lds, the maximum frequency deviation is given by Equation 18.25. (18.27) Aq==B (18.23) C = C (1+kv) (18.28) (18.24) (18.29) AF = = B fm (18.25) To achieve linearity, it is therefore necessary for the frequency In PM the phase deviation is proportional to the modulating deviation to be kept to a small fraction of the centre frequency. Since voltage; therefore B is independent of its frequency and the fre- the frequency depends on device parameters, which may change, a quency deviation is proportional to it. In FM, the frequency devia- frequency stabilisation loop is usually added to control the centre tion is proportional to the modulating voltage; therefore the frequency from a crystal oscillator. deviation frequency is independent of its frequency and B is inver- Phase modulation can be generated by Armstrong's method sely proportional to it. Thus, for FM, the modulation index may be shown in Figure 18.8. A product modulator produces a DSBSC defined as in Equation 18.26. deviation of carrier baseband frequency (18.26) In angle modulation, the information is conveyed by the instanta- neous phase of the signal. Consequently, phase distortion in the transmission path causes attenuation distortion of the received sig- nal. The differential delay of the transmission path must therefore be closely controlled over the bandwidth required to transmit the signal. However, since the information is not conveyed by the amplitude of the signal, the receiver can contain a limiter to main- tain a constant signal amplitude. Consequently, non linear distortion in the transmission path does not cause distortion of the demodu- lated output signal; nor does attenuation/frequency distortion. Use of a limiter in the receiver also removes amplitude variations due to noise and so enables angle modulation to give a better output signal to noise ratio than AM. Moreover, since the amplitude of an angle modulated wave is constant, the transmitter can deliver its full Figure 18.8 Phase modulation by Armstrong's method 18/8 Analogue modulatio