CBSE NOTES CLASS 12 PHYSICS
CHAPTER 15 COMMUNICATION SYSTEMS
Production and transmission of amplitude modulated wave
The modulating signal Am sin ωmt is added to the carrier signal Ac sin ωct to produce the signal x(t). This signal, x(t) = Am sin ωmt + Ac sin ωct is passed through a square law device which is a non-linear device which produces an output y(t) = Bx(t ) + Cx2(t) , where B and C are constants. Thus,
Using the trigonometric relation,
There is a DC term C/2(Am2+Ac2) and sinusoids of frequencies ωm, 2ωm, ωc, 2ωc, ωc – ωm and ωc + ωm. This signal is passed through a band pass filter which rejects DC and the sinusoids of frequencies ωm, 2ωm and 2ωc and retains the frequencies ωc, ωc – ωm and ωc + ωm. The device which only passes frequencies within a certain range and rejects frequencies outside that range is called band pass filter. The output of the band pass filter is of the same form as an AM wave.
The modulator is to be followed by a power amplifier which provides the necessary power and then the modulated signal is fed to an antenna of appropriate size for radiation.