Frequency Modulation and Demodulation

Frequency Modulation and Demodulation using Frequency modulator IC 8038 and Phase Locked Loop IC  – 565. OBJECTIVE:

1. To generate frequency modulated signal and determine the modulated index and bandwidth for various values of amplitude and frequency of modulated signal. 2. To demodulate a Frequency Modulated Signal using FM detector. CIRCUIT DIAGRAM:

FM Modulation Using IC 8038.

(made by proteus)

FM Demodulator using IC LM 565.

(made by proteus)

OBSERVATION:

Table no. 1

Table No. 2

GRAPH/CURVES:

Channel 2 is modulated wave, Channel 1 is message signal

Message signal after demodulation. DISCUSSION:  

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the output signal coming was 64.5 KHz in IC 8038 when no message signal was given. Due to noise, inaccurate resistor and capacitor are used so that the exact tuning couldn’t be done That’s why demodulated wave (message signal) is attenuated. And in the frequency modulation the modulation signal is as expected. i.e. when the amplitude of message signal is positive then time period is less, and when it is negative time period is large. We can use IC 555 also but we have used IC 8038 for frequency modulation, Because of this IC can give three type of output sine/ triangular/square. In IC 555 the message signal is given to control voltage pin, that pin control the timing of 555 by overriding 2/3Vcc level of voltage divider. By applying a voltage to this pin we can change the width of output. As the message signal amplitude is less than 2/3 Vcc, therefore when message has  –ve phase then net voltage is less and therefore time period is high and it other phase it is just opposite. In this way the message signal is modulated. To get good demodulated output wave (from IC565), we can use low pass filter of a particular cut -off frequency, the frequency we can find by trial and error.