Balanced Bandpass Filter with Reconfigurable Frequency and Bandwidth Based on Multi-Mode Resonator

In this paper, a balanced filter with controllable frequency and bandwidth is proposed. The filter is based on a multi-mode resonator loaded with varactors. With the change of varactor capacitance, the odd mode and even mode resonant frequencies of differential mode can be changed simultaneously, then the frequency can be adjusted. On the other hand, the even mode resonant frequency of differential mode can be changed independently, which causes the bandwidth reconfigurable. In order to verify the correctness of the design theory, the proposed filter is fabricated and measured. The measured results show that the frequency of the balanced filter can be changed from 1.92 GHz to 2.5 GHz, the 3 dB bandwidth can be changed from 129 MHz to 182 MHz, and the common mode rejection in the passband can reach 20.5 dB. There is little difference between the measured results and the simulation results, which verifies the validity of the design theory.