Amplitude modulation

When the amplitude of high frequency carrier wave is changed in accordance with the intensity of the signal, it is called amplitude modulation.

The following points are to be noted in amplitude modulation .

1. The amplitude of the carrier wave changes according to the intensity of the signal.
2. The amplitude variations of the carrier wave is at the signal frequency f_S.

3. The frequency of the amplitude modulated wave remains the same ie.carrier frequency f_C.

Modulation factor

The ratio of change of amplitude of carrier wave to the amplitude of normal carrier wave is called modulation factor.

m=(amplitude change of carrier wave) / normal carrier wave(unchanged)

Modulation factor is very important since it determines the strength and quality of the transmitted signal. The greater the degree of modulation, the stronger and clearer will be the audio signal. It should be noted that if the carrier is overmodulated (ie m>1) distortion will occur at reception.

Analysis of amplitude modulated wave

• A carrier wave is represented by e_c = E_ccosw_ct-------------------(1)

Where e_{c ------}instantaneous voltage of carrier.

                       E_c -----amplitude of carrier.   

• In amplitude modulation, the amplitude E_C of the carrier wave is varied in accordance with intensity of the signal as shown in figure.

• Suppose m=modulation index, then change in carrier amplitude =mE_c.

Amplitude or E_max of the signal = mE_c.

e_{s =}mE_ccosw_st---------------------------------(2)

where mE_c is the amplitude of the signal.

e_{s ---------}instantaneous voltage of the signal.

The amplitude of the carrier varies at signal frequency fs. Therefore the amplitude of AM wave is given by,

                              E_c +mE_ccosw_st = E_c(1+mcosw_st)

• The instantaneous voltage of AM wave is,

e =  Amplitude x cosw_ct

• The AM wave is equivalent ot thesummatoin of theree sinusoidal waves:  aone having amplitude E_C and frequency f_c, the second having amplitde mE_c/2 and frequency (f_c + f_s) and the third having amplitude mE_c/2 and frequency f_c – f_s..
• The AM wave consists three frequencies viz, f_c, f_c+f_s . The first frequency is the carrier frequency. Thus the process of modulation doesnot change the original carrier frequency but produces two new frequencies f_c+f_s  and f_c – f_s. which are called sideband frequencies.
• In amplitude modulation the bandwidth is from f_c – f_s. to f_c+f_s  ie 2f_s ie twice the signal frequency.
• Frequency spectrum of an amplitude modulated wave is shown in figure below.