Dual-mode dual-band microstrip bandstop filter design with independently tunable center frequencies.

In this article, a novel dual-mode dual-band bandstop filter design with independently tunable center frequencies is presented by using dual-mode square loop resonators. Tuning operation can be achieved with a constant fractional bandwidth. Varactor diodes are used instead of perturbation and reference elements. Two dual-mode square loop resonators in different electrical lengths are coupled to a transmission line connecting input and output ports. Center frequencies of both stopbands can be independently tuned by changing the capacitances of varactor diodes of both perturbation and reference elements. While the perturbation and reference varactors tune even mode resonance frequency together, odd mode resonance frequency can only be tuned by reference varactors. The designed filter was also fabricated for the experimental studies and it exhibits acceptable tunability operations in measurements. Center frequencies of the first and second stopbands can be tuned between 2.2 and 2.54 GHz and between 3 and 3.38 GHz, respectively. Rejection levels for both stopbands were obtained as better than −13 dB in the whole tuning range. [ABSTRACT FROM AUTHOR]