Planar Balanced Quadruplet Bandpass Filter With Extended Common- and Differential-Mode Two-Port Quasi-Reflectionless Ranges

A class of RF planar balanced fourth-order bandpass filter (BPF) with transmission zeros (TZs) and extended two-port quasi-absorptive bandwidths for both the differential-mode (DM) and common-mode (CM) operations is presented. It is composed of two replicas of a devised type of balanced-to-single-ended (BTSE) magic-T-based feeding stage with DM/CM input quasi-reflectionless behavior, which are back-to-back connected through a symmetrical-absorptive single-ended (SE) BPF unit. This SE BPF unit, which consists of a quadruplet with absorptive resistively terminated second-order bandstop filter (BSF) branches loaded at its input/output accesses, determines the total DM bandpass filtering transfer function. At the same time, the spectrally enlarged DM/CM two-port reflectionless characteristics of the overall balanced RF BPF device are obtained from the combination of the RF quasi-absorption mechanisms inherent to its BTSE magic-T-based feeding stages and its SE BPF unit. For experimental validation purposes, a 2-GHz proof-of-concept microstrip prototype is designed from its ideal transmission-line-based equivalent circuit model, manufactured, and characterized.