A novel ultra-wideband (UWB) bandpass filter (BPF) using three multi-mode resonator (MMR) units in series is proposed in this paper. Each of the units is a resonator made by attaching a ring open stub in shunt to a high impedance microstrip line. The units form a MMR to improve the selectivity. By properly adjusting the radius of the circles of the ring stubs and the length of the coupling segments, the resonant modes of this units can be allocated within the 3.1-to-10.6 GHz UWB band while the upper-stopband is also wide. Simulation results are achieved and analyzed. The filter shows good in-band and out-band performance and sharp selectivity.
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