Exploiting Hi-Lo Inter-Digital DGS for High-Order Microstrip Bandpass Filters

With the fast developments of the new generation of communication systems and the limited spectrum, the needs of bandpass filters with sharp roll-off responses increase rapidly in recent years. In this research, we propose an inter-digital defected ground structure (DGS) for planar microstrip bandpass filters. The bottom ground plane of the microstrip line is composed of two rectangular opening slots and each of them contains a finger-like high-low (Hi-Lo) impedance strip. The high and low impedance strips represent an individual parallel resonator with different resonant frequencies. In other words, one Hi-LO impedance strip accounts for two parallel resonators. The two separated Hi-Lo impedance strips are coupled by a capacitive gap of the top microstrip line to form a fifth-order bandpass response. For a compact DGS, the order of the proposed bandpass filters can be increased easily by adding more Hi-Lo impedance strips in inter-digital arrangements. A prototype was implemented with a FR-4 substrate and fabricated with a standard PCB lithography. The dimensions of the fabricated device and the DGS were 0.18 A by 0.041s and 0.04 A by 0.03 A, respectively. The measured frequency response was centered at 2.13GHz with a fractional bandwidth of 38.6% and the roll-off was 0.03 dB/Hz.