Studies on synthesis design of ultra-wideband parallel-coupled line bandpass filters with Chebyshev responses

In this paper, synthesis design of a class of ultra-wideband parallel-coupled line bandpass filters with Chebyshev responses is presented. Through the exact synthesis approach with virtue to closed-form insertion loss function, characteristic impedances of each coupled and connecting line section are explicitly determined. Based on the obtained impedance values, the realizable range of filtering bandwidth is extensively investigated. Moreover, under the restriction in fabrication and material, physical dimensions of the coupled line section are discussed using the modified design formulas. Finally, a five-pole parallel-coupled line bandpass filter with a fractional bandwidth of 90.1% is designed, fabricated and measured to provide an experimental validation on the presented synthesis approach.