At the same time, the transmission rate of high-speed PCB design wiring system increases steadily, but also brings some anti-interference vulnerability. This is because the higher the frequency of transmitting information, the more sensitive the signals, and their energy is weaker, the more susceptible the wiring system is to interference.
Interference is ubiquitous. Cables and equipment will interfere with other components or be seriously interfered by other interference sources, such as computer screens, mobile phones, motors, radio relay equipment, data transmission and power cables. In addition, the number of potential eavesdroppers, cybercriminals, and hackers is increasing because their interception of UTP cable transmissions can cause significant damage and loss.
Especially when high-speed data networks are used, the time required to intercept large amounts of information is significantly lower than that required to intercept slow data transmission. The stranded pair in the data twisted pair can resist external interference and crosstalk between the pair by its own twisting at low frequency, but in the case of high frequency (especially when the frequency exceeds 250MHz), the purpose of anti-interference cannot be achieved only by the pair twisting, and only by shielding can resist external interference.
The cable shield acts like a Faraday shield, in that interference signals get into the shield but not into the conductor. As a result, data transfer can run without failure. Because the shielded cable has lower radiation emission than the unshielded cable, the network transmission is prevented from being intercepted. Shielding networks (shielded cables and components) can significantly reduce the level of electromagnetic energy emitted into the environment that can be intercepted.
Different interference field shielding selection interference field mainly includes electromagnetic interference and radio frequency interference two kinds. Electromagnetic interference (EMI) is mainly low-frequency interference. Motors, fluorescent lamps and power cables are common sources of EMI. Radio frequency interference (RFI) refers to radio frequency interference, mainly high frequency interference. Radio, television, radar and other wireless communications are common sources of RF interference.
To resist electromagnetic interference, braided shielding is the most effective because of its low critical resistance. For RF interference, foil shielding is the most effective, because braided shielding depends on the change of wave length, the gap generated by braided shielding allows high-frequency signals to freely enter and leave the conductor; For the high and low frequency mixed interference field, the combination of foil layer and braided net with broadband coverage should be used. Generally, the higher the mesh shielding coverage, the better the shielding effect.
How to reasonably design and layout PCB circuit board components?
PCB circuit board needs reasonable design and layout of components, in order to better play out the functional advantages of the circuit board. The following Jingbang Xiaobian will introduce the rational layout method of components in PCB circuit board design:
First, beauty should not only consider the orderly placement of components, but also consider the beautiful flow of the line.
Because the general layman sometimes more emphasis on the former, in order to one-sided evaluation of the merits of circuit design, in order to the image of the product, in the performance requirements are not harsh to give priority to the former. However, in the occasion of high performance, if you have to use dual panels, but also packaged in the inside, usually can not see, you should give priority to emphasize the beauty of the line.
Two, pcb circuit board should be able to withstand the installation and work by a variety of external forces and vibration.
Therefore, the circuit board should have a reasonable shape, and the position of various holes on the board (screw holes, special-shaped holes) should be arranged reasonably. Generally, the distance between the hole and the edge of the plate should be at least larger than the diameter of the hole. At the same time, it should be noted that the weakest section of the plate caused by special-shaped holes should also have enough bending strength. In particular, the connectors that "extend" directly out of the equipment shell on the board should be reasonably fixed to ensure long-term reliability.
Three, heat for high-power, serious heating devices, in addition to ensuring heat dissipation conditions, but also pay attention to placing in the appropriate position.
Especially in sophisticated analog systems, particular attention should be paid to the adverse effect of the temperature field generated by these devices on the fragile pre-amplifier circuit. Generally, the part with very large power should be made into a separate module, and certain thermal isolation measures should be taken between the signal processing circuit and it.
Fourth, signal interference is the most important factor to be considered in PCB layout design.
A few of the most basic aspects are: weak signal circuit and strong signal circuit separation or isolation; The AC part is separated from the DC part; The high-frequency part is separated from the low-frequency part; Pay attention to the direction of the signal line; The arrangement of ground wire; Appropriate screening, filtering and other measures. These are things that have been emphasized over and over again by a lot of arguments, and they won't be repeated here.