How to prevent and solve the electromagnetic interference problem of connector? (II)

Common-mode noise in the I/O region

There is no universal solution for all types of I/O interfaces. The main goal of designers is to design circuits well, often ignoring details that are considered simple. Some basic rules can minimize noise before reaching the connector:

1) set the decoupling capacitor close to the load.

2) the pulse current along the front and back of the rapidly changing pulse shall have the minimum circuit size.

3) keep large current devices (i.e. drives and ASIC) away from I/O ports.

4) measure signal integrity to ensure minimum overshoot and undershoot, especially for critical signals with large current (such as clock, bus).

5) local filtering, such as RF ferrite, can absorb RF interference.

6) provide a low impedance lap to the bottom plate or a reference in the I/O area on the bottom plate. Rf noise and connectors

Even if engineers take many of the precautions listed above to reduce RF noise in the I/O area, there is no guarantee that these precautions will be successful enough to meet the launch requirements. Some of the noise is conduction interference, that is, flow by common mode current on the internal circuit board. The interference source is between the bottom plate and the circuit, etc.

Therefore, the RF current must flow through the path of the lowest impedance (between the bottom plate and the signal line). If the connector does not exhibit low enough impedance (at the base plate's lap), this RF power flows through the stray capacitance. When the RF current flows through the cable, transmission is inevitable.

Another mechanism by which co-mode current is injected into I/O is the coupling of nearby strong interference sources. Even some "shielded" connectors are useless because the source of interference is near the connector, such as in a PC environment. If there is a gap between the connector and the substrate, the RF voltage induced here can degrade EMC performance.

The method of shield connector is to add finger reed or gasket. The lap of the connector is to fill the space between the connector and the housing. This method requires a pad. Metal liners are better if handled properly, that is, as long as the surface is not contaminated, as long as the hand is not touching or damaging the liner and as long as there is sufficient pressure to maintain good, low impedance contact.

The alternative is to install the connector plate or to install the connector on the housing. At this point, the maximum contact surface is slightly smaller, and the size and elasticity of the joint piece should be strictly controlled. When installing the shielded connector, it should open on the shell and remove the grease on one side of the opening. If the tolerance is not appropriate, the connector will sink too deep in the shell and the lap will be interrupted. Every EMC engineer knows that in an "excellent" system, this problem must meet the launch requirements and be checked on the production line in time. Unsecured or bent gaskets mounted on grease in critical areas will fail.