Dual-band reconfigurable bandstop filter with independently controlled stopbands and constant absolute bandwidths

This paper presents a novel reconfigurable dual-band bandstop filter with independently controlled stopbands and constant absolute bandwidths (ABWs). The fundamental structure is based on λ/4 varactor-loaded resonators, and the key to constant ABW is choosing proper coupling regions between resonators. Theoretical analysis and calculation are carried out to determine physical parameters of the coupling regions. For demonstration, a second-order dual-band bandstop filter is implemented with 23.6% fractional tuning range of the first stopband from 1.27 to 1.57 GHz and 18.1% of the second stopband from 1.98 to 2.34 GHz, and 3-dB ABWs are 60 ± 3 MHz and 103 ± 5 MHz, respectively.