Adjacent-channel interference of a second-order PLL-FM-demodulator

In FM communication systems the receiver in general processes in addition to the useful channel also adjacent channels; thus interference disturbances are generated if no prefilter is used. The adjacent-channel interference has been investigated up to now in a second-order PLL-FM-demodulator but only in very special cases. For a deeper insight into the system behaviour the PLL is described in the paper by its nonlinear differential-equation in consideration of modulated signals. The solution of this equation by numerical time-domain simulations yields as characteristic data the critical amplitude-ratios of the involved carriers for the switch of synchronization. From these results the need of a prefilter in front of the demodulator is derived and its required stop band attenuation can be deduced. The theoretical results are confirmed experimentally