A study on ultra-wideband bandpass filter using inverter lines and inter-digital finger structure

An ultra-wideband bandpass filter (UWB BPF) which consists of inter-digital finger resonators and impedance inverter lines was studied in this paper. The performances of the UWB BPF of three-fingers were calculated and its performances had the attenuation pole at both passband edges. The stopband attenuation and passband reflection were improved by inserting the inverter lines between the finger structure and the input and output (I/O) ports. By inserting the inverter lines, the impedance of the finger structure was well matched to that of the I/O ports in the passband. To improve the skirt characteristics, the UWB BPF of five-fingers was examined and achieved good performances with the 10 dB return loss in the passband from 3.1 to 10.6 GHz and the 50 dB attenuation in the lower stopband.