Q. Process of Amplitude Modulation?

In AM the message signal is impressed on the amplitude of the carrier signal. There are several different ways of amplitude modulating the carrier signal by the message signal, each of which results in different spectral characteristics for the transmitted signal. Four methods are:

• Conventional (standard) double-sideband AM

• Double-sideband suppressed-carrier AM

• Single-sideband AM

• Vestigial-sideband AM.

AM, as its name implies, carries the modulating signal x(t) ina time-varying amplitude of the form

a(t) = A_c[1 + m_Ax(t)]

where the constant A_c stands for the unmodulated carrier amplitude, and m_A is the modulation index. The resulting modulated wave is

whereas the sinusoidal carrier waveform is given by

c(t) = A_c cos(ω_ct + φ_c) = A_ccos(2πf_ct + φ_c)

Sometimes the standard AM signal is expressed in the form of

x_c(t) = s_AM(t) = [A_c + f(t)] cos(ω_ct + φ_c)

in which the total amplitude of the carrier [A_c +f(t)] is a linear function of the message. Figure illustrates the AM waveforms.