Design of millimeter-wave bandpass filter using microstrip stepped-impedance stubs

In this paper, a novel compact millimeter-wave bandpass microstrip filter using a three-mode resonator with two shunt stepped impedance open stubs is proposed. The resonator produces two resonant frequencies and two attenuation poles near the edges of the passband, so a sharp rejection and a wide bandwidth larger than 30% can be obtained. Simulated results indicate that the filter characterizes a low insertion loss in the passband and a wide suppression outside the band. A cascaded filter is designed to improve the out-of-band performance further. The simulated insertion loss |S₂₁|is less than 1.2dB, and the return loss is greater than -17.3dB in the passband. Moreover, the attenuation below -20dB is from 42 to 90GHz.