An Electronically Reconfigurable/Tunable Dual Band Bandstop Filter for Cognitive Radio Applications

This study presents an electronically reconfigurable and tunable dual-band bandstop filter (DBSF) employing nested dual-mode resonators (DMRs). Varactor diodes (VDs) (SMV2020/2019-079-LF) are utilized instead of conductor patches (CPs) that act as perturbation and reference elements on nested DMRs, thereby enabling its multifunctional application in cognitive radio (CR) systems. The proposed configuration consists of two dual wideband bandstop filters (WBSFs) and a notched band (NB), where the passband (PB) bandwidth between the stopbands is reconfigurable, and both the passband and NB operating frequencies are tunable. Thus, adjusting the capacitances of the VDs enables two distinct filtering responses. To examine the topology, even-odd mode analysis of a single DMR with VDs is extracted. The filter design is simulated, fabricated, and two filters are measured based on this design. The measured results exhibit good agreement with the simulations, and the fabricated filter achieves a compact size of $0.45 \lambda \mathrm{~g} \times 0.31 \lambda \mathrm{~g}$, where $\lambda \mathrm{g}$ denotes the guided wavelength.