Design of coplanar bandstop filter based on open-loop-ring resonator and DGS for WLAN and UWB applications

Design of a novel compact bandstop filter composed on an open-loop-ring resonator and a defected ground structure (DGS) for coplanar waveguide is proposed and investigated. A simple coplanar open-loop-ring resonator has been designed and tested for the stop-band from 3.9 GHz to 4.3 GHz with tree transmission zeroes inside the stop-band. Two almost symmetrical poles are obtained on both sides of the stop-band. Then the proposed broad band bandstop filter based on the one open-loop-ring resonator has been transformed to the bandstop filter for WLAN (f_m=3.6 GHz) applications (f_m=3.6 GHz) with a narrower stop-band but with improved pass- and stop-band characteristics. It is realized by adding a conventional rectangular DGS slot inside the open-loop-ring. The equivalent circuit model has been proposed to describe the behaviour of the bandstop filters. The improved bandstop filter introduces the stop-band from 2.9 GHz to 4.2 GHz with two transmission zeroes within and more than -20 dB rejection. After that the proposed bandstop filter has been modified to the lowpass filter for ultra wide band (UWB) applications. The total size of the proposed structure is around 13.0×13.0 mm². All bandstop filters have been fabricated and measured. Measurements are in good agreement with simulation results.