THE FREQUENCY MODULATED RADIO RECEIVER In amplitude modulation, the frequency of the carrier is kept constant while its amplitude is changed in accordance with the amplitude of the modulating signal. In frequency modulation, the amplitude of the carrier is kept constant and its frequency is changed in accordance with the amplitude of the modulating signal. It is evident that, if a circuit could be found which will convert changes in frequency to changes in amplitude, the techniques used for detecting AM can be used for FM as well | 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 5 THE FREQUENCY MODULATED RADIO RECEIVER INTRODUCTION In amplitude modulation the frequency of the carrier is kept constant while its amplitude is changed in accordance with the amplitude of the modulating signal. In frequency modulation the amplitude of the carrier is kept constant and its frequency is changed in accordance with the amplitude of the modulating signal. It is evident that if a circuit could be found which will convert changes in frequency to changes in amplitude the techniques used for detecting AM can be used for FM as well. In Section three frequency-to-amplitude conversion circuits were discussed and their performance in terms of linearity and dynamic range were examined. It therefore follows that the FM receiver must have the same basic form as the AM receiver. The structure of the FM receiver is as shown in Figure . The superheterodyne technique is used in FM for the same reasons it is used in AM it translates all incoming frequencies to a fixed intermediate frequency at which the filtering process can be carried out effectively. The antenna is responsible for capturing part of the electromagnetic energy propagated by the transmitter. The basic rules of antenna design apply but because in commercial FM radio the frequency of the electromagnetic energy is between 88 and 108 MHz it is practical to have antennas whose physical dimensions are within tolerable limits. The radio-frequency amplifier raises the power level to a point where it can be used in a mixer or frequency changer to change the center frequency to a lower frequency - the intermediate frequency IF . The mixer in conjunction with the local oscillator translate the incoming radio frequency to an intermediate frequency of MHz. There is nothing special about an intermediate frequency of MHz except .
