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-pass filter (LPF) with single attenuation pole at finite frequency using Defected ground structures (DGSs). The designed LPF consists of two Dumb-bell-shaped Defected ground structures (DB-DGS) and a Spiral-shaped Defected ground structure (SP-DGS). The prototype quasi-elliptic LPF is realized by introducing a capacitive admittance inverter into a Chebyshev LPF, and is optimized for the best compromise between the pass band insertion loss and the stop band rejection based on circuit simulation approach. The DB-DGS is used to provide the required inductance of the LPF, while the SP-DGS is used to provide the required attenuation pole. Accurate curve-fitting and the successive design approach are given to determine the required size of DGSs corresponding to the prototype elements. Experimental results of a five-pole LPF for 2.4GHz show that the designed filter has a much sharper cutoff frequency response than conventional one. A very wide stop band from 3.65GHz to 12.7GHz with rejection better than −23dB is achieved. In addition, this filter is only 13.61mm long, which is less than one-sixth wavelength of the microstrip mode at the design frequency.