Proposal of Coplanar Stripline Series Stub Structure for Wideband Bandpass Filters

In this article, a simple kind of coplanar stripline (CPS) open-circuited series stub structure and its application in wideband bandpass filters (BPFs) are proposed and developed. Compared with its conventional dual-shunted CPS series stub counterparts, the proposed one is capable of achieving desired characteristics as the specified transmission-line (TL) prototypes behave. Based on such equivalent prototypes, three types of CPS open-circuited series stubs are then investigated to reveal the immunity against unexpected resonance caused by the underlying surface-wave-like mode. For demonstration, two examples, a second-order and a third-order wideband BPFs, are synthetically designed for equal-ripple Chebyshev response with prescribed fractional bandwidth (FBW) and return loss (RL). Finally, these two proposed filters with a respective FBW of 49%/70%, an RL of 16.426 dB, and a central frequency of 2 GHz are synthesized, simulated, fabricated, and measured. Measured and simulated results are reasonably in good agreement with each other, thus firmly validating the flexibility of the presented CPS series stub structure in the application of wideband BPFs.