Compatible EMI Filters

A vexing problem in electromagnetic compatibility is the effective filtering of conducted interference from power-supply lines. Because of unavoidable and severe mismatch, conventional suppression filters operate only conditionally; such filters are often so large that they are omitted from the system. Three new classes of power-line filters without these limitations are described. They are active, truly lossy, and ruggedized ceramic filters, covering the frequency range from direct current to microwaves. Present filter test methods are shown to be misleading and are replaced by a rather simple realistic test. There is a set of filter classes that is rapidly growing in importance and, by necessity, is characterized by the absence of impedance matching. Without matching, all the elegant filter theories developed invalidate the very premise upon which they were based, and the theories are wholly inadequate and misleading. Such conditions exist for filters inserted into power lines; power wiring is contrasted with impedance-matched cabling that interconnects subsystems for irformation handling and for which conventional filtering is fully adequate. Suppression of conducted EMI (electromagnetic interference) constitutes an essential part of EMC (electromagnetic compatibility) and has, thus far, been frustrating and inadequate. Conventional EMI or RFI (radio-frequency interference) filters, because of interface mismatching are, in general, undependable. Another basic difference of power-line filters, as contrasted with information handling filters, is that they are strongly biased by the very power they have to pass. Three generic types of filters are described and discussed in some detail.