Question

In the process of amplitude modulation, the modulating (message) signal m(t)[= A_m sin(ω_mt)] is added to the carrier wave c(t)[= Ac sin((ω_ct)], to produce the output signal x(t) [= A_m sin(ω_ct) + A_c sin (ω_ct)]. The devices, through which this signal is made to pass, before being fed into the transmitting antenna (as an AM wave), are sequence wise, the

(1) power amplifier, square law device and band pass filter. 

(2) band pass filter, power amplifier and square law device. 

(3) square law device, band pass filter and power amplifier. 

(4) power amplifier, band pass filter and square law device

Answer 1

Correct option: (3) square law device, band pass filter and power amplifier.

Explanation: 

The sequence of (amplitude) modulation, of the carrier wave by the message signal is as per the block diagram shown here.

After adding the message signal m (t) to the carrier wave c (t), the signal x (t) is produced. This is passed into a square law device whose output contains a d.c. term and sinusoidal of frequencies ω_m , 2ω_m, ω_c , 2ω_c, (ω_c –ω_m) and (ω_c +ω_m). The band pass filter than rejects the d.c. term and sinusoids of frequencies ω_c , (ω_c –ω_m) and (ω_c + ω_m). This output is the AM wave which has its central frequency ω_c an its side band frequencies (ω_c –ω_m) and (ω_c +ω_m). 

The output of the band pass filter is followed by a power amplifier. This provides it the necessary power. This power argumentative signal is than fed into an appropriate sized transmitting antenna which than radiates it out into space.