Achieving Filter Effectiveness and Insuring Reliability

This paper presents recently employed methods and techniques for properly designing and testing electromagnetic compatibility (EMC) filters from the viewpoints of EMC and reliability. Because of the application of current probes in the latest EMC specifications, the use of attenuation as a filter design criteria has less value to the EMC engineer. A more applicable characteristic has been found to be the Transfer Impedance. High transfer impedance is best obtained if the filter presents a high input and output impedance in the stop band. Therefore, the T section filter is well suited for the job. The transfer impedance equation of a T section filter, using series inductors and a shunt capacitor, is as follows: --Z L) + jX T K~ C L (2 ~ L). C -- For frequencies well above cutoff and for the worse case condition of generator impedance approaching zero this can be reduced to: -- X 2 | T| XC-- As with attenuation, the transfer impedance is more easily given in logarithmic terms, or decibels above one ohm. With the transfer impedance characteristics and a predicted circuit noise voltage, it is easy to arrive at the final value of short circuit current that the current probe will see. In a T section filter, the values of transfer impedance are characteristically higher than those of attenuation in the stop band.