Compact Tunable Balanced Bandpass Filter Using a Folded S-Shaped Slotline Resonator (FSSR)

Abstract

A compact tunable balanced bandpass filter (BPF) using a folded s-shaped slotline resonator (FSSR) is proposed in this paper. The FSSR can be approximately viewed as a half-wavelength resonator, which determines the self-resonant frequency. By changing the reverse bias voltage of the varactors loaded on the FSSR, the resonant frequency of the proposed filter can be tuned. The balanced stepped-impedance microstrip-slotline transition structures are adopted to block the common-mode (CM) signals transmission and transfer the differential mode (DM) signals successfully. Therefore, the superior common-mode (CM) suppression can be achieved. To verify the design method, a compact tunable balanced BPF with frequencies ranging from 0.66 GHz to 0.96 GHz is fabricated and measured. A good agreement can be achieved between the results of measurement and simulation.