Broadband microstrip filter apparatus having inteleaved resonator

Abstract

A broadband microstrip filter structure employs a first transmission line which basically consists of a series of capacitor plates which extend between a dielectric layer from a first location to a second location and each of the series of plates are connected together with a top plate on the top surface of the dielectric and a bottom plate located a given distance beneath the dielectric layer. A second transmission line alternates between the top and bottom layers of the dielectric and consists of a second series of capacitive plates whereby each capacitor is connected to an adjacent capacitor with the top plate of the first connecting to the bottom plate of the second and the bottom plate of the first connecting to the top plate of the second and so on. This pattern is repeated so that the conductor path alternates from the top to the bottom plate. Various capacitors are selected to provide predetermined length resonators while various other capacitors are provided to provide coupling sections. In this configuration each of the above-noted lines provide switching from the top to the bottom so that each conductor averages the same distance from the ground plane insuring identical impedances in each line. The structure allows tight coupling and enables the even and odd mode phase velocity differences to be compensated for due to the fact that the odd mode travels between the two conductors and the even mode travels between the conductor and the ground plane. In this manner the odd mode travels faster but further, thus the even mode and the odd mode move down the structure in synchronism.