Wideband balanced bandpass filter using slot resonators and back-to-back structure

A wideband balanced bandpass filter with three layers is presented in this paper. In differential mode, the microstrip layers are back-to-back short-circuited SIR, which form the differential passband with the slot resonators, meanwhile introducing two transmission zeros out of the passband. Whereas, the back-to-back SIR resonates outside the differential band in common-mode and produce a transmission zero at the center frequency to achieve rejection. A proposed filter operating at 6GHz with a 3-dB bandwidth of 67% is designed and fabricated. Measurement and simulation results are in good agreement.