Modulation index of FM

Calculate the modulation index in an FM signal when fm (modulating frequency) is 250Hz and Δf (frequency deviation) is 5KHz.


a) 20
b) 35
c) 50
d) 75

Correct Answer: a) 20


Modulation index is the measure of how much the modulation parameter changes from its un modulated value. The modulation index of FM is given by
μ = frequency deviation/ modulating frequency
= Δf/ fm
Where Δf is the peak frequency deviation i.e. the deviation in the instantaneous value of the frequency with modulating signal.
fm is the value of modulating frequency
μ =5000/250
= 20
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