Compact UWB bandpass filter with improved upper-stopband performance using multiple-mode resonator

In this paper, a novel ultra-wideband (UWB) bandpass filter (BPF) with compact size and improved upper-stopband performance has been studied and designed using multiple-mode resonator (MMR). The MMR is formed by attaching three pairs of ring open stubs in shunt to a high impedance microstrip line. By simply adjusting the radius of the circles of the ring stubs, the resonant modes of the MMR can be roughly allocated within the 3.1-10.6 GHz UWB band while suppressing the spurious harmonics in the upper-stopband. In order to enhance the coupling degree, two interdigital coupled-lines are used in the input and output sides. Thus, a predicted UWB passband is realized. Meanwhile, the insertion loss is higher than 20.0 dB in the upper-stopband from 11.7 to 33.5 GHz. The design is successfully realized in theory and verified by full-wave EM-simulation.