Open-/Short- Circuited Coupled-Line Structures for the Design of High-Selectivity Bandpass Filter

Abstract

A new class of compact high selectivity bandpass filter (BPF) is reported in this paper by making use of open-/short-circuited coupled-lines which are connected at the input and output filter stages and a pair of symmetrical parallel-coupled-lines which are connected to a pair of open stepped-impedance resonators (SIRs). As a result six finite transmission zeros (TZs) at both sides of the passband are introduced. To analyze the characteristics of the proposed design, even-, odd-mode and ABCD analysis is used. A prototype BPF covering the entire S-band (2-4 GHz) with 3- dB fractional bandwidth (FBW) of 60% was designed, fabricated and tested. The filter exhibits measured insertion loss less than 0.8 dB in the entire passband, while the measured return loss within the passband is greater than 16 dB. The fabricated filter is very compact, with an appealing circuit size of less than 0.09λ <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">g</sub> <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> and at the same time it presents upper stopband suppression level greater than 40 dB.