Patent ID: 8248191

Claim:
A low-pass filter, comprising: a circuit board comprising at least one metal layer, and a dielectric layer positioned below the at least one metal layer; and a low-pass filter circuit defined in the metal layer, the low-pass filter circuit comprising a main line that has two parallel portions, and a modulating circuit serving as a capacitor connected to the main line; wherein the main line of the low-pass filter circuit comprises an input portion, a transmission portion and an output portion which are successively connected in series; the modulating circuit includes a first low impedance portion, a second low impedance portion, a third low impedance portion, and an extending portion that is connected to an end of the transmission portion; the first low impedance portion interconnects the input portion and the transmission portion; the second low impedance portion and the third low impedance portion are connected respectively to the extending portion and the output portion through two branching portions that respectively perpendicularly extend from the extending portion and the output portion; wherein the input portion comprises a first vertical segment and a first horizontal segment; the first horizontal segment perpendicularly extends from a terminal end of the vertical segment to the first low impedance portion; the first horizontal segment is shorter than the first vertical segment in length.