High-selectivity wideband balanced bandpass filters using symmetrical multi-mode resonators

Two novel high-selectivity wideband balanced bandpass filters using symmetrical multi-mode resonators are presented. The proposed novel multi-mode resonator is used to realise a six-order wide passband in the differential mode. In addition, wide stopbands with insertion loss greater than 15 and 20 dB for the commode mode (0-18.5 GHz, 0-19 GHz) can also be achieved by the multi-mode resonators. The even/odd-modes of the symmetrical multi-mode resonators can be adjusted by changing the characteristic impedance of the shorted/open stubs. Two planar wideband balanced filters with 3 dB fractional bandwidths of 67.6 and 63.7% are designed and fabricated. The theoretical and measured results are in good agreement and show good in-band filtering performances and high selectivity.