High CM suppression balanced broadband BPF with hybrid hairpin-interdigital structure

A compact, high selectivity and good common-mode (CM) suppression balanced broadband bandpass filter (BPF) is proposed using hybrid hairpin-interdigital structure. Under the differential-mode (DM) excitation, the filtering topology is formed by n+1 quarter-wavelength resonators (QWRs) and n hairpin resonators (HRs) instead of QWRs with coupled-line I/O coupling structure. But, the DM circuit still maintains high selectivity and broadband characteristics of the interdigital BPF. By reasonably choosing the coupling length and the length of the HR, the balanced BPF can not only obtain the desired DM frequency response, but the CM harmonic generated by the HR can be well suppressed by the CM transmission zero created by the coupling between the HR and half-wavelength resonator (HWR). No via-hole ground for the balanced broadband BPF is required, reducing the implementation complexity. Finally, the third-, fifth-, and seventh-order balanced broadband BPF prototypes are designed and fabricated, and the simulated and measured results show a good agreement. The CM signal for seventh-order case has been suppressed below -46.5dB in the whole DM frequency band.