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THE AMPLITUDE MODULATED RADIO RECEIVER The electromagnetic disturbance created by the transmitter is propagated by the transmitter antenna and travels at the speed of light as described in Chapter 2. It is evident that, if the electromagnetic wave encounters a conductor, a current will be induced in the conductor. How much current is induced will depend on the strength of the electromagnetic field, the size and shape of the conductor and its orientation to the direction of propagation of the wave | Telecommunication Circuit Design Second Edition. Patrick D. van der Puije Copyright 2002 John Wiley Sons Inc. ISBNs 0-471-41542-1 Hardback 0-471-22153-8 Electronic 3 THE AMPLITUDE MODULATED RADIO RECEIVER INTRODUCTION The electromagnetic disturbance created by the transmitter is propagated by the transmitter antenna and travels at the speed of light as described in Chapter 2. It is evident that if the electromagnetic wave encounters a conductor a current will be induced in the conductor. How much current is induced will depend on the strength of the electromagnetic field the size and shape of the conductor and its orientation to the direction of propagation of the wave. The conductor will then capture some of the power present in the wave and hence it will be acting as a receiver antenna. However other electromagnetic waves emanating from all other radio transmitters will also induce some current in the antenna. The two basic functions of the radio receiver are 1 to separate the signal induced in the antenna by the transmission which we wish to receive from all the other signals present 2 to recover the message signal which was used to modulate the transmitter carrier. THE BASIC RECEIVER SYSTEM DESIGN In order to separate the required signal from all the other signals captured by the antenna we use a bandpass filter centered on the carrier frequency with sufficient bandwidth to accommodate the upper and lower sidebands but with a sufficiently high Q factor so that all other carriers and their sidebands are attenuated to a level where they will not cause interference. This is most easily achieved by using an LC tuned circuit whose resonant frequency is that of the carrier. 79 80 THE AMPLITUDE MODULATED RADIO RECEIVER Figure . a The envelope detector circuit. The diode half-wave rectifies the AM wave and the RC time-constant follows the envelope with a slight ripple. b The input signal to the envelope detector. c The output signal of the envelope detector.