Why network transformer is the standard design of Ethernet port of communication equipment？

As the physical layer interconnection component of the Ethernet network transmission system, the Ethernet hardware interface has gradually formed a set of standard interface design and specifications in the long-term hardware system design. As one of the key components, the network transformer also has its clear index definition requirements, which are closely related to the PHY drive part in the system board-level design. Some inadvertent changes in use may cause unpredictable consequences.

In our communication equipment, what is the role of the network transformer, and whether it must be selected?

The answer is less "I" is fine, using "I" is better. From the working principle, it can be considered that it is not necessary to connect directly to the RJ45 connector, and it is also workable. However, the price of the whole machine will be greatly reduced.

1. The transmission distance is very limited, and it will also have an impact when inputting network ports of different levels.

2. The interference and direct damage to the PHY from external factors are also great.

When the PHY output signal of the device is sent to a place of 100 meters or more, there will be a great loss of DC components. And if the external network cable is directly connected to the chip, the large pulse voltage and static electricity from the outside can easily cause damage to the chip. The network transformer filters the differential signal sent by the PHY with a differential mode coupling coil coupling filter to enhance the signal, and couples to the other end of the connecting network cable through the conversion of the electromagnetic field. In this way, there is no physical connection between the network cable and the PHY, but the signal is transferred, and the DC component in the signal is cut off, and data can also be transmitted in the device.

Common mode signal suppression of network transformer

When the differential mode signal passes through the network transformer when the common-mode signal passes through the transformer

When the common-mode signal passes through the network transformer, it presents a high impedance g to the common-mode signal. The common-mode signal refers to the input signals of the same magnitude and the same polarity at the two input terminals, so the common-mode signal will not be transmitted to the secondary circuit. However, the coupling capacitance Cww between the transformers provides a path for the transmission of common-mode signals:

To prevent the common-mode signal on the transformer from being coupled to the secondary through the coupling capacitor Cww channel, we use the center tap method in the design to provide a path for the common-mode signal to bleed. Use the center tap to increase the filter circuit, or reserve the position of the common-mode inductance on the differential signal trace of the network port):

Summary: In the network port design of communication equipment, the network transformer can not only play a role in anti-interference protection for the back-end circuit but also suppress common-mode radiation noise under the premise of satisfying the function (differential signal transmission).

Therefore, the network transformer is the standard design of the communication equipment Ethernet port!