Novel wideband balanced bandpass filter based on multi-mode resonator

A novel wideband balanced bandpass filter based on a symmetrical multi-mode resonator is presented in this paper. A six-order wide passband for the differential mode can be easily realized by the multi-mode resonator with three shorted stubs. In addition, a wide stopband with insertion loss greater than 15 dB for the commode mode (0-18.4 GHz) can be also achieved by the multi-mode resonator with three open stubs. The even/odd-modes of the symmetrical multi-mode resonator can be adjusted by changing the characteristic impedance of the shorted/open stubs. A planar wideband balanced filter with 3-dB fractional bandwidth of 67.6% is designed and fabricated. The theoretical and measured results are in good agreement and show good in-band filtering performance and high selectivity.