signal . In doing so, we take the help of a high frequency signal, known as the carrier wave , and a process known as modulation which attaches information to it. The carrier wave may be continuous (sinusoidal) or in the form of pulses as shown in Fig. .
. A sinusoidal carrier wave can be represented as c(t) = A c sin ( , c t + - ) ( . ) where c ( t ) is the signal strength (voltage or current), A c is the amplitude, , c ( = + c ) is the angular frequency and - is the initial phase of the carrier wave. During the process of modulation, any of the three parameters, viz A c , , c and - , of the carrier wave can be controlled by the message or FIGURE .
(a) Sinusoidal, and (b) pulse shaped signals. information signal. This results in three types of modulation: (i) Amplitude modulation (AM), (ii) Frequency modulation (FM) and (iii) Phase modulation (PM), as shown in Fig. .
. FIGURE . Modulation of a carrier wave: (a) a sinusoidal carrier wave; (b) a modulating signal; (c) amplitude modulation; (d) frequency modulation; and (e) phase modulation. Similarly, the significant characteristics of a pulse are: pulse amplitude, pulse duration or pulse Width, and pulse position (denoting the time of rise or fall of the pulse amplitude) as shown in Fig.
. (b). Hence, different types of pulse modulation are: (a) pulse amplitude modulation (PAM), (b) pulse duration modulation (PDM) or pulse width modulation (PWM), and (c) pulse position modulation (PPM). In this chapter, we shall confine to amplitude modulation on ly.