Question

Write two factors justifying the need of modulating a signal. A carrier wave of peak voltage 12 V is used to transmit a message signal. What should be the peak voltage of the modulating signal in order to have a modulation index of 75%?

Answer 1

Need of modulation : 

(i) audio/video signals do not have sufficiently high energy to travel upto long distances, because of their lower frequency

(ii) For effective transmission, the size of the antenna should be at least of the size λ/4,where λ is wavelength of signal to be sent. For an e.m. wave of the frequency of the order of audio signal, we need an antenna of size 3.75 km, which is practically impossible. Hence these low frequency base band signals are first converted into high frequencies, through modulation

Modulation index, μ = A_m/A_c

So peak voltage of modulating signal, Am = μ A_c

∴ A_m = 0.75 × 12 = 9 V