Multilayered input-reflectionless quasi-elliptic-type wideband bandpass filtering devices on diplexer-based structures

Abstract

Classes of input-reflectionless wideband bandpass filters (BPFs) and balun BPF with quasi-elliptic-type (QET) responses are reported. They consist of two signal-transmission parts in multilayered diplexer-based topologies, as follows: 1) a BPF channel shaped by a two-/three-port reflective-type BPF/balun on a microstrip-to-microstrip vertical transition and 2) an input-absorptive bandstop filter (BSF) channel built with a shunt resistively terminated lowpass filter (LPF) that is composed of hybrid microstrip sections and open-circuit-ended microstrip stubs. Two real-frequency out-of-band transmission zeros (TZs) are generated in these RF filtering devices using a cascaded short-circuit-ended two-section microstrip line and two shunt open-circuit-ended half-wavelength microstrip lines, respectively. Using a higher order LPF network, enhanced passband amplitude flatness and improved stopband power absorption ratio levels for the devised BPFs are attained. As design examples, two third-order BPFs with a shunt resistively terminated microstrip T-junction and a $\pi $ -shape structure, respectively, are first provided. Afterward, a higher order BPF based on two cascaded replicas of a third-order BPF unit is designed to show highly increased stopband power attenuation levels and enhanced power absorption ratio profile within the stopband-to-passband transitions. Subsequently, their application to an input-absorptive QET fourth-order wideband balun BPF is presented. The operational principles of these BPFs and balun are detailed by the developed design procedures, in which their associated impedance-type design parameters are synthetically determined. As practical validation, four microstrip prototypes corresponding to three 2-GHz wideband BPFs and one 1-GHz broadband balun BPF are manufactured and tested. These input-reflectionless wideband filtering components experimentally feature the desired merits in terms of QET responses, enhanced passband amplitude flatness, and improved stopband power absorption ratio levels.