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

Today, electronic systems have clock frequencies of a few hundred megahertz, the front and back 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 an ongoing challenge in engineering. So how to prevent and solve the problem of electromagnetic interference of connectors deserves 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, solving electromagnetic compatibility (EMC) is even more difficult today than before.

Before the two nodes of the circuit, the rapidly changing pulse current represents a so-called differential mode noise source, and the electromagnetic field around the circuit can be coupled to other components and invade the connection. The inductive or capacitively coupled noise is common mode interference. The RF interference currents are identical to each other and the system can be modeled as consisting of a noise source, a "damaged circuit" or "recipient" and a loop (usually a backplane). Several factors are used to describe the magnitude of the interference: the strength of the noise source, the size of the surrounding area of the interference current, and the rate of change.

Thus, although there is a high probability of unwanted interference in the circuit, the noise is almost always co-modeled. Once the cable is plugged in between the input/output (I/O) connector and the chassis or ground plane, some RF voltages can cause a few milliamps of RF current to exceed the allowable emission level.

Noise coupling and propagation:

Common mode noise is due to unreasonable design. Some typical reasons are the length of individual wires in different pairs, or the distance to the power plane or chassis. Another cause is component defects such as magnetic induction coils and transformers, capacitors and active devices (such as application specific integrated circuits (ASICs)).

Magnetic components, particularly so-called "core choke" type energy storage inductors, are used in power converters to always generate an electromagnetic field. The air gap in the magnetic circuit is equivalent to a large resistance in the series circuit, where more power is consumed.

Thus, the core choke coil is wound on the ferrite rod, creating a strong electromagnetic field around the rod and having the strongest field strength near the electrode. In a switching power supply using a retrace structure, there must be a gap in the transformer with a strong magnetic field. The most suitable component in which the magnetic field is maintained is a spiral tube that distributes the electromagnetic field along the length of the die. This is one of the reasons why the magnetic element operating at high frequencies preferably has a spiral structure.

Inappropriate decoupling circuits often also become sources of interference. If the circuit requires a large pulse current and local decoupling does not guarantee a small capacitance or a very high internal resistance, the voltage generated by the power supply circuit drops. This is equivalent to ripple, or equivalent to a rapid change in voltage between terminals. Due to the stray capacitance of the package, interference can be coupled into other circuits, causing common mode problems.

When common mode currents contaminate the I/O interface circuit, this problem must be resolved before passing through the connector. Different applications suggest different ways to solve this problem. In the video circuit, where the I/O signals are single-ended and share the same common loop, to solve it, use a small LC filter to filter out the noise.

In a low frequency serial interface network, some stray capacitance is sufficient to shunt noise onto the backplane. Differential drive interfaces, such as Ethernet, are typically coupled to the I/O area by a transformer that is coupled at the center tap on one or both sides of the transformer. These center taps are connected to the backplane via a high voltage capacitor to shunt common mode noise to the backplane so that the signal is not distorted.