FREQUENCY MODULATED RADIO TRANSMITTER In Chapter 2, the amplitude of a high-frequency (carrier) sinusoidal signal was varied in accordance with the waveform of an audio-frequency (modulating) signal to give an amplitude modulated (AM) wave which could be transmitted, received, and demodulated to recover the original audio frequency signal. In frequency modulated (FM) radio, the frequency of the carrier is varied about a fixed value in accordance with the amplitude of the audio frequency. | 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 4 FREQUENCY MODULATED RADIO TRANSMITTER INTRODUCTION In Chapter 2 the amplitude of a high-frequency carrier sinusoidal signal was varied in accordance with the waveform of an audio-frequency modulating signal to give an amplitude modulated AM wave which could be transmitted received and demodulated to recover the original audio frequency signal. In frequency modulated FM radio the frequency of the carrier is varied about a fixed value in accordance with the amplitude of the audio frequency. The amplitude of the carrier is kept constant. The waveform of a sinusoidal carrier modulated by a saw-tooth wave is shown in Figure . All signals carried on any transmission system will sooner or later be contaminated by noise so the susceptibility of the communication system to noise is an important consideration. The noise can be defined as a random variation superimposed on the signal. In AM systems the information to be transmitted is contained in the envelope of the carrier signal. The noise therefore appears on the envelope and has a direct role in corrupting the signal. In FM systems the information to be transmitted is contained in the variation of the frequency of the carrier about a pre-set value. The amplitude of the FM signal is kept constant and indeed if there are changes in the amplitude of the FM signal they are removed by clipping before demodulation. By comparison FM systems are less susceptible to degradation by noise. FREQUENCY MODULATION THEORY While a saw-tooth modulating signal provides a simple picture of the FM signal a sinusoidal modulating signal is the simplest for the derivation of the mathematical expressions to describe the FM signal. 111 112 FREQUENCY MODULATED RADIO TRANSMITTER Figure . The sawtooth waveform vs frequency modulates a carrier to give the output vfm. Note .
