Compact wideband bandpass filter based on coupled line stub with high selectivity

Two bandpass filters with the cross shaped resonator and parallel coupled lines are proposed. It has two fixed transmission zeros (TZ) which are located at 0 and 2pi. With the assistance of two types of parallel coupled line, two bandpass filters have other two TZs. Two TZs only depend on the odd and even mode impedance of open parallel coupled line stubs or short parallel coupled line stubs. By adjusting the odd- and even-mode impedance values, two TZs can be adjusted and the frequency selectivity of the bandpass filters are improved. Two bandpass filters are fabricated and measured. The experimental results are basically consistent with the simulation results, which verifies the filter design method.