An investigation of `windowing´ phenomena seen during EM susceptibility profiling

When electronic equipment is tested for its susceptibility to electromagnetic interference, the borderline has to be established between the equipment functioning normally and it beginning to malfunction. The interference power or current is usually measured at this borderline and plotted as a graph against frequency. Undocumented reports have been made of cases where this change-over from functioning to non-functioning has not been a simple border, but has involved multiple transitions. It was decided to investigate this effect and its causes by building and testing a simple circuit. It was hoped that the results of this would be useful in improving the accuracy of future susceptibility tests on `real´ systems