High Selectivity Cascaded Quadruplet Bandpass Filter with Three Pairs of Transmission Zeros

A novel microstrip Cascaded quadruplet (CQ) bandpass filter using quarter wavelength resonators and three-quarter wavelength resonators is proposed. Based on cross coupling and source-load coupling, three pairs of finite transmission zeros are generated at the stopband. As a result, the out-of-band rejection can be enhanced and high selectivity is achieved. Based on the proposed structure, a cascaded quadruplet filter centering at 3.0GHz is designed and fabricated. Good agreement between the measured and simulated data is obtained.