Input-Reflectionless Quasi-Elliptic-Type Single- and Dual-Band Bandpass Filters Based on Passive Channelized Principles

Abstract

An approach to design quasi-elliptic-type planar filters with single- and dual-band bandpass transfer functions and input-absorptive capabilities is presented. Two-branch channelized passive circuit configurations are exploited for this purpose, in which the low-order reflective-type filtering profile of their branches is converted into the desired sharp-rejection input-reflectionless filtering action in the overall circuit. This is achieved by means of fully-destructive and frequency-selective transversal signal-interference effects at the total input and output accesses of the channelized filter, respectively. The theoretical operational principles of the proposed concept of single/dual-passband input-reflectionless two-branch channelized filter are detailed, along with design considerations for their RF transmission-line-based implementation. Besides, its generalization to <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\boldsymbol {N}$ </tex-math></inline-formula> -channel architectures is also analyzed. Furthermore, as the fundamental elements of the devised channelized filtering philosophy to increase selectivity, alternative solutions to realize their output phase-delay sections for a more-flexible control of the transmission zeros in the overall transfer function are discussed. For experimental-demonstration purposes, two microstrip proof-of-concept prototypes are developed and measured. They consist of 3-GHz single-band and <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$2.58/3.43$ </tex-math></inline-formula> -GHz dual-band designs with input-quasi-reflectionless spectral ratio above 2.5:1 and 3:1, respectively.