Compact and Sharp-Rejection Bandstop Filter Using Uniplanar Double Spiral Resonant Cells

A novel compact bandstop filter composed of three cascaded uniplanar double spiral resonant cells (UDSRCs) for high attenuation rates is presented. Through the equivalent circuit prediction and parametric analysis, it is found that the UDSRC exhibits two controllable transmission zeros with great design flexibility through tuning the geometry parameters in a small range. Then, the influence of the stage separation between each UDSRC is investigated in order to get the appropriate stage separation. After optimization, a demonstration bandstop filter has been fabricated and measured. The results show that the attenuation rates on the lower and upper sides are 95 dB/GHz and 155 dB/GHz, respectively. Without any shunt stubs introduced, the length and width of the three cells are 28% and 4% of the guided wavelength at the midstopband frequency.