Theoretical Design of UWB Bandpass Filters Using Multistage Three-Line-Units

摘要

An improved compound three-line unit, which can also be treated as the one constructed by inserting a 5/4-wavelength stepped-impedance resonator (SIR) into between parallel-coupled two lines, is proposed to implement wideband bandpass filters with high skirt-selectivity and out-of-band characteristics. A corresponding design/synthesis scheme is proposed after the mechanism of the proposed structures is investigated. Two pairs of transmission zeros are found in lower and upper stopbands due to the inserted 5/4-wavelength SIR, whereas a pair of spurious passbands are found near the transmission zeros farther from the passband. To suppress it, two open-circuited stubs, whose transmission zeros are the same frequencies of the spurious passbands, are used to replace the short-circuited ones loaded at input- and output-ports. 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.