A Novel Ring Type Wideband Quasi-Elliptic Bandstop Filter With Controllable Equal Ripple Performance and Extra-High Attenuation Rate

In this article, a novel ring type wideband quasi-elliptic bandstop filter (BSF) is presented. The proposed structure consists of two parallel signal electrical paths between input and output ports, where one path has three cascaded transmission lines (TLs) and the other path employs two identical coupled lines (CLs) and one TL in the middle. Based on the coupling direction of CLs, there are two types of BSF topologies with quasi-elliptic response, where coupling direction only determines the number of transmission zeros (TZs), rather than the number of reflection zeros (RZs). By using the proposed quasi-elliptic synthesis approach for BSF, several design examples exhibit that equal ripple performance for both S-21 and S-11 can be controlled with extrahigh attenuation rate; meanwhile, the fractional bandwidth (FBWs) of S-21 could be adjusted flexibly. Compared with the former works, the proposed wideband BSF not only provides quasi-elliptic response of S-21 and S-11 with controllable equal ripple performance but also achieves better performance, such as extrahigh attenuation rate and compact circuit size. To prove the proposed concepts, three types of topologies operating at 1 GHz have been designed, fabricated, and measured in the experiment. Excellent agreement between the simulated and measured results is achieved to successfully prove the accuracy of the proposed synthesis approach.