t of the VCO causes the coherent demodu- lator to produce an output voltage. This is fed back to the VCO to (18.38) change its frequency and SO reduce the phase error. In this way, the frequency of the VCO tracks the varying frequency of the incoming carrier. If the characteristic of the VCO is linear, its control voltage 18.3.4 Interference and noise is thus proportional to the incoming carrier frequency and provides a demodulated baseband output signal. The low pass filter elimi- In FM, the amplitude of the signal conveys no information. The nates noise and interference at frequencies above the baseband. receiver contains a limiter, so amplitude variations due to inter- The phase locked loop demodulator is less complicated than a ference and noise (unless very severe) have no effect. However, discriminator because it has no tuned circuits to align. It also has a interference and noise also perturb the phase of the signal and thus better output signal to noise performance at low values of input its instantaneous frequency. 18/10 Analogue modulation Table 18.1 Bessel functions of the first kind J1(B) J2(B) J3(B) J4(B) J5(B) J6(B) Jg(B) J10(B) 1.000 0.990 0.099 0.005 0.960 0.196 0.019 0.001 0.912 0.286 0.043 0.004 0.846 0.368 0.075 0.010 0.001 0.765 0.440 0.114 0.019 0.002 0.223 0.576 0.352 0.128 0.034 0.007 0.001 -0.260 0.339 0.486 0.309 0.132 0.043 0.011 0.002 -0.397 -0.066 0.364 0.430 0.281 0.132 0.049 0.015 0.004 -0.177 -0.327 0.046 0.364 0.391 0.261 0.131 0.053 0.018 0.005 0.001 0.150 -0.276 -0.242 0.114 0.357 0.362 0.245 0.129 0.056 0.021 0.006 0.300 -0.004 -0.301 -0.167 0.157 0.347 0.339 0.233 0.128 0.058 0.023 0.171 0.234 -0.113 -0.291 -0.105 0.185 0.337 0.320 0.223 0.126 0.060 -0.090 0.245 0.144 -0.180 -0.265 -0.055 0.204 0.327 0.305 0.214 0.124 -0.245 0.045 0.254 0.058 -0.219 -0.234 -0.014 0.216 0.317 0.291 0.207 The phasor diagram in Figure 18.14(a) represents an unmodulated If the carrier is frequency modulated, the signal output voltage is carrier with a small interfering signal separated from it by a