Equipment, through the PHY then RJ45, the middle will add a network transformer. Some transformer center tap power, and some then connected to the ground. And then the power supply, the power value can be different, 3.3V, 2.5V, 1.8V have. What is the role of this transformer in the end it?
Here is a rough answer:
1, why do some of the middle tap power supply? Some grounding This is mainly used with the PHY chip UTP port drive type decision, RJ45 with transformer this drive type there are two, voltage drive and current drive. Voltage drive will be connected to the power supply; current-driven directly to a capacitor to the ground! So for different chips, the center of the tap connection, and PHY is closely related, the specific reference to the chip datasheet and reference design.
2, why then the power supply, and then take a different voltage? This is also used by the PHY chip data specified in the UTP port level decision. To determine what level, you have to take the corresponding voltage. That is, if it is 2.5v on the pull up to 2.5v, if it is 3.3v on the pull to 3.3v.
3. What is the role of this transformer in the end, can not take it. In theory, it is not necessary to take the transformer, directly connected to the RJ45, but also to work properly. However, the transmission distance is very limited, and when receiving different levels of network port, there will be impact. And the external interference on the chip is also great. When the network transformer is connected, it is mainly used for signal level coupling. One, can enhance the signal, so that the transmission distance farther; Second, the chip side and external isolation, anti-jamming capability greatly enhanced, and the chip to increase the protection of a great (such as lightning); Third, when the To different levels (if some PHY chip is 2.5V, and some PHY chip is 3.3V) of the network port, will not affect each other's equipment.