A Graphical Method for the Analysis and Synthesis of Electromagnetic Interference Filters

This paper provides the filter designer with a set of nomographs which enables him to immediately arrive at the insertion loss characteristic for any one, two, or three element electromagnetic interference filter. Conversely, the element values required to produce a desired amount of insertion loss at any given frequency can be quickly and accurately determined. Design information available in texts and handbooks is discussed, and the limitations and drawbacks of these familiar methods are considered. The need for a design method which includes provisions for dealing with non-ideally damped filter sections is demonstrated. General equations for the insertion loss of one, two, and three element filters are presented. From these general equations, expressions for cut-off frequency, damping ratio, and element values are developed. Ideal damping is defined, and the effect of non-ideal damping is examined. Based on the equations, a set of nine nomographs (the Craig Charts) is presented. Detailed instructions for the use of the charts are provided, along with illustrative examples. Actual insertion loss curves for several real filters are presented and compared with the results predicted by the Craig Charts.