Analytical determination of the interference of commerical FM stations with airborne communication and navigation receivers and experimental verifications

This paper summarizes the work performed under contract FA77WA-3932 sponsored by the Federal Aviation Administration (FAA). The primary objective of the research sponsored here is the determination of an analytical model capable of predicting the effects of strong interfering FM broadcast stations on air-borne navigational (specifically the localizer receiver) and communication receivers. An analytical model of the RF-amplifier stage, using a third order nonlinearity, of airborne communication and navigation receivers capable of predicting the effects of multiple interfering FM stations is developed. The effects of receiver input filters and IF filtering are modeled. Measurement techniques useful in determining the various parameters of the model are described and evaluated. The effects of different signal powers, different signal frequencies, etc. on the parameters are investigated. Limits are postulated on the accuracy of the model and regions of validity defined. Brute-force interference due to strong interfering FM stations which saturates the front end and intermodulation distortion due to the interaction of two or more FM stations are considered. Various signal characteristics are considered for evaluating the degree of distortion introduced by interfering FM stations. The effects of filtering due to the receiver are considered and the resulting FM-to-AM conversion analytically described. Signal characteristics considered for evaluating distortion include spectral plots of the IF output, peak (cross-) modulation indices, RMS output of an envelope detector, etc. The effects of receiver desensitation due to saturation of the front end by the interfering signals are considered. Experimental results are given to verify theoretical results. A computer program was developed and is decribed here which theoretically predicts the effects of multiple interfering FM stations. Input data include FM station frequencies, powers, location, etc. and the output consist of interfering signal spectral plots at the IF output, peak cross-modulation indices, RMS output of AM detectors, etc. Tradeoff studies indicating the effects of the creation of additional FM stations and/or the results to be expected when one or more stations change their power levels, location, antenna patterns, etc. are illustrated by example. The number of FM stations that can be considered is limited only by the computing facilities available to the user. Consideration into decision criteria that can be reliably used in predicting potential interfering sources are considered. The results of the interference on the Course Deviation Meter (CDI) of a localizer receiver are investigated using single tone FM modulation of the carriers. The results are applied to specific airports where interference problems have occurred and the results obtained by the theoretical model are analyzed.