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EMC problems in PCB design
Visit:793 Date:2020-08-24
With the increase of PCB routing speed, electromagnetic compatibility (EMC) design is an issue that our electronic engineers have to consider. In the face of electromagnetic compatibility (EMC) design, when performing an electromagnetic compatibility (EMC) analysis of a product and design, the following five important attributes need to be considered:

  (1) Dimensions of key components:
   The physical size of the radiation emitting device. 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 characteristic of interference signal:
   Is the problem a continuous (periodic signal) event or only exists in a specific operating 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 is the energy level of the source and how much potential it has for harmful interference.
  (5) Frequency characteristics of interference signal:
  Using a spectrum analyzer to observe the waveform, where the problem is observed in the spectrum, it is easy to find the problem.
   In addition, some low-frequency circuit design habits need attention. For example, the single-point grounding I used to be very suitable for low-frequency applications, but later 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 (EMC) issues.
   We should also pay attention to the direction of current flow in circuit components. With circuit knowledge, we know that current flows from high voltage to low voltage, and 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 source 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. At high frequencies, there is 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 electromagnetic compatibility (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). When designing carefully, the wiring becomes a high-performance antenna, which makes the later debugging more difficult.
  PCB layout problem:
   First, consider the size of the PCB.
   When the PCB size is too large, the system's anti-interference ability will decrease and the cost will increase with the increase of the traces, and the too small size will easily cause heat dissipation and mutual interference problems.
  Second, determine the location of special components.
   Like clock components, it is best not to lay the ground around the clock traces and not run above and below the key signal lines to avoid interference.
   Third, according to the circuit function, layout the PCB as a whole.
  In the component layout, the related components should be as close as possible, so that a better anti-interference effect can be obtained.

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