Compact Tunable Balanced Bandpass Filter With Novel Multi-Mode Resonator

A novel multi-mode resonator is presented to design compact tunable balanced bandpass filters (BPFs). From the theoretical analysis, the common-mode (CM) resonant frequencies are different from that of the differential-mode (DM) response, so the CM suppression can be achieved without adding any extra lumped elements. For the DM response, the modal frequencies of odd and even modes can be adjusted simultaneously by varying the varactors. This helps to design the DM response with tunable passband center frequency. Moreover, the even-mode resonant frequency can be tuned independently, leading to controllable passband bandwidth of the DM. To verify the design method, a compact tunable balanced BPF is developed, fabricated and measured. Good agreement can be attained between the measurement and simulation.