(a) The modulation is needed due to
(i) Transmission of audiofrequency electrical signals need long impracticable antenna.
(ii) The power radiated at audio frequency is quite small, hence transmission is quite lossy.
(iii) The various information signals transmitted at low frequency get mixed and hence can not be distinguished.
(b) Amplitude Modulation:
The block diagram is shown in fig
The modulating signal is superposed on carrier wave of high frequency (≈ MHz). The resultant wave so obtained is sent to square law device which producses wave
y(t) = Bx(t) + Cx2(t)
This is finally sent to Bandpass filter which rejects dc and sinusoids of frequencies ωm 2ωm, 2ωc and allows wave of frequency ωc, ωc - ωm and ωc + ωm. The output of Bandpass filter is AM wave.
