Q16 of 26 Page 1

Define the term ‘amplitude modulation’. Explain any two factors which justify the need for modulating a low frequency base-band signal.

Amplitude Modulation: -


In this type of modulation, the message signal is super imposed on the carrier wave and the modulated wave is transmitted


The carrier wave is,



The message signal is,



So the modulated signal cm(t) can be written as,




we can write,



Here,



Is called the modulation index, Using the trigonometric relation,



we can write,




The Reasons for modulating a low frequency signal is,


1. The size of the antenna which is necessary for transmission and reception of the signal must be comparable to the wavelength of the signal so as to properly sense the time variation, for a signal with low frequency it has a very large wavelength and thus constructing such large antennas is not feasible and practically not possible.


2. The signals with low frequency have low energies, so the long distance transmission of the signals is not possible due to attenuation and hence the need for modulation arises.


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