Coupled microstrip-line bandpass filters with wide upper stopband and high frequency selectivity

In this paper, a class of coupled microstrip-line bandpass filters with harmonic suppression and high selectivity are studied by exciting and utilizing the transmission zeros of parallel-coupled line sections in an inhomogeneous medium. A two-stage coupled microstrip-line bandpass filter is proposed and analyzed, and its prototype circuit is designed and fabricated to verify the proposed principle. As demonstrated in theory and experiment, two transmission zeros can be produced near the core passband so as to improve the filter selectivity and to suppress the first-order harmonic passband. In final, a three-stage bandpass filter with much wide upper stopband and frequency selectivity is designed and its predicted performance is well verified in experiment.