rier and generate only the sidebands, as shown in superimposed on the collector supply voltage. Figure 18.1(c). This is known as double sideband suppressed carrier If the gain of the amplifier is given by Equation 18.9 and the modulation (DSBSC). The modulator acts as a switch, which multi- carrier and modulating waveforms by Equations 18.2 and 18.3 then plies the baseband signal by a quasi square wave carrier. Its output Amplitude modulation 18/5 Figure 18.5 Envelope demodulator waveforms: (a) input; (b) output thus contains upper and lower sidebands about the fundamental and envelope demodulator used with simple AM. If the incoming harmonics of the carrier frequency. As shown in Figure 18.6(a), a DSBSC signal is given by Equation 18.12 and the coherent demodu- bandpass filter is used to remove all components except the wanted lator multiplies this with a local carrier given by Equation 18.13, its sidebands. Thus, for a sinusoidal baseband input signal, the output output voltage is as in Equation 18.14. signal is given by Equation 18.11. (18.12) (18.13) (18.14) The components at frequencies are removed by a low pass filter and the baseband output signal is given by Equation 18.15. (18.15) Figure 18.6 Applications of balanced modulator: (a) product modulator; (b) coherent demodulator Thus V represents the original baseband signal, provided that the phase 0 of the local carrier is stable. A further economy in power, and a halving in bandwidth, can be obtained by producing a single sideband suppressed carrier (SSBSC) signal, as shown in Figure 18.1(d). If the upper sideband is used, the effect of the modulator is simply to produce a frequency translation of the baseband signal to a position in the frequency spectrum determined by the carrier frequency. If the lower sideband A balanced modulator used in this way is often called a product is used, the band is inverted as well as translated. modulator. The SSBSC signal requires the minimum possible bandwidth for To demodulate a DSBSC signal, it is necessary to use a c