Patent ID: 7683743

Claim:
A filtering circuit, comprising: an input terminal; an output terminal; a resonant circuit, coupled between the input terminal and the output terminal, the resonant circuit comprising M resonators arranged in sequence for transmitting a signal received by the input terminal to the output terminal via the resonant circuit, wherein M is a natural number greater than or equal to 3, the adjacent resonators are coupled with each other, the 1st resonator is coupled to the input terminal, and the M th resonator is coupled to the output terminal, wherein the resonant circuit further comprises M−1 transmission lines, respectively having a first terminal and a second terminal, wherein the k th transmission line couples between the k th resonator and the (k+1) th resonator, and k is a natural number and k<=M; a first coupling portion, coupled to the i th resonator; and a second coupling portion, coupled to the j th resonator, wherein the difference between i and j is greater than or equal to 2, and a part of th the signal received by the input terminal is transmitted from the 1st resonator to the i th resonator and then coupled to the second coupling portion via the first coupling portion.