Mumbai University > Computer Engineering > Sem 3 > Electronic Circuits and Communication Fundamentals

Marks: 10 Marks

Year: May 2014

1. Amplitude modulation is a process of varying the amplitude of high frequency carrier signal in accordance with the amplitude of the low frequency modulating or information signal, and keeping frequency and phase of the carrier signal as constant.

2. Let the carrier voltage and modulating voltage be vcvc and vmvm, both represented as,

   vc=Vcsinωctvm=Vmsinωmtvc=Vcsinωctvm=Vmsinωmt

3. In amplitude modulation, amplitude of unmodulated carrier VcVc is varied proportional to the instantaneous modulating voltage VmVmsin ωmtωmt.

4. When there is no modulation, the amplitude of the carrier is equal to its unmodulated value and when modulation is present, amplitude of carrier is varied by its instantaneous value. This is explained with the help of Fig1 given below.

• The time domain representation of an amplitude modulated wave is given below in Fig2., which shows the modulation of the carrier signal by the modulating signal.
• In AM wave, both the carrier and modulating waves are sinusoidal in nature but the modulated wave is not a sine wave. The amplitude of the AM wave is given as, vAM=Vcsinωct+mVc2cos(ωc−ωm)t−mVc2cos(ωc+ωm)tvAM=Vcsinωct+mVc2cos⁡(ωc−ωm)t−mVc2cos⁡(ωc+ωm)t

• Thus we can say that apart from the original signal there are two additional sine waves which are one above the carrier and the other below the carrier frequency. Therefore, the complete AM signal consists of a carrier wave and two additional frequencies one on each side which are called the side frequencies.
• The frequency which is above the carrier frequency is called the upper sideband and the frequency below the carrier frequency is called the lower sideband.
• The frequency of the upper sideband (USB) is fc+fmfc+fm and the frequency of lower sideband (LSB) is fc−fmfc−fm. The spectrum of amplitude modulated wave is given below in the Fig3.

• The central frequency has the highest amplitude which is the carrier frequency fcfc. Adjoining both the sides of the carrier frequencies are the sideband frequencies with lower amplitude. The bandwidth of the amplitude modulated wave is given by,

$$BW=(fc+fm)−(fc−fm)=2fm$$