Sharp-rejection and ultra-wide bandstop filter with five transmission zero frequencies

This study proposes a new modified optimum wideband bandstop filter (BSF). Detailed theoretical analysis reveals that five transmission zero frequencies appear in the proposed BSF's response to compose an ultra-wide BSF with stiff rejection. Moreover, the filter's bandwidth, stopband rejection level and passband performance can be tuned flexibly by changing the impedances of the configuration. Both the simulations and measurements of the fabricated filter demonstrate good wideband BSF performance including an ultra-wide 20 dB rejection bandwidth of 140%, sharp attenuation rates more than 227.3 dB/GHz at both skirts, lower passband insertion loss less than 1.5 dB at both lower and upper passband and very compact size of λg2.