When designing routers and switches, we will use a very common device between the Ethernet PHY chip and the RJ45 interface-a network transformer, also called data mercury. (Some network transformers are integrated in the RJ45. Pay attention to the selection. We rarely use this one at present) The main functions of the network transformer are signal transmission, impedance matching, waveform repair, signal clutter suppression and voltage isolation.
Theoretically speaking, it is not necessary to connect the network transformer. We directly connect the PHY chip to the RJ45, and the device can work normally, but the transmission distance will be limited at this time. When the network transformer is connected, one, It can strengthen the signal to make the transmission distance longer; secondly, the chip end is isolated from the outside, the anti-interference ability is greatly enhanced, and the chip has a great protective effect. At present, if we do not add other protection chips on the network port , When there is a network transformer, it can pass 2KV of static electricity and lightning; third, when it is connected to the network port of different levels (such as 2.5V for some PHY chips, and 3.3V or 1.8V for some), they will not interfere with each other The equipment has an impact.
As shown in the figure below, two 49.9 or 50Ω (accuracy 1%) terminating resistors will be connected in parallel between the transmitting differential line and the receiving differential line. The function of this resistor is to achieve impedance matching. For the first time ratio 1:1 The ratio of the input resistance to the output resistance of the transformer is also 1:1. The result of this parallel connection is a matching resistance of 100Ω from the output end. The characteristic impedance of the twisted pair we use now is mostly 100Ω.
Please note that in the SCH of our different chips, some of the center taps of the network transformer are connected to a 3.3V level, some are connected to 2.5V or 1.8V, and some are suspended. In fact, this is mainly determined by the UTP port drive type of the PHY chip. There are two types of this drive, voltage drive and current drive.
The voltage drive is connected to the power supply, and the current drive is directly connected to the capacitor to ground. As for why the connected voltage will be different when the power is connected, this is determined by the UTP port level specified by the PHY chip used. Therefore, for different PHY chips, the center tap of the network transformer will have different connections. We need to carefully check the chip information and reference design when designing. Once again, if we choose a current-driven PHY, and the middle tap of the external network transformer is connected to the power supply, the function will be affected and it will not even be usable! The power supply should be connected to 3.3V, but not 2.5V and 1.8V. Please pay attention to this!
The routing of the network port differential pair, the network port filter capacitor and the center-tap power supply terminal magnetic bead model, and the common mode suppression ratio of the network transformer itself will directly affect the effect of the board network port end EMI. This part should be paid attention to at the moment of design.
In addition, mention the RJ45 end. Our RJ45 connector is divided into two types, upward contact type and downward contact type. As shown below. The above is the RJ45 package that we commonly use upward contact. The following is our commonly used downward contact RJ45 package.
When we were drawing the board, in response to the structural requirements, we often encountered the problem of interchange of the two packages. To prevent errors in the connection. One of the easiest way is to do it in PIN order. That is, the first pair of the first pin of each network port. Until the 8th foot. Of course, the correspondence between SCH and PCB should be carefully checked after the theoretical change. To avoid errors.