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

Common mode noise in the I/O area

There is no general way to solve the problem of all types of I/O interfaces. The main goal of the designers is to design the circuit, often ignoring some of the details that are considered simple. Some basic rules allow noise to be minimized before it reaches the connector:

1) Set the decoupling capacitor close to the load.

2) The pulse current of the rapidly changing front and rear edges should have a minimum loop size.

3) Keep high current devices (ie, drivers and ASICs) away from the I/O ports.

4) Determine signal integrity to minimize overshoot and undershoot, especially for critical signals (such as clocks, buses) for high currents.

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

6) Provide a low impedance bond to the backplane or the reference in the I/O area on the backplane. 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 noise is conducted interference, which is a common mode current flowing on the internal circuit board. This source of interference is between the backplane and the circuit.

Thus, this RF current must flow through the path of the lowest impedance (between the backplane and the load signal line). If the connector does not exhibit a sufficiently low impedance (overlap with the backplane), this RF current flows through the stray capacitance. When this RF current flows through the cable, emission is inevitably generated.

Another mechanism for injecting common mode current into the I/O region is the coupling of strong sources of interference nearby. Even some "shield" connectors are useless because the source of interference is in the vicinity of the connector, such as a PC environment. If there is a gap between the connector and the backplane, the RF voltage induced here can degrade EMC performance.

The method of shielding the connector is to add a finger spring or a spacer. The lap joint of the connector is filled between the connector and the casing. This method requires a pad. Metal liners are preferred as long as the treatment is suitable, that is, as long as the surface is not contaminated, as long as the hand does not touch or damage the liner and as long as there is sufficient pressure to maintain good, low impedance contact.

Another method is to attach the connector to the connector or mount the connector on the chassis. At this time, the maximum contact surface is slightly smaller, and the size and elasticity of the tab should be strictly controlled. When installing the shielded connector, open it on the casing, and remove the oil on the side of the opening. Carefully make it. If the tolerance is not suitable, the connector will fall too deep in the casing, causing the overlap to be interrupted. Every EMC engineer knows that in an "excellent" system, this problem must meet the launch requirements and be checked in time on the production line. Untightened or bent gaskets that are installed on oil in critical areas will fail.

The EMI connector was chosen for the following reasons:

1) Conductive foamed plastic is extremely soft and can be placed around the entire connector. This eliminates the problems associated with another chassis and liner.

2) Mechanical engineers can install connectors within the tolerances that the system enclosure can accept.

3) The connector and the case achieve low impedance bonding to ensure good contact. The liner on the inside of the casing wall can be made of a softer material when it is required to be painted.

4) For designs requiring forced cooling, the liner preferably has another feature: the joint between the connector and the wall of the casing should be sealed to reduce air leakage. In dusty environments, the liner should be kept clean inside the system.