Design of Quasi-Elliptic Low-Pass Filter with Single Attenuation Pole Using Defected Ground Structures

This paper presents a design of a microstrip quasi-elliptic Low-passfilter (LPF) with single attenuation pole at finite frequency usingDefected ground structures (DGSs). The designedLPF consists of two Dumb-bell-shaped Defected ground structures(DB-DGS) and a Spiral-shaped Defected ground structure (SP-DGS). Theprototype quasi-elliptic LPF is realized by introducing a capacitiveadmittance inverter into a Chebyshev LPF, and is optimized for the bestcompromise between the pass band insertion loss and the stop bandrejection based on circuit simulation approach. The DB-DGS is used toprovide the required inductance of the LPF, while the SP-DGS is used toprovide the required attenuation pole. Accurate curve-fitting and thesuccessive design approach are given to determine the required size ofDGSs corresponding to the prototype elements. Experimental results of afive-pole LPF for 2.4GHz show that the designed filter has a muchsharper cutoff frequency response than conventional one. A very widestop band from 3.65GHz to 12.7GHz with rejection better than -23dBis achieved. In addition, this filter is only 13.61mm long, which isless than one-sixth wavelength of the microstrip mode at the designfrequency.