Compact millimeter-wave ultra-wideband bandpass filter using dual-mode ring resonator and multiple- mode parallel-coupled line structure

A compact millimeter-wave ultra-wideband bandpass filter is proposed for vehicular radar systems. The filter is built by cascading a dual-mode square ring resonator with a multiple-mode parallel-coupled line structure. The ring resonator is loaded with two 3lambda₀/4 open stubs to realize a passband width of about 30%. Both the dual-mode ring resonator and the multiple-mode parallel-coupled line structure produce a couple of transmission zeros in the lower and upper stopbands of the filter. As a consequence, the filter has not only a sharp rejection near the passband, but also an extremely wide stopband. The measured response of the filter agrees well with the design simulation, and shows low insertion loss within the passband.