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.

This article is an expanded version from the 2020 Asia-Pacific Microwave Conference (APMC2020), Hong Kong, Dec. 8–11, 2020 [DOI: 10.1109/APMC47863.2020.9331419]. (Corresponding authors: Li Yang; Roberto Gómez-García.)

Agencia Estatal de Investigación

European Comission