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A design methodology and equations are described for lumped-element filter prototypes having low-pass, high-pass, bandpass, or bandstop characteristics with theoretically perfect input- and output-match at all frequencies. Such filters are a useful building block in a wide variety of systems in which the highly reactive out-of-band termination presented by a conventional filter is undesirable. The filter topology is first derived from basic principles. The relative merits of several implementations and tunings are then compared via simulation. Finally, measured data on low-pass and bandpass filter examples are presented, which illustrate the practical advantages, as well as showing excellent agreement between measurement and theory.