ABSTRACT
A communications system is intended to transmit a signal carrying information from its source to its destination. In its most basic form, it is formed by three stages: transmitter, communications channel, and receiver. The transmission of the message can be carried out in baseband or bandwidth. The first step in a bandwidth communications system is performed by the transmitter, which has the function of adapting the signal so that it can be transmitted in a communication channel. In analog communications, the signal adaptation consists of the modulation process, which represents a frequency offset of the signal to be transmitted in a communications channel, called the message signal. According to the parameters of the generic sinusoidal signal, the modulation can be realized in three distinct ways, they are: amplitude modulation, frequency modulation and phase modulation. The chapter discusses the amplitude, frequency and phase modulations, with emphasis on their modulator and demodulator circuits.
