Today, the clock frequency of electronic systems is several hundred megahertz, and the front and back edges of the pulses used are in the sub nanosecond range. High quality video circuits are also used at sub nanosecond pixel rates. These higher processing speeds indicate continuous challenges in engineering. So how to prevent and solve the problem of electromagnetic interference in connectors is worth our attention.

The oscillation rate on the circuit becomes faster (rise/fall time), the voltage/current amplitude becomes larger, and the problems become more frequent. Therefore, addressing electromagnetic compatibility (EMC) is even more challenging today compared to before.

Before the two wave nodes of the circuit, the rapidly changing pulse current represents the so-called differential mode noise source, and the electromagnetic field around the circuit can be coupled to other components and invade the connection part. Noise coupled by inductance or capacitance is common mode interference. The radio frequency interference current is the same as each other, and the system can be modeled as consisting of a noise source, a "victim circuit" or "receiver", and a loop (usually a backplane). Describe the magnitude of interference using several factors: the intensity of the noise source, the size of the interference current surrounding area, and the rate of change.

So, although there is a high possibility of unwanted interference in the circuit, the noise is almost always co model. Once a cable is inserted between the input/output (I/O) connector and the chassis or ground plane, certain RF voltages can occur, causing a few milliamperes of RF current to exceed the allowable emission level.

Coupling and propagation of noise

Common mode noise is caused by unreasonable design. Some typical reasons are different lengths of individual wires in different pairs, or different distances from the power supply plane or chassis. Another reason is the defects of components, such as magnetic induction coils and transformers, capacitors and active devices (such as special integrated circuits (ASICs) for applications).

Magnetic components, especially so-called "iron core choke" type energy storage inductors, are used in power converters and always generate electromagnetic fields. The air gap in the magnetic circuit is equivalent to a large resistor in a series circuit, which consumes a lot of electrical energy.

So, the iron core choke is wound around the ferrite rod, generating a strong electromagnetic field around the rod and having the strongest field strength near the electrode. In a switching power supply using a retracing structure, there must be a gap on the transformer with a strong magnetic field between them. The most suitable component for maintaining a magnetic field is a spiral tube, which distributes the electromagnetic field along the length of the tube core. This is one of the reasons for optimizing the spiral structure of magnetic components operating at high frequencies.

Improper decoupling circuits often become sources of interference. If the circuit requires a large pulse current and cannot guarantee small capacitors or very high internal resistance during local decoupling, the voltage generated by the power circuit will decrease. This is equivalent to ripple or rapid voltage changes between terminals. Due to the stray capacitance of the packaging, interference can be coupled to other circuits, causing common mode problems.

When common mode current contaminates the I/O interface circuit, this issue must be resolved before passing through the connector. It is recommended to use different methods to solve this problem for different applications. In video circuits, where the I/O signal is single ended and shares the same common circuit, to solve it, a small LC filter is used to filter out noise.

In low-frequency serial interface networks, some stray capacitors are sufficient to divert noise onto the substrate. Differential drive interfaces, such as Ethernet, are typically coupled to the I/O area through a transformer, providing coupling at the center tap on one or both sides of the transformer. These center taps are connected to the bottom plate through high-voltage capacitors, diverting common mode noise onto the bottom plate to prevent signal distortion.

Common mode noise within the I/O region

There is no universal solution to all types of I/O interface issues. The main goal of designers is to design the circuit well, often neglecting some details that are considered simple. Some basic principles can minimize noise before reaching the connector:

1) Set the decoupling capacitor close to the load.

2) The loop size of the rapidly changing pulse current at the front and back edges should be minimized.

3) Keep high current devices (i.e. drivers and ASICs) away from I/O ports.

4) Measure the integrity of the signal to ensure minimal overshoot and undershoot, especially for critical signals with high currents (such as clocks and buses).

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

6) Provide low impedance bonding on the base plate or reference in the I/O area on the base plate. RF noise and connectors

Even if engineers take many of the preventive measures listed above to reduce RF noise in the I/O area, it cannot be guaranteed that these preventive measures will be successful enough to meet the emission requirements. Some noise is conducted interference, which flows in common mode current on the internal circuit board. This interference source is between the backplane and the circuit, etc.

Therefore, this RF current must flow through the path with the lowest impedance (between the base plate and the carrier signal line). If the connector does not exhibit sufficiently low impedance (at the overlap with the base plate), this RF current flows through the stray capacitance. When this RF current flows through the cable, it inevitably generates emissions.

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

There are methods for shielding connectors, such as adding finger shaped springs or gaskets. The overlap of connectors is to fill the gap between the connector and the casing. This method requires a pad. Metal pads are good, as long as they are properly treated, that is, as long as the surface is not contaminated, as long as hands do not touch or damage the pads, and as long as there is sufficient pressure to maintain good, low impedance contact.

Another method is to install connectors with connectors or install connectors on the casing. 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 shielded connectors, open an opening on the casing, remove oil stains on one side of the opening, and carefully make it. If the tolerance is not appropriate, the connector may fall too deep inside the casing, causing the overlap to be interrupted. Every EMC engineer knows that in an "excellent" system, this issue must meet the launch requirements and be promptly checked on the production line. Unfastened or bent gaskets installed on oil stains in critical areas will fail.

EMI connectors were selected due to the following reasons

1) Conductive foam plastic is extremely soft and can be placed around the entire circumference of the connector. This eliminates the issue related to the other casing and gasket.

2) Mechanical engineers can install connectors within the acceptable tolerance range of the system casing.

3) The connector achieves low impedance overlap with the casing to ensure good contact. The lining on the inner side of the casing wall can be made of softer materials when there are shielding requirements for painting.

4) For designs that require forced cooling, it is best for the gasket to have another feature: the gap between the connector and the casing wall should be sealed to reduce air leakage. In dusty environments, the gasket should be kept clean inside the system.