Question: Q. 2. (i) How is amplitude modulation achieved ?

(ii) The frequencies of two side bands in an $A M$ wave are $640 \mathrm{kHz}$ and $660 \mathrm{kHz}$ respectively. Find the frequencies of carrier and modulating signal. What is the bandwidth required for amplitude modulation? $\mathrm{R}[2017$, OD set-1]

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Solution:

Ans. (i) Amplitude modulation can be achieved by applying the message signal, and the carrier wave, to a non linear (square law device) followed by a band pass filter.

(Alternatively, The student may just draw the block diagram.)

$A_{c} \sin \omega_{c}{ }^{t}$

(Alternatively, Amplitude modulation is achieved by superposing a message signal on a carrier wave in a way that causes the amplitude of the carrier wave to change in accordance with the message signal.) (ii) Frequencies of side bands are :

$\therefore$ $\left(v_{c}+v_{m}\right)$ and $\left(v_{c}-v_{m}\right)$
and $v_{c}+v_{m}$ $=660 \mathrm{kHz}$
$\therefore$ $v_{c}-v_{m}$ $=640 \mathrm{kHz}$
$\therefore$ $v_{c}$ $=650 \mathrm{kHz}$
$v_{m}$ $=10 \mathrm{kHz}$
Bandwidth $=(660-640) \mathrm{kHz}$
$=20 \mathrm{kHz}$


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