Principle of Amplitude ModulationRoll NO 140002016 KARTIKEYA SURENDRA SINGH

WHAT IS MODULATION?

The process of impressing a low frequency signal which is to be transmitted on a high frequency carrier signal, by changing the characteristics of the carrier signal like amplitude, frequency or phase is called modulation.

WHY DO WE NEED MODULATION?

The advantages of using modulation are

1) High frequency signal is able to travel faster

2) The length of antenna is given by

L=λ=u/ϑ=(3∗108)/ϑHz

It is inversely proportional to frequency so the required length reduces to practically possible values

Amplitude Modulation

The modulation in which only the amplitude of the carrier wave is changed in accordance with the signal to be transmitted is called amplitude modulation. The frequency of modulated wave remains the same as carrier frequency

Derivation of AM wave

Here E is the amplitude of the modulated wave.

Now, e = (Ec + em) sin wct

= (Ec + Em sin wmt) sin wct

= Ec sin wct + Em sin wmt sin wct

(Because 2sinA sinB = cos (B-A) - cos (B+A))

Side band frequencies

As we can see above two new frequencies are produced. They are known as side band frequencies.  nc +nm is called the upper side band frequency (USB). nc - nm is called the lower side-band frequency (LSB). The two side-band lie on either side of the carrier frequency at equal frequency interval nm. So, band width is 2nm.

Why Carrier signal frequency should be high?

If the carrier signal frequency is high the required antenna length would be lesser, therefore the power loss at antenna will be reduced significantly. Furthermore high frequency signal undergo lesser dispersion and can travel faster than low frequency signal.

Demodulation

Demodulators are used to separate the carrier signal and the modulating signal. For AM synchronous detector is used