Printed circuit board capable of reducing return loss of differential signal and electronic device using the same

1. A printed circuit board (PCB) capable of reducing return loss of differential signals, the PCB comprising:

a receiver component;

a transmission line comprising a positive signal transmission line and a negative signal transmission line;

a transmitter component configured to transmit a differential signal to the receiver component via the transmission line; and

a capacitance unit spaced from the receiver component for a predetermined distance S, wherein, the predetermined distance S is calculated by a formula S=V×(n*UI−(C 1 +C 2 )×Zo/2)/2, where V is a speed of the transmission line; C 1 is an equivalent capacitance value of the receiver component, C 2 is a capacitance value of the capacitance unit, Zo is a resistance value of the transmission line, UI is a half of one cycle of the differential signal, and n is a value less than or equal to 1.5 and equal to or greater than 0.5.

2. The PCB according to claim 1 , wherein the differential signal output by the transmitter component is reflected by both the receiver component and the capacitance unit; the receiver component produces a first reflected signal and the capacitance unit then produces a second reflected signal, a phase difference between the first reflected signal and the second reflected signal is 180 degree when the receiver component and the capacitance unit are spaced from each other for the predetermined distance.

3. The PCB according to claim 2 , wherein the capacitance unit comprises a first capacitor, the first capacitor is connected between the positive signal transmission line and the negative signal transmission line, and is spaced from the receiver component for the predetermined distance, a capacitance value of the first capacitor is the capacitance value of the capacitance unit.

4. The PCB according to claim 2 , wherein the capacitance unit comprises a first via and a second via, the first via is located on a path of the positive signal transmission line, and the second via is located on a path of the negative signal transmission line, the first via is equivalent to a second capacitor connected between the positive signal transmission line and ground, the second via is equivalent to a third capacitor connected between the negative signal transmission line and ground; the first via and the second via are spaced from the receiver component for the predetermined distance and a sum of equivalent capacitance values of the first via and the second via is equal to the capacitance value of the capacitance unit.

5. The PCB according to claim 2 , wherein n=1.

6. The PCB according to claim 1 , wherein the receiver component is a processor, a memory, or a codec.

7. The PCB according to claim 6 , wherein the transmitter component is a processor, a memory, or a codec.

8. An electronic device comprising:

a printed circuit board (PCB) comprising:

a receiver component;

a transmission line comprising a positive signal transmission line and a negative signal transmission line;

a transmitter component configured to transmit a differential signal to the receiver component via the transmission line; and

a capacitance unit spaced from the receiver component for a predetermined distance S, wherein, the predetermined distance S is calculated by a formula S=V×(n*UI−(C 1 +C 2 )×Zo/2)/2, where V is a speed of the transmission line; C 1 is an equivalent capacitance value of the receiver component, C 2 is a capacitance value of the capacitance unit, Zo is a resistance value of the transmission line, UI is a half of one cycle of the differential signal, and n is a value less than 1.5 and greater than 0.5.

9. The electronic device according to claim 8 , wherein the differential signal output by the transmitter component is reflected by both the receiver component and the capacitance unit; the receiver component produces a first reflected signal and the capacitance unit then produces a second reflected signal, a phase difference between the first reflected signal and the second reflected signal is 180 degree when the receiver component and the capacitance unit are spaced from each other for the predetermined distance.

10. The electronic device according to claim 9 , wherein the capacitance unit comprises a first capacitor, the first capacitor is connected between the positive signal transmission line and the negative signal transmission line, and is spaced from the receiver component for the predetermined distance, a capacitance value of the first capacitor is the capacitance value of the capacitance unit.

11. The electronic device according to claim 9 , wherein the capacitance unit comprises a first via and a second via, the first via is located on a path of the positive signal transmission line, and the second via is located on a path of the negative signal transmission line, the first via is equivalent to a second capacitor connected between the positive signal transmission line and ground, the second via is equivalent to a third capacitor connected between the negative signal transmission line and ground; the first via and the second via are spaced from the receiver component for the predetermined distance and a sum of equivalent capacitance values of the first via and the second via is equal to the capacitance value of the capacitance unit.

12. The electronic device according to claim 9 , wherein n=1.

13. The electronic device according to claim 8 , wherein the receiver component is a processor, a memory, or a codec.

14. The electronic device according to claim 13 , wherein the transmitter component is a processor, a memory, or a codec.

15. The electronic device according to claim 8 , wherein the electronic device is a table computer, a mobile phone, a digital camera, a digital photo frame.