High selectivity UWB bandpass filter using dual-mode resonators

A novel ultra-wideband bandpass filter using multilayer technology is reported. A dual-mode resonator is designed on the middle layer to generate a transmission zero at the upper stopband, and a short-circuited coupling line is designed on the top layer for generating a transmission zero at the lower stopband. These incorporated transmission zeros result in high selectivity for the proposed filter. The broadside coupling technique is adopted for the strong coupling to realise an ultra-wide passband. The proposed filter has been investigated using full-wave simulation and fabricated using multilayer liquid crystal polymer technology. Good agreement between simulated and measured results is observed. The measured UWB filter has a 3 dB bandwidth from 3 to 9 GHz with a small in-band insertion loss, a flat group delay, high selectivity and a compact size of 4.8 by 11 mm.