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5 important attributes of EMC analysis
2020-03-23
 
It has been said that there are only two types of electronic engineers in the world: those who have experienced electromagnetic interference and those who have not. With the increase of PCB signal frequency, electromagnetic compatibility design is a problem that our electronic engineers have to consider. In the face of a design, when conducting an EMC analysis of a product and design, there are five important attributes to consider:

(1) Key device size: The physical size of the radiation-emitting device. Radio frequency (RF) current will generate an electromagnetic field that will leak out of the case through the case. The length of the trace on the PCB as a transmission path has a direct effect on the RF current.

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

(3) Temporal characteristics of the interference signal: Is the problem a continuous (periodic signal) event or is it only in a specific operating cycle (for example, a single event may be a key operation or power-on interference, periodic disk drive operation, or network Burst).

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

(5) Frequency characteristics of the interference signal: Use the spectrum analyzer to observe the waveform and observe where the problem appears in the frequency spectrum, which is convenient for finding the problem.

In addition, some low-frequency circuit design habits need attention. For example, my customary single-point grounding is very suitable for low-frequency applications, but I chatted with the company and found that it is not suitable for RF signal occasions because there are more EMI problems in RF signal occasions. I believe that some engineers will apply single-point grounding to all product designs without realizing that using this grounding method may create more or more complex EMC problems.

We should also pay attention to the current flow in the circuit components. From the knowledge of the circuit, we know that the current flows from the place of high voltage to the place of low voltage, and the current always flows in a closed loop circuit through one or more paths, so there is an important rule: design a minimum loop. For those directions where the interference current is measured, modify the PCB traces so that they do not affect the load or sensitive circuits. 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.

We also need to pay attention to the PCB traces. The impedance of the wire or trace includes the resistance R and the inductive reactance. There is an impedance at high frequencies, and there is no capacitive reactance. When the trace frequency is higher than 100kHz, the wire or trace becomes an inductor. Wires or traces working above audio may become radio frequency antennas. In EMC specifications, wires or traces are not allowed to work below λ / 20 at a specific frequency (the design length of the antenna is equal to λ / 4 or λ / 2 of a specific frequency). If you accidentally design it like that, the wiring becomes a high-performance antenna, which makes the later debugging more difficult.

Finally, talk about the layout of the PCB:
First: We must consider the size of the PCB. When the size of the PCB is too large, the anti-interference ability of the system will decrease with the growth of the wiring, and the cost will increase. Too small size will easily cause problems of heat dissipation and mutual interference.

Second: Then determine the location of special components (such as clock components) (the clock traces should preferably not be grounded or walked above and below key signal lines to avoid interference).

Third: According to the circuit function, the overall layout of the PCB. In component layout, related components should be as close as possible, so as to obtain better anti-interference effect.
 
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