A compact UWB bandpass filter with improved upper-stopband performance

A compact microstrip-line ultra-wideband (UWB) bandpass filter (BPF) using a multiple-mode resonator (MMR) is presented. This MMR consists of a folded stepped-impedance resonator (FSIR) and three open-ended stubs in shunt to it. In the design, the first four resonant frequencies of this MMR are properly adjusted to be placed evenly within the UWB while the fifth resonant frequency is raised above 16.0 GHz. Then the combination of capacitive-ended interdigital coupled lines and backside-apertures at two sides, by which the tight line spacing is greatly relaxed, are used to raise the coupling degree with the coupling peak near the center of the UWB and create an transmission zero to cancel the fifth resonant mode of the MMR to make a wide upper-stopband. After optimization of this filter, a good UWB bandpass behavior is theoretically realized and experimentally confirmed .The results shows that the insertion loss is higher than 20 dB in the upper-stopband (11.7-19.0 GHz), and the bandwidth is 3.3-10.7 GHz.