Input-Reflectionless Low-Pass Filter on Multilayered Diplexer-Based Topology

A class of input-reflectionless low-pass filter (LPF) using a multilayered diplexer-based architecture is presented. It consists of two signal-transmission parts: 1) a reflective-type LPF network built with an open-ended microstrip resonator and a shunt microstrip section and 2) a resistively terminated wide-band microstrip-to-microstrip vertical transition. Thus, based on the in-band transmission poles of the wideband transition at which the RF input signal energy not transmitted by the LPF part is dissipated by the terminating 50-Ω resistor, a higher selectivity LPF response with broadband reflectionless behavior is attained. To further increase the stopband bandwidth, as well as the attenuation levels and cut-off slope sharpness, in-series cascaded multi-LPF unit designs are engineered. The main RF operational principles and theoretical design examples of the proposed input-reflectionless LPF approach are detailed. Moreover, for practical validation purposes, a two-layered two-LPF unit microstrip prototype with extended stopband and broadband input-reflectionless characteristics is designed, fabricated, and tested.