Quintuple-Mode UWB Bandpass Filter With Sharp Roll-Off and Super-Wide Upper Stopband

This letter presents a quintuple-mode ultra-wideband (UWB) bandpass filter (BPF), using a multiple-stub-loaded resonator, consisting of multiple loaded stubs to a uniform transmission-line resonator, i.e., two asymmetrical stepped-impedance stubs at the center and four uniform-impedance stubs at the symmetrical side locations. Six modes including two odd and four even modes are controllable, while the first five modes are allocated within the UWB band. Two transmission zeros generated by the asymmetrical stepped-impedance stub are used to improve the passband selectivity greatly. The resonant mode and transmission zero chart are given, indicating that the higher modes could be suppressed by the transmission zeros generated by the stubs and the parallel-coupled lines. A compact planar UWB BPF is simulated, fabricated and measured, and the 20 dB stopband performance is extended to 29.7 GHz.