Modulation index of FM

Q.  Calculate the modulation index in an FM signal when fm (modulating frequency) is 250Hz and  Δf (frequency deviation) is 5KHz.
- Published on 14 Oct 15

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

ANSWER: 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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