Design of low-pass filters with ultra-wide stopband using asymmetric DGS

A novel microstrip low-pass filter (LPF) with asymmetric defected ground structure (DGS) resonators is proposed. The asymmetric DGS is composed of a schematic Π-shaped DGS and is coupled to a T-shaped resonator at both terminators. By using this structure, the filter not only supports conventional DGS performances with a sharp rejection, but also exhibits an ultra-wide stopband compared with the traditional dumbbell-shaped DGS (DB-DGS) and the existing asymmetric DB-DGS. The successfully designed, implemented, and measured asymmetric DGS-LPF with 2 unit cells has a compact size of 36.8×12 mm² and operates with an insertion loss of less than 2 dB from dc to 1.9 GHz. From 2.6 GHz to 6.3 GHz, the rejection is better than 30 dB. Using the Π-shaped resonator in the DGS-LPF design makes the LPF flexible in terms of controlling resonances, bandwidth, and the stopband. The proposed asymmetric DGS-LPF is easily integrated with radio-frequency/microwave circuitry at a low manufacturing cost, especially for wireless communication applications. In this letter, the design of the DGS-LPF is presented and discussed, as well as the improvements in the S-parameter response of the DGS-LPF.