. The transmitted signal is a some systems use two separate oscillators, instead of one whose fully modulated carrier of amplitude 1/2V 2 SO that the power at the frequency is shifted. However, this is not true frequency modula- carrier frequency, fc, corresponds to Therefore, half of the tion, since there is no continuity of phase at the instants of frequency total transmitted power is in the carrier and half is in the sidebands. transition. It may be termed non-coherent FSK. ASK can be demodulated by an envelope demodulator, as shown For each of the constituent ASK signals in Figures 19.5(c) and in Figure 18.5 (Chapter 18). However, at poor input signal/noise 19.5(d), half of the power lies in the sidebands and half in the carrier. ratios, a coherent demodulator gives a better output signal/noise Therefore the complete FSK signal has half of its power in side- ratio and SO a smaller bit error rate. Comparing Figures 18.6(b) bands and one quarter at f and at (Chapter 18) and Figure 19.4 shows that the coherent demodulator If the frequency deviation is large compared with the digit rate is an optimum detector for the signal s(t) = V cos W t. (high index f.m.), the resulting spectrum is that corresponding to For binary coded data, a coherent demodulator gives an output of amplitude shift keying of f and with the energy concentrated +V for '1' and zero for '0'. The error rate thus corresponds to that about these two frequencies as shown in Figure 19.6(a). If the of a unipolar baseband system and is given by Equation 19.27, frequency deviation is reduced to become comparable to the digit where the signal/noise ratio is that at the output of the demodulator. rate (low index f.m.), the sidebands about f and f merge, as shown It is shown in Chapter 18 that the output signal/noise ratio, So/No, in Figure 19.6(b). Consequently, little more bandwidth is required of the coherent demodulator is twice the input signal/noise ratio, than for ASK. S./N., when the latter is calculated from the sideband power alone. A high value 