An Ultra-Wideband Bandpass Filter Using Defected Ground Structure

An Ultra-Wideband bandpass filter (BPF) using novel circle-shaped defected ground structure (DGS) resonators and inter-digital capacitors is proposed in this paper. The BPF is designed by cascading a novel microstrip highpass filter (HPF) and a lowpass filter (LPF). The LPF with a 3 dB cutoff frequency of 6.1 GHz is constructed by a ladder-shaped transmission line on the top plane and the combined open loop DGS on the bottom one, resulting high selectivity and good stopband performance. The HPF consists of a ladder-shaped transmission line with zigzag slot on the top plane and circle-shaped DGS on the bottom one, providing two transmission zeros and steep rejection slope. The designed BPF with a 3 dB bandwidth of 2.8 GHz and center frequency of 4.7 GHz exhibits wide passband and sharp cutoff frequency response. The BPF also shows good stopband performance with rejection better than 28 dB from DC to 3.1 GHz in lower stopband and with rejection better than 15 dB from 6.3 to 18 GHz in upper stopband, respectively.