Insertion Loss of Mismatched EMI Suppressors

The insertion loss (IL) of EMI suppressors operating in systems with mismatched load and source impedances is studied. The design of an EMI suppressor is difficult because the actual impedances connected to it are often unknown. It has been recognized for a long time and by many authors [1]-[7] that the IL defined in a 50-ohm system (MIL-STD-220) is rather meaningless for general application. It may happen that the line voltage delivered to systems changes strongly after the insertion of the suppressor [8]. The IL under operating conditions may be quite different from that predicted. The source and load impedances are the variable parameters; for variations of one of them, the maximum and minimum IL's are calculated. Some general guidelines are also given in order to improve the perfonnance of suppressors.