Q26 of 30 Page 1

Write two basic modes of communication. Explain the process of amplitude modulation. Draw a schematic sketch showing how amplitude modulated signal is obtained by superposing a modulating signal over a sinusoidal carrier wave.

The two basic modes of communication are: -


1. Point-to-point communication


2. Broadcast communication


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,



Here and are respectively called the lower side and upperside frequencies. The modulated signal now consists of thecarrier waveof frequency plus two sinusoidal waves each with a frequency slightly different from, known as side bands. The frequency spectrum of the amplitude modulated signal is shown below,


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Draw a labelled diagram of Van de Graaff generator. State its working principle to show how by introducing a small charged sphere into a larger sphere, a large amount of charge can be transferred to the outer sphere. State the use of this machine and also point out its limitations.

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(i) Find out the ratio of the electric flux through them.


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