The maximum deviation allowed in an FM broadcast system is 75KHz. If the modulating signal is a single tone sinusoidal signal of 20 KHz, find the bandwidth of FM signal using Carson's rule. What will be the change in band width if modulating signal frequency is doubled? Determine the bandwidth when modulating signal amplitude is doubled.

Subject : Principle of Communication Engineering

Topic : Angle modulation and demodulation

Difficulty : High

Given data:

∆f =75 kHz,

fm=20kHz

We have BW=2(Δf+fm)

        =2(75+20)=190kHz

When modulating frequency is doubled, the bandwidth changes slightly,

BW = 2(75 + 2x 20)

  = 230 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.