Systematic Design of Wideband Bandpass Filters Based on Short-Circuited Stubs and λ/2 Transmission Lines

Abstract

In this letter, a new technique to design wideband bandpass filters (BPFs) is proposed. It is based on the classical filter topology formed by shunt short-circuited stubs connected by transmission lines, where the λ/4 connecting lines are replaced by λ/2 lines. In this way, the connecting lines have a double functionality: to control the coupling between resonators and to add additional poles, increasing the filter order up to 2N-1, where N is the number of short-circuited stubs. In addition, their function as inverters is, theoretically, for all spectrum, avoiding the limitation of the λ/4 lines working as inverters or the coupled-line coupling mechanisms of classical configurations, which are of narrowband nature. Design equations are provided with up to N degrees of freedom, which allows for a proper selection of impedance values for the filter design. As a verification, a fifth-order BPF of fractional bandwidth 50% centered at 2.5 GHz and with a Chebyshev response is implemented, fullfiling the expections and validating, in this way, the proposed approach.