This current source circuit then passes charges the capacitor C1 connected to the pin 7 and the resistor R1 inside the chip controls the amount of current. The output voltage of the capacitor is connected to a Schmitt trigger circuit inside the IC. The voltage across the capacitor is discharged into the Schmitt trigger circuit and the output of the Schmitt goes high. When the output of the Schmitt trigger circuit is high it turns on the current switch inside the chip and therefore the charging of the capacitor is stopped. Once the output of the Schmitt trigger goes low, the current switch is turned off and the capacitor starts charging again and the process repeats over and over again. This in effect produces square wave signal at the output of the Schmitt trigger whose frequency is constant since only dc bias current is applied and no modulating signal at the pin 5 is applied. If modulation signal is applied to the pin then it rides on the dc bias current and into the current source circuit that in effect produces square wave output with varying frequency which is linearly related to the amplitude of the applied modulating signal(message signal). 

Thus to generate an FM signal using the LM566 VCO IC, we need to apply the modulating signal to the input of the VCO. The modulating signal can be any type of signal, such as an audio signal or a digital signal. The modulating signal is then used to modulate the frequency of the output of the VCO.

The VCO frequency formula is given by the following equation:

\(f_o = \frac{2.4(V_{cc}-V{cn})}{R_1C_1V_{cc}}\)

With Vcc=12V, Vcn=10.5V, R1=6KOhm and C1=1nF the frequency of the FM signal generated by the VCO integrated circuit is 50KHz.

To calculate the component values for particular frequency we can use the online 566 Voltage Controlled Oscillator(VCO) Calculator.

Thus, the output frequency of the VCO is determined by the values of R1 and C1. The modulating signal is applied to the input of the VCO through coupling capacitor C3. In the circuit, the capacitor C2 of 1nF (suggested by the manufacturer) is used to reduce the chances of possible oscillation created inside the chip.

LM566 square and triangle wave

The output of the LM566 VCO IC can then be fed to a power amplifier to increase the output power of the FM signal. The FM signal can then be transmitted through an antenna to a receiver.

The following video demonstrates generation of FM signal using LM566 VCO IC.

 

In conclusion, the LM566 VCO IC is a versatile IC that can be used to generate an FM signal. It is easy to build an FM transmitter with this single integrated chip. The output frequency of the VCO is controlled by an external voltage, making it ideal for use in communication systems. By applying a modulating signal to the input of the VCO, we can generate an FM signal that can be transmitted through an antenna to a receiver.

See next Frequency Modulation with Varactor Diode.

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