High-selectivity microstrip bandpass filters using stub-loaded resonators

Abstract

In this paper, high selectivity microstrip bandpass filter (BPF) with loaded stubs is proposed. The filter A is consisted of a half-wavelength transmission line with two different open-ended stubs at the line center. Compared with conventional uniform stub, the circular impedance-stepped stub can improve the performance of the upper-stopband. This filter can generate three operation modes, which can be easily adjusted individually. Owing to its symmetrical structure, which can be analysed by the odd-even-mode method. When the coupling is weak, the even-mode frequency can be flexibly controlled by the open-ended stubs. Due to the two unsymmetrical stubs, two transmission zeros are brought out at low and high rejection bands, which leads to high selectivity. In order to improve the filter's in-passband and out-passband performance, another two identical open-ended stubs are introduced. Simulated results show that two transmission poles are added in passband and two transmission zeros are added in up-stopband. The measured performance of two kinds of filter agrees well with the result of simulation.