Ultra-wideband (UWB) bandpass filter with sharp selectivity and wide upper stopband

A compact quadruple-mode ultra-wideband (UWB) bandpass filter with sharp selectivity and extremely wide upper stopband is proposed in this paper, using a multi-stage stepped-impedance resonator, two open-circuited ends and a pair of high-low impedance stubs. The characteristics of the proposed multimode resonator are analyzed by simulation, demonstrating that the first four resonant modes can be appropriately located in the desired passband and other resonant modes in the upper stopband can be suppressed. To improve the skirt selectivity and out-of-band performance, two identical high-low impedance stubs are introduced into the design, without changing any resonant characteristic of the filter. Finally, the improved filter is fabricated and measured, and simulated and measured results are in good agreement to show that the proposed filter has a sharp selectivity with a -21.5 dB rejection stopband extended to over 30 GHz.