A Dual-Band Tunable Balanced Filter With Independently Tuning Bands

A dual-band tunable balanced filter based on one hybrid microstrip-cavity structure is proposed in this brief. The cavity acts as one multimode resonator, and the differential feeding networks are printed on the PCBs. According to the electromagnetic (EM) field theory, TE101 and TE011 modes are generated under differential-mode (DM) excitation, and highly suppressed under common-mode (CM) excitation. The dielectric posts in the cavity can independently shift the resonant frequencies of the two modes. Moreover, the inter-resonator coupling of each band is independently controlled by the cross-shaped coupling slot. Therefore, good filtering differential response can be realized with in a wide band. For demonstration, a dual-band tunable balanced filter is designed and measured. The measured results indicate that the center frequencies of the two bands are tuned from 7.51 GHz to 8.22 GHz and 9.34 GHz to 10.56 GHz, respectively. Over the tuning range, the insertion losses vary from 0.5 dB to 1.5 dB and the CM suppressions in both passbands are under 50 dB and 42 dB. In this way, the proposed hybrid microstrip-cavity structure is feasible to design a reconfigurable balanced filter with wide tuning range and high CM suppression.