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 (f_zeros) and do not degrade the insertion loss at the resonant frequency (f_r). The tunable reactive elements enable control of the f_r. The prototype of the resonator is fabricated, and the measured characteristics demonstrate the feasibility of the proposed resonator. The f_r 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 f_zeros.