Varactor-tuned bandstop filters with tunable center frequency and bandwidth

This paper presents a new tunable bandstop resonator architecture which allows for both center frequency and bandwidth tuning. It consists of a varactor-loaded transmission line resonator with odd symmetry about one axis coupled twice to a through line. No tuning elements are placed in the signal through path. It is based upon a recently developed proposed bandstop resonator coupling topology [2] where the bandwidth is determined not only from the strength of the couplings but also from the electrical length of the associated through line. Bandwidth tuning is achieved by differentially two varactors attached at opposite ends of the resonator, and center frequency tuning is achieved by tuning both varactors simultaneously.