Planar filter with a zero-degree feed structure

摘要

A planar filter includes a ground plane underlying a dielectric substrate, and input and output resonators provided on the dielectric substrate. Each of the input and output resonators has a feed point provided on a transmission line conductor that is divided by the feed point into a long transmission line segment and a short transmission line segment, each which has an open end. A first signal path is defined from a first feed point provided on the input resonator to a second feed point provided on the output resonator. A second signal path is defined from the second feed point to the first feed point. A phase difference between the first and second signal paths is substantially equal to zero degree when the filter operates at a central operating frequency thereof.