It has been said that there are only two kinds of electronic engineers in the world: those who have experienced electromagnetic interference and those who have not. With the increase of PCB signal frequency, EMC design is a problem that we electronic engineers have to consider.
1、5 Important Attributes to Consider in EMC Analysis
In the face of a design, there are five important attributes to consider when conducting an EMC analysis of a product and design.
- Key device dimensions.
The physical size of the transmitter device that generates radiation. Radio frequency (RF) current will generate electromagnetic field, the electromagnetic field will leak through the enclosure and out of the enclosure. length of PCB alignment as a transmission path has a direct impact on RF current.
2, impedance matching.
The impedance of the source and receiver, and the transmission impedance between the two.
3, the time characteristics of the interference signal.
The issue is a continuous (periodic signal) event, or just the presence of a specific operating cycle (for example, a single event may be a keystroke operation or power-up interference, periodic disk drive operation or network burst transmission).
- Strength of the interfering signal.
How strong the source energy level is, and how much potential it has to generate harmful interference.
5, the frequency characteristics of the interfering signal.
Use a spectrum meter for waveform observation to observe where the problem appears in the spectrum to facilitate finding the problem.
In addition, some low-frequency circuit design habits need to be noted. For example, my usual single point of grounding for low-frequency applications is very suitable, but chat with the company bull, found not suitable for RF signal occasions, because there are more EMI problems in RF signal occasions.
It is believed that some engineers will apply single-point grounding to all product designs without recognizing that using this grounding method may create additional or more complex EMC problems.
We should also pay attention to the direction of current flow within circuit components. From circuit knowledge we know that current flows from where the voltage is high to where it is low, and that current always flows through one or more paths in a closed-loop circuit, so there is a very important rule: design a minimum circuit.
For those directions where interference currents are measured, the PCB alignment is modified so that it does not affect the load or sensitive circuitry. 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 the PCB alignment. The impedance of the wire or alignment contains resistance R and inductance, and at high frequencies there is impedance, no capacitive resistance. When the alignment frequency is higher than 100kHz or more, the wire or alignment becomes inductive. In the audio above the work of the wire or alignment may become RF antenna.
In the EMC specification, it is not allowed to operate the wire or alignment below λ/20 of a particular frequency (the antenna is designed with a length equal to λ/4 or λ/2 of a particular frequency). If accidentally designed that way, then the alignment becomes a high performance antenna, which makes the later debugging more tricky.
2、PCB layout
Finally, on the layout of the PCB.
First: to consider the size of the PCB size. the size of the PCB is too large, with the growth of the alignment so that the system anti-interference ability to reduce the cost, while the size is too small to easily cause heat dissipation and mutual interference problems.
Second: then determine the location of special components (such as clock components) (clock alignment is best around not laying the ground and do not go up and down in the key signal lines to avoid interference).
Third: based on the function of the circuit, the overall layout of the PCB. In the component layout, the relevant components as close as possible, so that you can get a better anti-interference effect.
