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The role of the network transformer filter on the center tap

Time:2022-09-02 Views:379
Regarding the connection method of the middle tap (some are grounded, some are connected to the power supply, and the level value may be different), there is the following Q&A on the Internet: Q: In an Ethernet device, when connecting RJ45 through PHY, a middle will be added in the middle. network transformer. Some transformer center taps are connected to the power supply, and some are connected to the capacitor to the ground. And when the power supply is connected, the power supply value can be different, 3.3V, 2.5V, and 1.8V are available. What exactly is the role of this network transformer filter?
A: The following is a rough answer:
1. Why do you connect different voltages when connecting to the power supply? This is also determined by the UTP port level specified in the PHY chip data used. What level is decided, you have to connect the corresponding voltage. That is, if it is 2.5v, it will be pulled up to 2.5v, and if it is 3.3v, it will be pulled up to 3.3v.
2. Why are some of the middle taps connected to the power supply? Some are grounded? This is mainly determined by the UTP port drive type of the PHY chip used. There are two types of drive types, voltage drive and current drive. The voltage-driven one should be connected to the power supply; the current-driven one should be directly connected to a capacitor to the ground!
For different chips, the connection method of the center tap is closely related to the PHY. For details, please refer to the datasheet and reference design of the chip.
3. What is the function of this transformer, can it not be connected? In theory, it is possible to connect it directly to the RJ45 without the need for a transformer, and it can work normally. However, the transmission distance is very limited, and when it is connected to a different level network port, it will also have an impact. And the external interference to the chip is also very large. When connected to the network transformer, it is mainly used for signal level coupling. First, it can enhance the signal and make the transmission distance longer; second, the chip end is isolated from the outside, the anti-interference ability is greatly enhanced, and a great protection effect (such as lightning strike) is added to the chip; When it reaches the network ports of different levels (if some PHY chips are 2.5V, some PHY chips are 3.3V), it will not affect each other‘s devices.
In general, the network transformer mainly has the functions of signal transmission, impedance matching, waveform repair, signal clutter suppression and high voltage isolation. Looking up the PHY chip information, I found no description of the UTP port level, so there are still doubts in this place.
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