Balanced bandpass filter using ring resonator

A balanced bandpass filter (BPF) using multi-mode ring resonator is proposed, which is configured by two pairs of balanced ports and a ring resonator with loaded resistors and shorted stubs. The proposed filter can get dual-mode differential-mode response with single resonator, and it has the advantages of simple structure, high common-mode suppression and finite controllability of bandwidth (4%-9%). A prototype at 1.87 GHz is designed on RO4003C substrate to verify the theoretical prediction. Measured results show that the 20-dB impedance matching bandwidth is 8.9%, the insertion loss at the center frequency is 0.86 dB and the common-mode suppression is greater than 30 dB from 1 GHz to 3 GHz.