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

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 lambda/4 connecting lines are replaced by lambda/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 lambda/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.

Automated summary

A new technique for designing wideband bandpass filters is proposed in this work, which involves using shorter connecting lines in the filter topology to control coupling between resonators and increase the number of poles. The method allows for a proper selection of impedance values for filter design and was validated through the successful implementation of a fifth-order BPF with Chebyshev response.