Solution to the problem of electromagnetic interference in PCB

- Aug 31, 2016-

In the face of a design, when a product and design of the EMC analysis, there are 5 important attributes to be considered:

(1) Key device dimensions: the physical dimensions of radiation emitting devices. Radio frequency (RF) current will generate electromagnetic field, the electromagnetic field will be separated from the casing leakage. The length of the transmission line on the PCB has a direct effect on the radio frequency current.

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

(3) The time characteristics of interference: this problem is continuous (periodic signal) events, or exist only in the specific operation period (for example, a single may be a key operation or electrical interference, periodic burst transmission or disk drive operation network).

(4) The intensity of the interfering signal: how strong the source energy level is, and how much it has the potential to cause harmful interference.

(5) The frequency characteristics of the interference signal: the use of spectrum analyzer to observe the waveform, the problem observed in the spectrum of which location, easy to find the problem.

In addition, some low frequency circuit design habits need to pay attention to. For example, my usual single point grounding for low frequency applications is very suitable, but later found not suitable for radio frequency signals, because the radio frequency signals, there are more EMI issues. Believe that some engineers will be single point grounding application to all product design, and did not realize that the use of this grounding method may produce more or more complex electromagnetic compatibility issues.

We should also pay attention to the current flow in the circuit component. A circuit knowledge we know, from the current high voltage of the local flow and low places, and the current is always through one or more paths in a closed-loop circuit flow, therefore a minimum loop and a very important law. For those measuring the direction of the interference current, by modifying the PCB to walk the line, so that it does not affect the load or the sensitivity of the circuit. Applications that require a high impedance path from the power to the load must consider all possible paths that return current can flow through.

There is also a PCB to take the issue of the line. A wire or wire impedance includes resistance R inductance, impedance at high frequency, there is no capacitance. When the line frequency is higher than 100kHz, the wire or cable into the inductor. A wire or cable working above the audio frequency may become a radio frequency antenna. In the EMC specification, does not allow the wire or wire is /20 at a particular frequency following work (length design of antenna is equal to a particular frequency lambda /4 or lambda /2), so when not careful design, walk the line into a high performance antenna, which makes the debugging become the more difficult.

Finally talk about the layout of the PCB. First, to consider the size of the size of PCB. When the PCB size is too large, with the growth of the line to make the system ability to resist interference, the cost increases, and the size is too small to cause heat dissipation and mutual interference problem. Second, to determine the specific components (such as clock components) position (clock line around the best floor and did not walk in the key signal lines, avoid interference). Third, according to the circuit function, the overall layout of the PCB. In the component layout, related components as close as possible, so as to obtain a better anti interference effect.

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