On RJ45 interface

1、 Interface Overview.

RJ45 Ethereum interface is the most widely used communication equipment interface at present. The electromagnetic compatibility performance of Ethereum is related to the stable operation of communication equipment.

The EMC design scheme of Ethereum is designed from three aspects: interface schematic diagram, structural design and cable design.

2、 EMC design of interface circuit schematic diagram.

Outline of interface loop design: in terms of EMC principle, the EMC problems related to interference suppression and sensitivity design are solved at the design level. At the same time, the loop is compatible with the lightning protection design of 100M Ethernet interface.

The lightning protection circuit design can pass the interface lightning protection test of common mode 2KV and contacting 1kV unshielded balanced signal in iec61000-4-5 or gb17626.5 standards.

Circuit EMC Design Description:

(1) Key points of circuit filter design:

In order to suppress RJ45 interface Through the common mode interference brought by the cable, it is suggested to change the conventional network transformer into the network transformer with common mode suppression interface in the design process. The schematic diagram of the transformer is as follows.

RJ45 interface The NC null pin must be designed by Bob Smith circuit to achieve signal impedance matching and suppress external interference. After testing, Bob Smith circuit has the effect of suppressing interference of about 10 dB.

The network transformer has isolation function, but in order to improve the distribution capacity of some pf between the primary coils of the transformer, in order to improve the isolation function of the transformer, it is recommended to increase the floor filter capacity on the secondary circuit of the transformer, such as C4-C7 in the circuit diagram, which is 5pf ~ 10PF.

In the transformer driving power supply circuit, LC filter is added to suppress the interference caused by the power supply system. For example, L1, C1, C2, C3 and L1 adopt magnetic beads on the circuit diagram, with a typical value of 600 ω/ 100MHz, capacity value is 0.01F ~ 0.1f.

In the design of 100M Ethernet, properly reducing the voltage level of network drive without affecting the communication quality helps to suppress EMC interference. 10 can also be connected on the difference line between the transmitting end and the receiving end of the secondary transformer ω To suppress interference.

(2) Key points of circuit lightning protection design:

In order to meet the lightning protection test requirements of iec61000-4-5 or gb17626.5, common mode 2KV and contact 1kV, the lowest cost design scheme is that the primary center tap of the transformer is grounded through lightning protection components, and the low-cost semiconductor discharge tube can be selected for D1 on the circuit diagram, However, the nominal voltage of the protective parts shall be more than 6V, the peak current of the protective parts shall be more than 50a, and the peak power of the protective parts shall be more than 300W. When selecting the semiconductor discharge tube, please note that the open circuit voltage and maintenance current of the equipment are greater than the circuit working voltage and current.

According to the requirements of the test standard, the gap test is not required for unshielded balance signals. Therefore, for the protection requirements within 1kV, the energy impact can be protected by the winding resistance of the transformer itself, and there is no need to add gap protection components.

Interface circuit design remarks:

If the equipment is a metal shell and the single board can divide the interface independently, the metal shell is directly electrically connected with the interface, and the single board is connected with the interface through a 1000pf capacitor.

3、 Connector design.

RJ45 Ethernet interface circuit design.

Overlap mode of connector and body:

(1) Precision milling technology is adopted for panel opening to make the opening shape suitable for the placement of connectors and avoid gaps in places with incorrect opening cutting. The test of reducing electromagnetic interference radiation shows that precision milling can improve electromagnetic compatibility by 12 ~ 18%

(2) Shrapnel is added at the joint between the machine body and the 100M Ethernet metal connector to maintain good electrical conductivity during joint.

4、 Cable design.

100M Ethernet interface signal cable.

Cable design requirements: general type of RJ45 metal connector.

Cable design:

(1) RJ45 signal cable adopts the shielding mode of mesh braided shielding layer, and the braided density of mesh braided layer is required to be more than 90%

(2) During internal wiring, the differential cable adopts twisted pair transmission, and the distance of twisted pair is generally 3 times of the cable diameter of signal cable.

(3) Magnetic ring treatment shall be added at both ends of the cable. The inner diameter of the magnetic ring shall be closely combined with the outer diameter of the cable, and thick magnetic ring shall be selected as far as possible.

Wiring design:

(1) When RJ45 cable is walking, it is required to stay away from other strong interference sources, such as power module

(2) It is better to walk alone or keep a distance of more than 10cm from other analog and power cables. Do not mix and bind with other cables.

Overlap of shielding layer and metal connector:

(1) The shielding layer of shielded cable is required to overlap with the metal connector by 360 °, as shown in the above figure

(2) Separate tail phenomenon shall be avoided in the shielding layer of shielded cable.