Do you know what electromagnetic radiation is? What harm does it have? For PCB technology, Altium Designer 20 has now ushered in, adding many new features. So how do we deal with the electromagnetic radiation problem in the PCB circuit board? These interferences cannot exceed a limit, which will cause problems. In the final analysis, these interferences still affect the signal integrity of the system.

When we come into contact with new things, we usually use our familiar knowledge to explain things we are not familiar with. EMC knowledge is more related to microwave and radio frequency. For a rookie like me who focuses on signal integrity but has little knowledge of EMC, at first, only some basic knowledge of SI can be used to open the door of EMC design. . In my understanding, EMI focuses on the radiation of electromagnetic energy, including the interference of the external electromagnetic environment to its own system, and the interference of the electromagnetic energy radiated by itself to the external system. None of these interferences can exceed a limit, beyond which they can cause problems, and ultimately affect the signal integrity of the system.

How is the electromagnetic energy on the circuit board radiated?

Speaking of which, I remembered the following picture, which is also my most basic impression of electromagnetic radiation.

The early PCB is a single-layer board, and the chips are connected by wires. There is no difference between the power line and the signal line, just the connected wires. This reminds me of my graduation project, which is a single-chip controlled LED Display. The system is very simple, with only a few ICs, color ring resistors and capacitors. The circuit can work with simple soldering. . Microstrip, stripline, twisted pair, coaxial cables are not used at all.

After learning high-speed design, I understood that as frequency goes up, so does the electromagnetic energy from signal transitions. Chips can no longer be connected so simply. The connection method like Figure 1 will make the loop inductance very large, and the loop inductance will be very large, which will make the inductive reactance of the AC signal very large, and the signal will not be honest at all. It propagates along the wire, but radiates into space.

How to correctly face the problem of electromagnetic radiation in PCB circuit boards?

In the eyes of SI engineers, the use of microstrip or stripline is to provide a low impedance transmission path for the signal. This is also the need for electromagnetic shielding in the eyes of EMC engineers. After the use of microstrip or stripline, the electromagnetic energy is contained in the medium between the conductors. Why is most of the electromagnetic energy bound in the medium after using microstrip and stripline? The main reason is that the signal path and the return path are closer, so that the inductance of the entire loop is reduced.If you don’t believe me, let’s use the software to calculate it

As can be seen from the above figure, the reference plane has a great influence on the effective inductance per unit length of the transmission line. It is conceivable that under high frequency conditions, if the signal has a good return path, the loop inductance it feels will be very small, and the signal will be transmitted from the transmitter to the receiver according to people’s wishes. A large loop inductance can cause radiation problems.

summary

At low frequencies, the problem of electromagnetic interference can be ignored. At low frequencies, the electromagnetic field around the wire does not change so strongly, and the inductance effect of the wire will not be so obvious. However, at high frequencies, the electromagnetic field changes drastically. The coupling between the signal path and the return path should be fully considered, and the coupling between the signal path and the return path should be used to reduce the inductance of the entire loop and control the electromagnetic energy emitted by the wire into the space. The above is some analysis of electromagnetic radiation, I hope it can help you.

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