Nowadays, the clock frequency of electronic systems is several hundred MHz, the leading and trailing edges of the pulses used are in the sub-nanosecond range, and high-quality video circuits are also used for sub-nanosecond pixel rates. These higher processing speeds represent constant challenges in engineering. So how to prevent and solve the problem of connector electromagnetic interference is worthy of our attention.

The oscillation rate on the circuit becomes faster (rise/fall time), the voltage/current amplitude becomes larger, and the problem becomes more. Therefore, it is more difficult to solve electromagnetic compatibility (EMC) today than before.

Before the two nodes of the circuit, the rapidly changing pulse current represents the so-called differential mode noise source. The electromagnetic field around the circuit can couple to other components and invade the connection part. Inductively or capacitively coupled noise is common mode interference. The radio frequency interference current is the same as each other, and the system can be modeled as: composed of a noise source, a "victim circuit" or "receiver" and a loop (usually a backplane). Several factors are used to describe the size of the interference: the intensity of the noise source, the size of the area around the interference current, and the rate of change.

Thus, although there is a possibility of undesired interference in the circuit, the noise is almost always co-model. Once a cable is connected between the input/output (I/O) connector and the chassis or ground plane, when some RF voltage appears, a few milliamps of RF current can be sufficient to exceed the allowable emission level.

Coupling and propagation of noise

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

Magnetic components, especially the 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, where more electrical energy is consumed.

Therefore, the iron core choke coil is wound on the ferrite rod, and a strong electromagnetic field is generated around the rod, and the strongest field strength is near the electrode. In a switching power supply using a retrace structure, there must be a gap on the transformer with a strong magnetic field in between. The most suitable element to maintain the magnetic field is the spiral tube, which makes the electromagnetic field distributed along the length of the tube core. This is one of the reasons why the spiral structure is preferred for magnetic elements operating at high frequencies.

Inappropriate decoupling circuits also often become sources of interference. If the circuit requires a large pulse current, and the need for small capacitance or very high internal resistance cannot be guaranteed during partial decoupling, the voltage generated by the power circuit will drop. This is equivalent to ripple, or equivalent to rapid voltage changes between terminals. Due to the stray capacitance of the package, interference can couple to other circuits, causing common mode problems.

When the common mode current contaminates the I/O interface circuit, the problem must be resolved before passing through the connector. Different applications are suggested to use different methods to solve this problem. In the video circuit, the I/O signal there is single-ended and shares the same common loop. To solve it, use a small LC filter to filter out the noise.

In a low-frequency series interface network, some stray capacitance is sufficient to shunt noise to the bottom board. Differentially driven interfaces, such as Ethernet, are usually coupled to the I/O area through a transformer, and the coupling is provided by the center taps on one or both sides of the transformer. These center taps are connected to the bottom plate via a high-voltage capacitor to shunt common mode noise to the bottom plate so that the signal does not distort.