Intermodulation distortion of the PLL FM demodulator in the presence of FM interference

The intermodulation (IM) distortion of the PLL FM demodulator in the presence of FM interference is evaluated using Monte Carlo simulation. Both the desired signal and the interferer are considered to be frequency modulated by Gaussian random signals. The simulation is based on solving the nonlinear integrodifferential equation of the PLL FM demodulator, considering the amplitude and distorted phase modulation of the filtered FM signals. The IM distortion of the second-order loop PLL FM demodulator with Butterworth and Bessel input bandpass filters is given for co-channel and adjacent channel FM interferers. Bessel filters of order greater than two are shown to compare favourably with Butterworth filters in front of the PLL FM demodulator in the presence of an adjacent channel FM interferer.