and phase Q. Thus there are three basic modu- lation methods: amplitude modulation (AM), frequency modulation (FM) and phase modulation (PM). (18.1) (18.4) When modulation is employed, a modulator is needed at the sending end of a channel and a demodulator at the receiving end If the modulating signal contains several components, f1, f2, ---, recovers the baseband signal from the modulated carrier. The com- etc., then the modulated signal contains f-f1,fc-f2, etc., and bination of modulator and demodulator at a terminal is often etc. in addition to f If the modulating signal consists referred to as a modem. of a band of frequencies, as shown in Figure 18.1(a), the modulated f--, m f + F m fc-Fm f + F Figure 18.1 Frequency spectra for amplitude modulation: (a) baseband signal; (b) simple amplitude modulation (AM); (c) double sideband suppressed carrier (DSBSC) modulation; (d) Single sideband suppressed carrier (SSBSC) modulation; (e) vestigial sideband (VSB) modulation 18/4 Analogue modulation signal consists of two sidebands, each occupying the same band- width as the baseband signal, as shown in Figure 18.1(b). In the upper sideband, the highest frequency corresponds to the highest frequency in the baseband; this is therefore known as an erect A.M. Output sideband. In the lower sideband, the highest frequency corresponds to the lowest frequency in the baseband; this is known as an inverted sideband. Simple amplitude modulation makes inefficient use of the trans- mitted power, as information is transmitted only in the sidebands but the majority of the power is contained in the carrier. If a carrier as in Equation 18.2 is modulated to a depth m by the sinusoidal baseband signal of Equation 18.3, the output power is given by Figure 18.3 High level amplitude modulator Equation 18.5. the output voltage is given by Equation 18.10, which is the required AM wave. -1/2(1+1m2) (18.5) 4=A,(1+kv_) (18.9) The maximum sideband power is obtained with 100% depth of modulation. The power in the sidebands is then a third of the t