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.
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.
Frequency Demodulated Output
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