Ring resonators for bandwidth and center frequency tunable filter

This paper presents a novel tunable resonator for a filter that independently changes its center frequency and bandwidth. The proposed resonator comprises a closed ring shaped transmission line with switches and three tunable reactive elements. The switches tune the frequencies at which transmission zeros appear (fzeros), and do not degrade the insertion loss at the resonant frequency (fr). The tunable reactive elements enable control of the fr. The prototype of the resonator is fabricated, and the measured characteristics demonstrate the feasibility of the proposed resonator. The fr of the resonator varies from 3.5 GHz to 5.1 GHz. A bandwidth tunable filter is also fabricated. Tuning range from 8% to 25% is obtained in the fractional bandwidth. These results approve the tuning of bandwidth by controlling fzeros.