se power, so no overall improvement is obtained. For simple AM, a coherent demodulator acts in the same way as for DSBSC. So does the envelope demodulator of Figure 18.4 at good signal to noise ratios. The diode acts as a switch operated by the incoming carrier and this multiplies the input signal by cos t, as does a coherent demodulator. At poor input signal to noise ratios, this is no longer true. The diode is switched by peaks of the noise voltage and the output signal to noise ratio is worse than for a coherent demodulator (Brown and Glazier, 1974; Coates, 1975). The signal to noise ratio performance of simple AM is, of course, worse than that of DSBSC because the input power includes that of the carrier as well as the sidebands. For 100% depth of modulation, the sideband power is only a third of the Figure 18.7 Quadrature method of SSBSC generation total power. Thus, for equal transmitter powers and receiver noise power densities, simple AM gives an output signal to noise ratio 4.8dB worse than for DSBSC or SSBSC transmission. one directly, and to the other after a phase shift of 90°. If the carrier By using SSBSC, it is possible to transmit two channels through and modulating signals are as in Equations 18.2 and 18.3, the the bandwidth needed by simple AM for a single channel; one uses outputs of the two modulators are given by Equations 18.16 and the upper sideband of the carrier and the other uses the lower 18.17 and V by Equation 18.18. sideband. This is known as independent sideband modulation and is used in h.f. radio communication (Hills, 1973). However, it is also (18.16) possible to do this using DSBSC. The transmitter uses two modula- tors whose carriers are in quadrature. The receiver uses two coher- ent demodulators whose local carriers are in quadrature. Equation (18.17) 18.15 shows that each demodulator produces a full output from the signal whose carrier is in phase (since cos 0=1) and zero output from (18.18) the signal whose carrier is in quadrature (since cos /2=0). This is called quadratur