Electronic Design - Set off the "hidden head" of PCB electromagnetic compatibility

Someone has said that there are only two types of electronic engineers in the world: those who have experienced electromagnetic interference and those who have not experienced electromagnetic interference. With the increase of PCB routing speed, electromagnetic compatibility design is a problem that our electronic engineers have to consider. Faced with a design, when performing an EMC analysis of a product and design, there are five important attributes to consider:

(1) Key device size: the physical size of the emitting device that generates radiation. The radio frequency (RF) current will generate an electromagnetic field, which will leak through the case and leave the case. The length of the trace on the PCB as a transmission path has a direct impact on the RF current.

(2) Impedance matching: the impedance of the source and receiver, and the transmission impedance between the two.

(3) Time characteristics of the interference signal: Is the problem a continuous (periodic signal) event or only exists in a specific operation cycle (for example, a single key operation or power-on interference, periodic disk drive operation or Network burst transmission).

(4) The strength of the interference signal: how strong the energy level of the source is, and how much potential it has for harmful interference.

(5) Frequency characteristics of interference signal: Use a spectrum analyzer to observe the waveform, and where the problem is in the spectrum, it is easy to find the problem.

In addition, some low-frequency circuit design habits need to be paid attention to. For example, my usual single-point grounding is very suitable for low-frequency applications, but chatting with the company's Daniel, I found that it is not suitable for RF signal occasions, because RF signal occasions have more EMI problems. I believe that some engineers apply single-point grounding to all product designs without realizing that using this grounding method may cause more or more complex electromagnetic compatibility issues.

We should also pay attention to the direction of current flow in circuit components. With circuit knowledge, we know that current flows from a place with a high voltage to a place with a low voltage, and the current always flows in a closed-loop circuit through one or more paths, so a minimum loop and a very important law. For those directions where the interference current is measured, the PCB traces are modified so that it does not affect the load or sensitive circuits. Those applications that require a high-impedance path from the power supply to the load must consider all possible paths through which the return current can flow.

There is also a PCB routing problem. The impedance of a wire or trace includes resistance R and inductive reactance. The impedance at high frequencies does not have capacitive reactance. When the trace frequency is higher than 100kHz, the wire or trace becomes inductance. Wires or traces that work above audio may become radio frequency antennas. In the EMC specification, wires or traces are not allowed to work below λ/20 of a certain frequency (the design length of the antenna is equal to λ/4 or λ/2 of a certain frequency)., The wiring becomes a high-performance antenna, which makes the later debugging more difficult.

Finally, talk about the layout of the PCB circuit board. First, consider the size of the PCB. When the size of the PCB is too large, the system's anti-interference ability will decrease and the cost will increase with the increase of the traces. However, the size of the PCB is too small to easily cause heat dissipation and mutual interference problems. Second, determine the location of special components (such as clock components) (the clock traces are best not to be grounded and not to walk above and below the key signal lines to avoid interference). Third, layout the PCB as a whole according to circuit functions. Fourth, the PCB factory should pay attention to the layout problem. In the component layout, the related components should be as close as possible, so that a better anti-interference effect can be obtained.