# Modulation index of FM

## Calculate the modulation index in an FM signal when f_{m} (modulating frequency) is 250Hz and Δf (frequency deviation) is 5KHz.

**Options:**

a) 20

b) 35

c) 50

d) 75

Correct Answer: a) 20

**Explanation:**

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/ f_{m}

Where Δf is the peak frequency deviation i.e. the deviation in the instantaneous value of the frequency with modulating signal.

f_{m} is the value of modulating frequency

μ =5000/250

= 20