What is the modulation index of an FM phone signal having a maximum frequency deviation of 3000 Hz either side of the carrier frequency if the highest modulating frequency is 1000 Hz?
Why Modulation index for FM is simply the peak deviation divided by the modulating frequency at that instant. Here 3000 Hz of deviation divided by a 1000 Hz modulating tone gives an index of 3. Note that the index changes with the modulating frequency: the same deviation with a higher audio tone yields a smaller index. (Deviation ratio is the related term that uses maximum deviation divided by the highest modulating frequency permitted in the system.)
Watch out The value of 0.3 comes from dividing the wrong way, modulating frequency over deviation, and 6 comes from using peak-to-peak deviation (6000 Hz) instead of deviation to one side of the carrier.
Index = deviation / audio frequency. Divide the big number by the small one: 3000/1000 = 3.