Lowpass-like Bandpass Filters

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  • Author: George Szentirmai
Dr. Ralph Levy in a paper described a method to design bandpass filters that look like lowpass ones, and have equal terminations. His method works for odd degree filters and he provided a short table for various degrees, bandwidths and passband loss ripple values. His method provides a closed form solution but relies on computing some esoteric functions leading to a fairly complicated design procedure. Here, I present a different method, based on iterative optimization, that is easy to implement, works for both even and odd degree functions and can also be extended in other directions. The procedure is now part of the S/Filsyn design program. This note will provide a brief description of the method and illustrate it with a pair of examples beyond the capability of Levy’s technique.