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

Today, the clock frequency of an electronic system is several hundred megahertz, and the pulse front and back of it is in the range of nanoseconds. The high quality video circuit is also used for the pixel rate of nanoseconds. These high processing speeds represent ongoing engineering challenges. So how to prevent and solve the problem of connector electromagnetic interference is worthy of our attention.

The circuit oscillates faster (up/down time), the voltage/current amplitude becomes greater, and the problems become more numerous. As a result, it is harder to solve electromagnetic compatibility (EMC) today than it was before.

Before the two 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 intruded into the connection. Sensible or capacitive coupling of noise is common-mode interference. Rf interference currents are identical to each other, and the system can be modeled as: composed of a source of noise, a "victim circuit" or a "receiver" and a loop (usually a substrate). The magnitude of the interference is described by several factors: the intensity of the source of the noise, the size of the area around which the interference current flows, and the rate of change.

Thus, although unwanted interference is likely to occur in the circuit, noise is almost always co-modeled. Once a cable is connected between the input/output (I/O) connector and the housing or ground plane, some RF voltage occurs, causing a few milliamps of RF current to exceed the permissible transmitting level.

Coupling and propagation of noise

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

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 a magnetic circuit is equivalent to a large resistance in a series circuit, where more electricity is consumed.

Thus, the core choke is wound around the ferrite bar to generate a strong electromagnetic field around the bar and the strongest field near the electrode. In the switching power supply using the retracting structure, there must be a gap on the transformer, in which there is a strong magnetic field. The most suitable element to keep the magnetic field is the spiral pipe, which makes the electromagnetic field distributed along the length of the tube core. This is one of the reasons why magnetic elements working at high frequencies preferred helical structures.

Improper decoupling circuits often become interference sources. If the circuit requires a large pulse current and local decoupling does not guarantee a small capacitance or very high internal resistance, the voltage generated by the power circuit will decrease. This is the equivalent of a ripple, or a rapid change in voltage between terminals. Because of the encapsulated stray capacitance, interference can be coupled 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 suggest different ways to solve this problem. In video circuits, where I/O signals are single-ended and share the same common circuit, to solve it, filter out the noise with a small LC filter.

In low frequency serial interface networks, some stray capacitors are sufficient to shunt noise onto the substrate. Differential drive interfaces, such as Ethernet, are usually coupled to the I/O area by a transformer and are provided by a central tap on one or both sides of the transformer. These central taps are connected by high voltage capacitors to the bottom plate, and the common-mode noise is shunted onto the bottom plate so that the signal is not distorted.