A novel ultra-wideband (UWB) bandpass filter is presented in this pape r,which is based on a circuit model for an optimum short-circuited stub transmi ssion-line filter,whose unit elements or connecting lines are substituted by th e stepped impedance hairpin resonator with the cascaded structure.

The fundamental wave response of a stepped impedance resonator and its high order spurious bandpass are analyzed,and a method of designing broad stopband bandpass filters by using parallel-coupled microstrip line stepped impedance resonator is proposed.

In this method,first,a stepped impedance resonator and a half-wavelength resonator are both employed to construct an assembled multiband resonator.

Here we designed a miniature coaxial-cavity using stepped impedance resonator(SIR) as resonator element.

According to the basic principle of stepped impedance resonator(SIR),λg/4-type SIRs are adopted as elements to compose cross-coupling multilevel coaxial bandpass filter(BPF).

This filter is composed of three stepped-impedance resonators(SIRs),and can decrease the length of resonator effectively.

Then used the theory of parallel-coupled filter to design a SIR bandpass filter,and their parameters are simulated and optimized using Angilent ADS 2003,a RF and microwave circuit CAD software.

It introduces the basic structure and characteristic of the uniformity transmission-line, the Uniformity Impedance Resonator (UIR) and the Step Impedance Resonator (SIR), and analyses how to lead transmission-zero into the stop-band.