DESIGN OF F M MODULATOR AND DEMODULATOR

INTRODUCTION

It is a process in which the frequency of the carrier is varied in accordance with the instantaneous value of modulating voltage. The amount of change in frequency is determined by the amplitude of the modulating signal.

FREQUENCY MODULATION VS AMPLITUDE MODULATION

The main advantages of FM over AM are: • Improved signal to noise ratio (about 25dB).• Smaller geographical interference between neighbouring stations. • Less radiated power. • Well defined service areas for given transmitter power.

Disadvantages of FM: • Much more Bandwidth (as much as 20 times as AM). • More complicated receiver and transmitter.

• MODULATION INDEX :It is defined as the ratio of frequency deviation to the frequency of the modulating signal.

m = ΔF/f m • FREQUENCY DEVIATION :The frequency deviation ΔF represents the maximum shift between the modulated signal frequency, over and under the frequency of the carrier

ΔF = (Fmax-Fmin)/2

BANDWIDTH OF FM

Carson's Bandwidth Rule states that 98% of the signal power is contained within a bandwidth equal to the deviation frequency, plus the modulation frequency doubled, i.e.:

• The bandwidth of a frequency modulated signal varies with both deviation and modulating frequency. • The frequency modulation bandwidth increases with modulation

frequency but it is not directly proportional to it.

CIRCUIT DIAGRAM

OBSERVATION

Frequency Modulated Output

Frequency Demodulated Output

CONCLUSION

We have designed a Frequency Generator which can produce Square, Triangular and Sinusoidal waveforms up to 15 kHz (approx..) frequency. When the Sinusoidal message signal is fed to the Frequency Modulator circuit we get a frequency modulated output. The modulated wave when passed through the Frequency Demodulator circuit we get back the message signal in a distorted form.

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