Electronic Design - PCB anti-interference technology design

1 Introduction

Electromagnetic compatibility EMC refers to the ability of electronic systems to work normally in accordance with the design requirements in the specified electromagnetic environment. The electromagnetic interference experienced by electronic systems not only comes from the radiation of electric and magnetic fields, but also has the influence of the common impedance of the line, the combination of wires and the structure of the circuit. When developing and designing circuits, we also hope that the printed circuit board designed is as less susceptible to external interference as possible, and that it will interfere with other electronic systems as little as possible. There are many factors that affect the anti-interference performance of printed boards, including the thickness of the copper foil, the width and length of the printed wires and the crosstalk between adjacent wires, the rationality of the layout of the components in the board, and the common impedance of the wires. Electromagnetic fields generated by wires and components in space, etc.

The primary task of designing a printed board is to analyze the circuit and determine the key circuit. This is to identify which circuits are interference sources and which circuits are sensitive circuits, and find out what paths the interference sources may use to interfere with sensitive circuits. In analog circuits, low-level analog circuits are often sensitive circuits, and power amplifiers are often sources of interference. When the working frequency is low, the interference source mainly interferes with the sensitive circuit through the inter-wire barrel connection; when the working frequency is high, the interference source mainly interferes with the sensitive circuit through electromagnetic radiation. In digital circuits, high-speed repetitive signals, such as clock signals, bus signals, etc., are rich in frequency components, which are the biggest source of interference and often pose a threat to sensitive circuits. Reset circuits, interrupt circuits, etc. are sensitive circuits, which are susceptible to interference from spikes, so digital circuits cannot work normally. The input/output circuit (1/0) is connected to the outside world, and special attention should also be paid. If the UO circuit is close to interference sources such as clock lines, the unnecessary high-frequency energy will be integrated into the output line, and the noise on the line will interfere with sensitive circuits near the cable through radiation or conduction.

On the basis of fully analyzing the circuit and determining the key circuit, the circuit must be properly arranged on the printed board. For digital circuits, high-speed circuits (such as clock circuits, high-speed logic circuits, etc.), medium and low-speed logic circuits, and UO circuits should be arranged in different areas, and the interference source and sensitive circuits should be separated in space as much as possible, so that the interference source can be separated. Radiation interference to sensitive circuits is greatly reduced.

2 Printed board anti-jamming design

The purpose of the anti-interference design of the PCB board is to reduce the electromagnetic radiation of the PCB board and the crosstalk between the circuits on the PCB board. In addition, the ground design of the PCB directly affects the common-mode voltage radiation of the 1/0 cable. Therefore, the anti-interference design of the PCB is of great significance for reducing the electromagnetic information radiation of the system.

2.1 PCB layout design

The density of the printed circuit board (PCB) is getting higher and higher, and the quality of the PCB design has a great influence on the anti-interference ability, so the layout of the PCB is in a very important position in the design.

Layout requirements for special components:

1. The shorter the wiring between high-frequency components, the better, and minimize the electromagnetic interference between each other; components that are susceptible to interference should not be too close; input and output components should be as far away as possible;

2. Some components have a higher potential difference, so the distance between them should be increased to reduce common mode radiation. Pay special attention to the rationality of the layout of components with high voltage;

3. The thermal element should be far away from the heating element;

4. The solution capacitor should be close to the power pin of the chip;

5. The layout of adjustable components such as potentiometers, adjustable inductance coils, variable capacitors, micro switches, etc. should be placed in an easy-to-adjust position as required;

6. The position occupied by the positioning hole of the printed board and the fixed bracket should be reserved.

Layout requirements for common components:

1. Place the components of each functional circuit unit according to the circuit flow, so that the signal flow direction is as consistent as possible;

2. Take the core components of each functional circuit as the center and lay out around it. The components should be evenly and neatly arranged on the PCB to minimize and shorten the leads and connections between the components;

3. For circuits operating at high frequencies, the interference between components should be considered. In general, the components should be arranged in parallel as far as possible to facilitate wiring;

4. The outplace line of the PCB is generally not less than 80 mils away from the edge of the circuit board. The best shape of the circuit board is rectangular. The aspect ratio is 3:2 or 4:30.

2.2 PCB layout design

The wiring density of PCB is increasing, so PCB wiring design is particularly important.

1. The power line layer of the four-layer board should be as close as possible to the ground line layer to obtain the minimum power impedance. From top to bottom are: signal wire, ground wire, power wire, signal wire. Considering electromagnetic compatibility, the best six-layer boards from top to bottom are: signal wire, ground wire, signal wire, power wire, ground wire, signal wire;

2. The clock line should be adjacent to the ground layer, and the line width should be as large as possible, and the line width of each clock line should be the same;

3. The signal layer adjacent to the ground wire is arranged with high-speed digital signal lines and low-level analog signal lines, and the farther layer is arranged with low-speed signal lines and high-level analog signal lines;

4. The wiring of the input and output terminals should be avoided as far as possible to avoid parallel connection to avoid feedback;

5. The bend of the printed wire is generally 135 degrees obtuse angle;

6. The line width of the power line and the ground line should be increased as much as possible, and the wiring width of the device with a 0.5mm pin pitch should not be less than 12mil;

7. The signal line width of general digital circuit is 8.il-10nul, and the pitch is 6mi1-8mil;

8. Leads of de-radiation capacitors should not be too long, especially for high-frequency bypass capacitors;

9. The digital ground and analog ground on the mixed-signal circuit board are separated. If the wiring crosses the separation gap, electromagnetic radiation and signal interference will increase sharply, causing electromagnetic compatibility problems. Therefore, PCB design generally adopts a unified ground, layout and wiring through digital circuits and analog circuits;

10. For some high-speed signals, differential pair wiring can be used to reduce electromagnetic radiation.

4 Conclusion

Multi-layer printed boards have unique anti-interference characteristics. With the continuous development of large-scale integrated circuits and very large-scale integrated circuits, people will increasingly adopt multi-layer printed boards. In modern electronic systems, with the increase of clock frequency and the increase of chip integration, the rationality and reliability of PCB design are becoming more and more important. In the design, specific problems need to be analyzed in order to obtain high-quality PCB design.