se modulation. A simple discriminator can use a parallel tuned circuit whose Frequency modulation may be produced by using a phase modu- resonant frequency is offset from the centre frequency of the incom- lator whose modulating signal is the integral of the baseband signal, ing FM signal. As shown in Figure 18.11(a), a deviation of the input as shown in Figure 18.10. Therefore Equations 18.30 and 18.31 may frequency varies the output voltage and this is rectified by an be obtained and this corresponds to FM. envelope demodulator to give a baseband output voltage. Improved forms of discriminator use a pair of tuned circuits, one resonant on each side of the centre frequency, to obtain linearity of conversion over a greater range of frequency deviation (Langford-Smith, 1960; Sturley, 1965). Since the output voltage of a discriminator is propor- (18.30) tional to the amplitude of the input signal as well as its frequency, the discriminator must be preceded by a limiter to ensure that its input is of constant amplitude. A tuned circuit can also produce a phase shift depending on the (18.31) frequency deviation, as shown in Figure 18.11(b). The baseband signal can therefore be recovered by a coherent demodulator. The incoming FM carrier is fed directly to the coherent demodulator, but The Armstrong modulator of Figure 18.8 can therefore be used to it is shifted 90° (by a small series capacitance) before being applied generate FM. Similarly, phase modulation may be generated with a to the discriminator. The circuit is therefore known as a quadrature frequency modulator by differentiating the baseband signal, as shown in Figure 18.10(b). The Armstrong circuit only produces low FM demodulator (Gosling, 1986). index modulation. The frequency deviation can be increased by A form of demodulator which is increasingly used for FM is the phase locked loop circuit shown in Figure 18.12. When the voltage Frequency and phase modulation 18/9 signal to noise ratio (Pappenfus et al., 1964; Viterbi, 1966; Roberts, 1977). 18.3.3 Freq