What is the role of the network transformer filter on the center tap?

Regarding how the middle taps are connected (some are connected to ground, some are connected to power, the level values ​​may be different), there is the following Q&A on the Internet: In Ethernet devices, when connecting to RJ45 through PHY, one is added to the middle network transformer. Some transformers connect the center tap to the power supply, and some transformers connect the capacitor to ground. When connecting the power supply, the power supply value can be different, there are 3.3V, 2.5V, 1.8V. What is the function of this network transformer filter?

network transformer

Here is a rough answer:

1, Why is it connected to a different voltage when connected to the power supply? This is also determined by the UTP port level specified in the PHY chip data used. What level determines, you have to connect the corresponding voltage. That is, if it‘s 2.5v, it will be pulled to 2.5v, and if it‘s 3.3v, it will be pulled to 3.3v.

2. Why are some central faucets connected to the power supply? Some are grounded? This mainly depends on the UTP port driver type of the PHY chip used. There are two drive types, voltage drive and current drive. For voltage drive, connect the power supply; for current drive, connect the capacitor directly to ground!

For different chips, the connection method of the center tap is closely related to the physical layer. See the chip‘s datasheet and reference design for details.

3. What is the purpose of this transformer, can it be left unconnected? Theoretically, you can connect the RJ45 directly without a transformer and it works fine. However, the transmission distance is very limited, and it also has an impact when it is connected to different levels of network ports. And the external interference to the chip is also very large. When the network transformer is connected, it is mainly used for signal level coupling. The first is to enhance the signal to make the transmission distance longer; the second is that the chip end is isolated from the outside world, and the anti-interference ability is greatly enhanced, adding a great protection to the chip (such as lightning strikes); third, when connected to different levels of network ports (such as some PHY chips are 2.5V, some PHY chips are 3.3V), it will not affect each other‘s devices.

Network transformers generally have the functions of signal transmission, impedance matching, waveform repair, signal clutter suppression and high-voltage isolation. When looking for the PHY chip data, there is no relevant description of the UTP port level, so the question remains here.