Low-pass filter

1. A low-pass filter comprising an input portion, an output portion, a third connecting portion, a high impedance transmission portion, a first low impedance transmission portion, a second low impedance transmission portion and a pair of parallel aligned metal plates, the high impedance transmission portion electrically connecting the input portion and the output portion, the high impedance transmission portion comprising a first high impedance transmission portion and a second high impedance transmission portion the first and second high impedance transmission portions being symmetrical about the third connecting portion, the first low impedance transmission portion electrically connecting the input portion and one end portion of the high impedance transmission portion, the second low impedance transmission portion electrically connecting the output portion and another end portion of the high impedance transmission portion, one of the pair of parallel aligned metal plates electrically connecting to the third connecting portion, and the other metal plate electrically connecting to ground of a PCB through a via.

2. The low-pass filter as recited in claim 1 , wherein the input portion and the output portion are arranged symmetrically about the pair of metal plates.

3. The low-pass filter as recited in claim 2 , wherein the input portion aligns with the output portion.

4. The low-pass filter as recited in claim 1 , wherein a slot is formed between the first and second low impedance transmission portions.

5. The low-pass filter as recited in claim 4 , wherein the slot is opposite to the pair of metal plates.

6. A low-pass filter comprising:

an input portion enabling input of an electromagnetic signal;

an output portion enabling output of the electromagnetic signal;

a high impedance transmission portion electrically connecting the input portion and the output portion and transmitting the electromagnetic signal therebetween;

a first low impedance transmission portion electrically connecting the input portion and one end portion of the high impedance transmission portion;

a second low impedance transmission portion electrically connecting the output portion and the other end portion of the high impedance transmission portion; and

a pair of parallel aligned metal plates, one of which electrically connects to the high impedance transmission portion, and the other metal plate electrically connecting to ground of a PCB through a via;

wherein the first and second low impedance transmission portions and the high impedance transmission portion generally surround a closed region in which the pair of metal plates are located;

wherein the high impedance transmission portion is symmetrical about the pair of metal plates.

7. The low-pass filter as recited in claim 6 , wherein the input portion and the output portion are arranged symmetrically about the pair of metal plates.

8. The low-pass filter as recited in claim 7 , wherein the input portion aligns with the output portion.

9. The low-pass filter as recited in claim 6 , wherein a slot is formed between the first and second low impedance transmission portions.

10. The low-pass filter as recited in claim 9 , wherein the slot is opposite to the pair of metal plates.

11. The low-pass filter as recited in claim 6 , wherein a capacitor is formed between the pair of metal plates.