Tunable Bandstop Filters Based on Varactor-loaded Multi-mode Resonators

This paper presents a tunable dual-band or single-band bandstop filter based on switchable varactor-loaded dual-mode resonator. The proposed tunable filters topologies consist of two switchable dual-mode resonators and main transmission line. When the switch is on, the switchable dual-mode resonator changes to dual-mode resonator, which can be used to design dual-mode tunable bandstop filter; when the switch is off, the switchable dual-mode resonator changes to single-mode resonator, which can be to realize single-mode tunable bandstop filter. The each mode of dual-mode resonator can be independent controlled by tuning loaded varactor, thus, the each stop-band can be independent adjusted. The lower stopband's (or first stopband) bandwidth of the dual-band tunable bandstop filter can be controlled by the coupling coefficient between main transmission line and dual-band resonator, the high stopband's (or second stopband) bandwidth can be independently tuned by the varactor between main transmission line line and dual-band resonator. For demonstration, a tunable dual-band or single-band bandstop filter is designed. The simulated results are presented to verify the theoretical predications.