Analytical Determination of the Interference of Commercial FM Stations with Airborne Communication and Navigation Receivers and Experimental Verifications.

摘要

The effects of multiple-interfering FM stations on airborne communication and navigation receivers are predicted using a third-order model of RF amplifier stage. The effects of receiver input filters and IF-filtering are modeled. Models of the detector circuits, 90 Hz and 150 Hz filters, along with the course deviation indicator (CDI) are developed. Measurement techniques useful in determining the various parameters of the model and for evaluating interference potentials are given and evaluated. The effects of different signal powers, different signal frequencies, etc. on the parameters are reported. Limits are given on the accuracy of the model and regions of validity defined. 'Brute-force' interferences due to strong interfering FM stations which saturate the front end and intermodulation distortion due to the interaction of two or more FM stations are included. The effects of filtering due to the receiver are determined and the resulting FM-to-AM conversion analytically derived. Signal characteristics considered for evaluating distortion include peak (cross-) modulation indices and RMS output of an envelope detector. The effects of receiver desensitation due to saturation of the front end by the interfering signals are determined. Experimental results are given to verify theoretical results. A computer program is developed and reported which theoretically predicts the effects of multiple interfering FM stations. Input data included FM station frequencies, powers, location, etc. and the output consists of the RMS output of the receiver filters.