PCB Blog - Electromagnetic Compatibility in PCB Board Development Technology

1. The overall layout of the printed circuit boards and device layout
1) Whether a product is successful or not, one must pay attention to the internal quality, and the other is to take into account the overall aesthetics. Only when both are perfect can the product be considered a success; on a PCB board, the layout requirements of components should be balanced and sparse. Dense and orderly, not top-heavy or heavy, with as few vias as possible; the shape of the circuit board is a rectangle. The aspect ratio is 3:2 or 4:3; the noise of 4-layer board is 20dB lower than that of double-layer board. The noise of 6-layer board is 10dB lower than that of 4-layer board. When economic conditions permit, try to use multi-layer board.
2) The circuit board is generally divided into analog circuit area (afraid of interference), digital circuit area (afraid of interference, and interference), power drive area (interference source), so the board should be reasonably divided into three areas.
3) The device generally chooses devices with low power consumption and good stability, and uses as few high-speed devices as possible.
4) The lines are exquisite: the lines that can be made wide should not be made thin; the high-voltage and high-frequency lines should be smooth, and there should be no sharp chamfers, and no right angles should be used for corners. The ground wire should be as wide as possible, and a large area of copper should be used, which greatly improves the connection point problem.
5) The external clock is a high-frequency noise source, which not only can cause interference to the application system, but also may cause interference to the outside world, making the electromagnetic compatibility detection not up to the standard. In the application system with high requirements on system reliability, the selection of low-frequency single-chip microcomputer is one of the principles to reduce system noise. In recent years, some manufacturers that produce 8051-compatible microcontrollers have also adopted some new technologies to reduce the demand for external clocks to 1/3 of the original without sacrificing computing speed. The internal phase-locked loop technology is widely used in 32-bit microcontrollers, which reduces the external clock frequency to 32KHz, while the internal bus speed is increased to 8MHz or even higher.
6) The wiring should have a reasonable direction: such as input/output, AC/DC, strong/weak signal, high frequency/low frequency, high voltage/low voltage, etc., their direction should be linear (or separated) and must not blend with each other. Its purpose is to prevent mutual interference. The trend is in a straight line, but it is generally not easy to achieve, and the unfavorable trend is circular. For DC, small signal, low voltage PCB board design requirements can be lower. So "reasonable" is relative. The direction of the wiring between the upper and lower layers is basically vertical. The whole board does not want to be uniform, and those that can be squeezed should not be crowded together.
7) In terms of device arrangement, like other logic circuits, the related devices should be placed as close as possible, so that a better anti-noise effect can be obtained. The clock generator, the crystal oscillator and the clock input of the CPU are prone to noise, so they should be close to each other, especially do not run signal lines under the crystal oscillator. Devices that are prone to noise, low-current circuits, and high-current circuits should be kept away from logic circuits as much as possible. If possible, another circuit board should be made.

2. Ground wire technology SkE safety regulations and electromagnetic compatibility network
1) Analog circuits and digital circuits have many similarities and differences in the design and wiring methods of component layout diagrams. In the analog circuit, due to the existence of the amplifier, the extremely small noise voltage generated by the wiring will cause serious distortion of the output signal. In the digital circuit, the TTL noise tolerance is 0.4V ~ 0.6V, and the CMOS noise tolerance is 0.3 Vcc. ~0.45 times, so the digital circuit has strong anti-interference ability. Reasonable selection of good power supply and ground bus mode is an important guarantee for the reliable operation of the instrument. Quite a lot of interference sources are generated by power supply and ground bus, among which the noise interference caused by ground wire.
2) The digital ground is separated from the analog ground (or grounded at one point), and the ground wire is widened. The wire width should be determined according to the current. Generally speaking, the thicker the better (a 100mil wire passes through a current of about 1 to 2A). Ground wire > power wire > signal wire is a reasonable choice of wire width.
3) The power line and the ground line should be as close as possible, and the power supply and ground on the entire printed board should be distributed in a "well" shape, so that the distribution line current can be balanced.
4) In order to reduce the crosstalk between the lines, the distance between the printed lines can be increased if necessary, and some zero-volt lines are placed in it as the isolation between the lines. Especially between input and output signals, three technologies of decoupling, filtering and isolation
a Decoupling, filtering, and isolation are the three commonly used measures for hardware anti-interference.
b The power input end is connected across an electrolytic capacitor of 10~100uf. If possible, it is better to connect more than 100uF; in principle, each integrated circuit chip should be arranged with a 0.01pF ceramic capacitor. But capacitors; for devices with weak anti-noise ability and large power changes when turned off, such as RAM and ROM storage devices, a decoupling capacitor should be directly connected between the power line and the ground line of the chip;
c Filtering refers to classifying various types of signals according to their frequency characteristics and controlling their direction. Commonly used are various low-pass filters, high-pass filters, band-pass filters. The low-pass filter is used on the incoming AC power line to allow the 50-cycle AC power to pass smoothly, and other high-frequency noises are introduced into the ground. The configuration index of the low-pass filter is the insertion loss. The selected low-pass filter insertion loss is too low to suppress the noise, and the high insertion loss will cause "leakage" and affect the personal safety of the system. High-pass and band-pass filters should be selected and used according to the signal processing requirements in the system.
d Typical signal isolation is optical isolation. Using optoelectronic isolation devices to isolate the input and output of the single-chip microcomputer, on the one hand, the interference signal cannot enter the single-chip microcomputer system, and on the other hand, the noise of the single-chip microcomputer system itself will not be transmitted by conduction. Shielding is used to isolate space radiation. For components with particularly large noise, such as switching power supplies, they are covered with metal boxes, which can reduce the interference of noise sources to the single-chip microcomputer system. For analog circuits that are particularly afraid of interference, such as high-sensitivity weak-signal amplifying circuits, they can be shielded. The important thing is that the metal shield itself must be connected to the real ground SkE safety regulation and electromagnetic compatibility network on PCB board.