Effect of impedance control of the response of aircraft digital subsystems to HIRF [high-incidence RF field]

Research into the effect that complex source and load impedances can have on the predicted radiated susceptibility for digital avionics systems as a function of frequency is described. Differing logic families have differing input and output device impedances. Consequently, these logic types can have profoundly different responses over any given bandwidth. Understanding the effect that these impedances can have on the upset levels in circuit boards employing these logic families can aid the designer early in the design cycle to minimize random and potentially dangerous circuit responses. Analytical models describing the effects of incident EM fields on cables over ground planes have been developed. The development grew out of a need to understand HIRF-type phenomena inside the fuselage of aircraft. High-incidence EM fields can generate significant induced fields on the aircraft cables. These fields can in turn develop large voltages at the inputs to avionics circuits. These levels can and do approach the upset thresholds. Knowing the frequency dependency of the impedances attached to the ends of aircraft cabling is of utmost importance