communication-systems Question 34
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 | $=(660-640) \mathrm{kHz}$ | ||
| $=20 \mathrm{kHz}$ |
