y and phase modulation 18/7 greater channel bandwidth than SSB. However, for a wideband rated power all the time, whereas for AM this only occurs for peak signal such as television, the bandwidth saving compared with DSB amplitudes of the baseband signal. This contributes a further im- is considerable. provement in signal to noise ratio. In PM, the frequency deviation (Bfm) is proportional to the fre- quency of the modulating signal as well as to its amplitude. Conse- 18.3 Frequency and phase modulation quently, for signals (such as speech) that have the major proportion of their energy at the lower end of the baseband, PM makes ineffi- 18.3.1 General cient use of the transmission path bandwidth compared with FM. Moreover, to demodulate a PM signal, the receiver must compare the phase of the incoming carrier with that of a locally generated The instantaneous angular frequency of an alternating voltage is carrier which must be very stable. FM is therefore preferred to PM given by Equation 18.21. This relationship between frequency and for the transmission of analogue signals. It is used whenever suffi- phase means that frequency modulation (FM) and phase modulation cient bandwidth can be provided. However, PM is widely used for (PM) are both forms of angle modulation. the transmission of digital signals as described in Chapter 19. radians / s (18.21) 18.3.2 Modulators and demodulators To generate FM, a voltage controlled oscillator is needed to enable A sinusoidal carrier modulated by a sinusoidal baseband signal the instantaneous frequency of the carrier to be varied by the may be represented by Equation 18.22 where B is the modulation baseband signal. This usually employs a varactor diode or a transis- index. tor reactance circuit to provide a voltage controlled capacitance (Coates, 1975). (18.22) The frequency of the oscillator is given by Equation 18.27. Thus, if C is given by Equation 18.28 then Equation 18.29 may be The maximum phase deviation is given by Equation 18.23 and, obtained. since Equation 18.24 ho