CH3-LINEAR-MODULATIONS

LINEAR MODULATION
AMPLITUDE MODULATION The transmission of an information-bearing signal (or the message signal) over a bandpass communication channel, such as a telephone line or a satellite channel, usually requires a shift of range of frequencies contained in the signal to another frequency range suitable for transmission. A shift in the signal frequency range is accomplished by modulation. Modulation is defined as the process by which some characteristic of a carrier signal is varied in accordance with a modulating signal. Here the message signal is referred to as the modulating signal, and the result of modulation is referred to as the modulated signal.


In continuous-wave modulation, a sinusoidal signal Ac cos (?ct + ?) is used as a carrier signal. Then a general modulated carrier signal can be represented mathematically as xc(t)=A(t) cos [?ct + ?(t)] ?c=2?fc


In Eq. (1.1), ?c is known as the carrier frequency. And A(t) and ?(t) are called the instantaneous amplitude and phase angle of the carrier, respectively. When A(t) is linearly related to the message signal m(t), the result is amplitude modulation. If ?(t) or its derivative is linearly related to m(t), then we have phase or frequency modulation.