Miniaturized CQ bandpass filter with mixed coupling

In this paper, a compact fourth-order cascade quadruplet (CQ) passband filter (BPF) with improved selectivity is proposed based on the dual-metal-plane structure. To further improve the selectivity, more finite frequency transmission zeros (TZs) can be employed via introducing mixed electric/magnetic (EM) coupling in the cross-coupled arm between the nonadjacent resonators of the conventional CQ filter. The mixed EM coupling matrix with frequency variable of the proposed filter is analyzed in the filter design. A new BPF centered at 2.2 GHz with 15% fractional bandwidth has been designed and fabricated to verify the validity of the proposed method. The measurement result shows four finite TZs in the stopband, located at 1.72GHz @43.25dB, 1.99GHz@18.01dB, 2.67GHz@31.35dB, 3.04GHz@49.83dB, respectively. The circuit only occupies 0.08λ_g×0.10λ_g.