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A novel type of multi-mode resonator (MMR) is proposed in this letter, which is constructed by cascading interdigital coupled microstrip line sections with short-ended stepped impedance stubs being loaded. Its transmission line model is set up and analyzed, which shows that it has seven resonances. Finally, an ultra-wideband filter based on this type of MMR is designed, fabricated and measured. The measured results show that the filter has a sharp skirt and an out-of-band rejection level as good as 40 dB. Meanwhile, return loss is better than 17 dB. Group delay is also flat in the passband.