Design of UWB filter with SIRs and parallel-coupled three lines

An ultra-wideband bandpass filter (UWB) consisting of stepped-impedance resonators cascaded by parallel-coupled two-line and three-line structures is designed to satisfy Federal Communications Commission (FCC)´s spectrum mask. Firstly, a simpler equivalent circuit for the parallel-coupled three lines is derived based on the transmission line theory. Then, after the derivation of the Chebyshev-type transfer function, the filter´s characteristic equation relating the circuit parameters to the frequency responses is derived mathematically. Accordingly, a UWB filter with a fractional bandwidth (FBW) up to 110% is synthesized. The designed filter is then simulated and fabricated. The measured results agree well with the predicted ones, which validates the effectiveness of our new models and the corresponding synthesis theory.