Principles of Transistor Circuits, 9th Edition

Chapter 12

Modulators, demodulators, mixers and receivers

Introduction

The first step in communicating by radio is to establish a carrier-signal link between the two points. One characteristic of this signal is then varied to send information over the link. In radio and television broadcasting the amplitude or the frequency of the carrier is varied in accordance with the instantaneous value of the audio or video signal, a process known as modulation. Alternatively the carrier can be amplitude-modulated by a pulse train and a characteristic of the pulses can be varied to achieve modulation. For example, the pulse amplitude can be varied but there are other methods of modulation in which the amplitude is kept constant and the information is transmitted ...