1. Key device size:
The physical dimensions of the emitting device that produces the radiation. The 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 effect 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 interfering signals:
Is the problem a continuous (periodic signal) event, or is it only a specific cycle of operations (eg a single event might be a keystroke or power-on glitch, periodic disk drive operations or network bursts).
4. The strength of the interference signal:
How strong is the source energy level and how much it has the potential to cause harmful interference.
5. Frequency characteristics of interference signals:
Use the spectrum analyzer to observe the waveform and observe where the problem occurs in the spectrum, so as to find the problem.
In addition, the design habits of some low-frequency circuits need attention. For example, my customary single-point grounding is very suitable for low-frequency applications, but I find it not suitable for RF signal applications, because there are more EMI problems in RF signal applications.
It is believed 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 issues.
We should also pay attention to the current flow within the circuit components. We know from circuit knowledge that 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 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.
We also need to pay attention to the PCB traces. The impedance of a wire or trace consists of resistance R and inductive reactance, with impedance at high frequencies and 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). If not carefully designed that way, the trace becomes a high-performance antenna, which makes later debugging more difficult.
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 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 position of special components (such as clock components) (the clock traces are best not to be paved around and not to walk above and below 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.
