Orsdxy

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 experienced electromagnetic interference. With the increase of PCB line express delivery, EMC design is a problem that our electronic engineers have to consider. Faced with a design, when conducting EMC analysis of a product and design, there are five important attributes to consider:
(1) Key device size: the physical size of the emitter that generates radiation. Radio frequency (RF) current will generate electromagnetic field, which will leak through the chassis and leave the chassis. The length of the line on PCB as the transmission path has a direct impact on the radio frequency current.
(2) Impedance matching: the impedance of source and receiver, and the transmission impedance between them.
(3) Time characteristics of interference signals: Is this a continuous (periodic) event or only exists in a specific operating cycle (for example, a single keystroke operation or power-on interference, periodic disk drive operation or network burst transmission).
(4) Intensity of interference signal: how strong is the energy level of the source and how great is its potential for generating harmful interference.
(5) Frequency characteristics of interference signals: the use of a spectrum analyzer for waveform observation, observed problems in which position of the spectrum, 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 Daniel, we found that it is not suitable for RF signal occasions, because there are more EMI problems in RF signal occasions. It is believed that some engineers have applied single-point grounding to all product designs without realizing that the use of this grounding method may lead to more or more complex EMC problems.
We should also pay attention to the current flow in the circuit components. With circuit knowledge, we know that current flows from high voltage to low voltage, and that current always flows through one or more paths in a closed-loop circuit, so a minimum circuit and an important law. For those directions in which the interference current is measured, the PCB routing is modified so that it does not affect the load or the sensitive circuit. For applications requiring high impedance paths from power to load, all possible paths through which the return current can flow must be considered.
There is also a PCB routing problem. The impedance of a conductor or a route includes resistance R and inductance. At high frequencies, there is no impedance. When the traveling frequency is higher than 100kHz, the conductor or traveling becomes inductance. Wires or routes working above audio frequency may become radio frequency antennas. In the EMC specification, it is not allowed for wires or routes to work below a specific frequency of lambda/20 (antenna design length is equal to a specific frequency of lambda/4 or lambda/2,), when carelessly designed, the routes become a high-performance antenna, which makes the later debugging more difficult.
Finally, the layout of PCB is discussed. First, consider the size of PCB. When the size of PCB is too large, the anti-jamming ability of the system decreases and the cost increases with the increase of the alignment. However, too small size can easily cause problems of heat dissipation and mutual interference. Secondly, the location of special components (such as clock components) should be determined (the clock routing should be around and not around the key signal lines, so as to avoid interference). Third, according to the circuit function, the overall layout of PCB. In the layout of components, the relevant components are as close as possible, so that better anti-interference effect can be achieved. http://www.pcbindex.com