Design of wideband filters with high skirt-selectivity using improved parallel-coupled three-line units

An improved compound three-line unit is proposed to implement wideband bandpass filters with high skirt-selectivity and out-of-band characteristics. A corresponding two-step design/synthesis scheme is proposed to design the filter. Firstly, a filter consisting of two cascaded parallel-coupled lines with two short-circuited stubs loaded at input and output-ports is synthesized based a Chebyshev Function. Then, in order to improve the skirt characteristics, two 5/4 wavelength SIRs are inserted into between the parallelcoupled lines, whereas twp two-section open-circuited stubs are designed to replace the short-circuited ones loaded at the ports to create multiple transmission zeros. Finally, a wideband filter with a bandwidth of 3-5GHz (FBW:50%) is designed to validate the effectiveness of the proposed structure and corresponding design scheme.