“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 experienced electromagnetic interference. With the increase of PCB signal frequency, electromagnetic compatibility design is an issue that our electronic engineers have to consider.
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 experienced electromagnetic interference. With the increase of PCB signal frequency, electromagnetic compatibility design is an issue that our electronic engineers have to consider. Facing a design, when performing an EMC analysis of a product and design, there are five important attributes to consider:
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 radio frequency current.
Impedance matching: the impedance of the source and receiver, and the transmission impedance between the two.
Time characteristics of the interference signal: Is the problem a continuous (periodic signal) event or only exists 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 transmission ).
The strength of the interfering signal: how strong the source energy level is, and how much potential it has for harmful interference.
The frequency characteristics of the interference signal: Use a spectrum analyzer to observe the waveform and observe where the problem occurs in the spectrum, so that it is easy to find the problem.
In addition, some low-frequency circuit design habits need attention. 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 will apply single-point grounding to all product designs without realizing that the use of this grounding method may cause more or more complex electromagnetic compatibility issues.
We should also pay attention to the direction of current flow within the circuit components. From the knowledge of the circuit, we know that the current flows from a place where the voltage is high to a place where the voltage is low, and the current always flows in a closed-loop circuit through one or more paths, so there is a very important rule: design a minimum loop. 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.
We also need to pay attention to PCB routing. The impedance of a wire or trace includes resistance R and inductive reactance. There is impedance at high frequencies, but no 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). If you accidentally design it like that, the wiring becomes a high-performance antenna, which makes later debugging more difficult.
Finally, talk about the layout of the PCB:
First: Consider the size of the PCB. When the size of the PCB is too large, the anti-interference ability of the system will decrease and the cost will increase with the increase of the traces. However, the size of the PCB is too small, which will easily cause the problems of heat dissipation and mutual interference.
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 the circuit function. In the component layout, the related components should be as close as possible, so that a better anti-interference effect can be obtained.