Ultra-Wideband (UWB) Microstrip Bandpass Filters with Improved Upper-Stopband and Miniaturized Size

An ultra-wideband (UWB) microstrip bandpass filter with improved upper-stopband and miniaturized size is presented. A modified multiple-mode resonator is formed by transversely attaching three pairs of non-uniform and folded stubs. Its configuration is then investigated, aiming to allocate the first three resonant frequencies within 3.1-10.6 GHz band while suppressing spurious harmonics at the upper-stopband. Plus, its high-impedance sections in two sides are linked with two feed lines via interdigital coupled lines, leading to make up an initial UWB filter. Then, two-stage UWB bandpass filter is constructed to sharpen the rejection skirt and deepen the upper-stopband. Finally, two filters are optimized, fabricated and measured to give us an evident confirmation on the improved UWB performance.