Ultra-wideband (UWB) bandpass filters with improved upper-stopband performance

A novel ultra-wideband (UWB) bandpass filter with improved upper-stopband performance is presented and constituted by integrating an initial UWB filter with two lowpass filters at its two sides. Firstly, a multiple-mode resonator is formed with first three resonant frequencies located within the 3.1–10.6GHz UWB band while its two-side high-impedance sections are excited by two interdigital coupled lines with enhanced coupling degree, leading to make up an initial UWB filter. Secondly, two 3^rd- or 5^th-order lowpass filters are designed with their cut-off frequency slightly higher than 10.6GHz (upper-end), aiming at suppressing the spurious upper-passband near the interested UWB passband. Finally, a resultant UWB bandpass filter is optimized, fabricated and measured to give us an evident confirmation on the improved upper-stopband performance.