TABLE OF CONTENTS The function of the receiver is to recover the original information that was used to modulate the transmitter. This process is referred to as demodulation, and the circuits that perform the recovery are called demodulators. The term detector is also used, and the demodulators in single superheterodyne receivers are sometimes called second detectors. However, today the term detector is seldom used this way.
Because of thermal, atmospheric, and man-made interference as well as transmission and circuit distortions, the demodulated signal is a distorted version of the modulating signal and is corrupted by the addition of noise. In the case of analog demodulators, we wish to minimize the distortion and noise so that the output signal waveform is as close to the original waveform as possible. In the case of digital demodulation, the objective is to produce a digital output of the same type as that at the transmitter input, with as few errors as possible in the digital output levels, with the correct signaling rate, and without addition or deletion of any symbols. Consequently, the performance measures for analog- and digital-signal demodulators differ. Often the digital demodulator is located separately in a modem, which also incorporates the digital modulator for a companion transmitter. In this chapter, we treat demodulators for analog and digital signals separately.
Modulation types for analog-modulated waves include: AM and its various derivatives (DSB, SSB, VSB, etc.), angle modulation (PM and FM), and various sampled pulse systems. Demodulation...
