Flexible Design of Dual-Band Common-Mode Filters Using Hairpin Ring Resonators and Defected Ground Slots

Adopting multimode hairpin ring resonator (HRR) and multiarm defected ground slot (DGS), a new class of dual-band common-mode filter with flexible frequency ratio and bandwidth is proposed in this article. First, the working theory of multimode short-ended HRR is introduced and its multidegree of design freedom is analyzed by changing the physical dimensions. It is noted that the frequency ratio of the second resonant mode to the fundamental resonant mode can be adjusted flexibly, i.e., less than 1.5 or greater than 3. Second, a novel multiarm DGS structure is designed and applied to feed differentially multimode HRR for the desired common-mode noise suppression performance. Third, a pair of microstrip stepped-impedance feeding lines loaded with the multiarm DGS are presented to meet the flexible bandwidth of common-mode filter design. Following these principles, three dual-band bandpass common-mode filters are implemented and fabricated with the flexible frequency ratios of 1.36/2.34/3.39, re-spectively. The common-mode suppression, which is below 30 dB, is from 1 to 9 GHz for electromagnetic interference rejection. Finally, measured results and simulated results are given to verify the flexibility of the proposed structure and design approach.