The maximum deviation allowed in a FM broadcast system is 75 kHz. If the modulation signal is a single tone sinusoidal of frequency 15kHz, find the bandwidth of the FM signal. How does the bandwidth change if the modulating frequency is doubled?

Subject : Principle of Communication Engineering

Topic : Angle modulation and demodulation

Difficulty : High

Given:

∆f =75 kHz,

fm=15 kHz

We have BW=2(Δf)+2fm

        = 75x2+15x2 = 180 kHz

When modulating frequency is doubled, the bandwidth changes slightly ,

       BW=2(75 + 15 x2)  = 210 kHz

When modulating signal amplitude is doubled, the frequency deviation becomes 2 x 75 = 150 kHz.

Now, the bandwidth becomes,

       BW=2 x 150 + 2 x 20 = 340 kHz

Thus, the bandwidth is almost doubled.