Direct synthesis of transmission-line elliptic bandpass filters

A direct synthesis method of elliptic function transmission-line filters is presented in this paper. The filter designs start from elliptic lumped prototype filters, and then Richards' transform is applied to obtain commensurate transmission-line circuits. To make the elliptic filters practicable in planar structures, two coupled-line structures and their equivalent circuits are introduced, and they can be assembled to realize the elliptic filters. Since the transmission-line filters are achieved with frequency scaling, passband filters with wide-bandwidth responses can be obtained directly. An example of fifth-order elliptic filter with a 40% fractional bandwidth is given. The simulation and measurement results have shown the feasibility of the proposed filter design procedure.