It has been said that there are only two kinds of electronic engineers in the world: those who have experienced EMI and those who have not. With the increase in
PCB board trace delivery, electromagnetic compatibility design is a problem that our electronic engineers have to consider. Faced with a design, when conducting an EMC analysis of a product and design, there are five important attributes to consider:
1) Critical Device Size: The physical size of the emitting device that produces the radiation. 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 the 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 disturbance 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 disturbance, periodic disk drive operation or network burst transfer).
4) Strength of the interfering signal: How strong is the source energy level, and how much potential it has to cause harmful interference.
5) Frequency characteristics of the interference signal: Use the spectrum analyzer to observe the waveform, where the observed problem is in the frequency spectrum, so that it is easy to find the problem.
In addition, the design habits of some low-frequency circuits need attention. For example, my usual single-point grounding is very suitable for low frequency applications, but later found it is not suitable for RF signal applications, because there are more EMI problems in RF signal applications. It is believed that some engineers apply single-point grounding to all product designs without realizing that using this grounding method may create more or more complex EMC issues.
We should also pay attention to the current flow within the circuit components. With circuit knowledge we know that current flows from where the voltage is high to where it is low, and current always flows in a closed loop circuit through one or more paths, hence a small loop and a very important law. For those directions where interference currents are measured, modify the PCB traces so that they do not affect the load or sensitive circuits. Those applications that require a high impedance path from the source to the load must consider all possible paths through which the return current can flow.
There is also a PCB board routing problem. The impedance of a wire or trace consists of resistance R and inductive reactance, and at high frequencies, there is no capacitive reactance. When the trace frequency is above 100kHz, the wire or trace becomes inductive. Wires or traces that work above audio can become RF 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 trace becomes a high-efficiency antenna, which makes later debugging more tricky.
Then talk about the layout of the PCB board. , to consider the size of the PCB board. When the size of the PCB board is too large, the anti-interference ability of the system will decrease with the growth of the traces, and the cost will increase. If the size is too small, it is easy to cause problems of heat dissipation and mutual interference. Second, determine the location of special components (such as clock components) (the clock traces should not be laid on the ground and not on the top and bottom of key signal lines to avoid interference). Third, according to the circuit function, the overall layout of the
PCB board is carried out. In the component layout, the related components should be as close as possible, so that a better anti-interference effect can be obtained.
