Electromagnetic compatibility design based on double-sided printed circuit board

Printed circuit boards (PCBs) are highly integrated components, power lines, and signal lines in electronic application systems. The quality of PCB design has a great impact on the electromagnetic compatibility of the system. Therefore, the design of PCBs is by no means a simple layout and arrangement of components and lines. Only by consciously strengthening the electromagnetic compatibility design can the anti-interference ability of the system be enhanced and the stability be improved.

For commonly used single-chip microcomputer systems, the clock frequency is generally 4 to 12 MHz, and the rest of the integrated circuits are mostly 74HC and 74LS series. If a single-sided printed circuit board is used, it is difficult to meet the needs, and the cost of using a multi-layer printed circuit board is too high, so most of them use double-sided printed circuit boards. When using double-sided printed circuit boards, as long as the electromagnetic compatibility problem is fully considered, it can meet the application requirements.

Of course, the electromagnetic compatibility of the single-chip microcomputer system involves many aspects. This article only briefly analyzes the layout of unit circuits (or components) and the arrangement of lines on double-sided printed circuit boards and other issues related to electromagnetic compatibility, and gives specific measures accordingly.

1 Layout of unit circuits on printed circuit boards

The relative positions of various unit circuits on a double-sided printed circuit board directly affect the electromagnetic compatibility of the system. Therefore, it is very important to identify the unit circuits to be used. Unit circuits are grouped according to their different sensitivity to electromagnetic compatibility during use. The purpose of grouping is to divide the printed circuit board area by group, so that the components of the same group are placed together, so as to ensure that the components of each group do not interfere with each other in space. Generally, they are grouped according to the speed of operation or the level of power supply voltage.

1.1 Group layout according to working speed

The higher the operating frequency of the unit circuit, the faster the speed and the richer the signal spectrum; the greater the proportion of high-frequency components, the stronger the external interference. According to the operating frequency of the unit circuit, it can be divided into high-speed circuits (such as microprocessors), medium-speed circuits (such as display processing), low-speed circuits (such as interfaces) and analog circuits (such as analog signal amplifiers). The layout of various speed circuits on the printed circuit board is generally shown in Figure 1.

1.2 Grouping layout according to the level of working power supply voltage

Generally speaking, different power supply voltages often lead to different types of circuits. For example, digital circuits mostly use 5V, while operational amplifiers in analog circuits mostly use 12V or 15V. If there are still digital and analog components in the circuits using the same power supply voltage, they can be further grouped. The level layout of different power supply voltages is shown in Figure 2.

Note: Components with different power supply voltage levels cannot be overlapped to prevent crosstalk. The rationality of component distribution is shown in Figure 3.

2. Arrangement of ground wire and power wire

From the perspective of solving electromagnetic compatibility, the ground wire is the most important wire on the printed circuit board, so the ground wire for double-sided printed circuit boards must be arranged particularly reasonably.

2.1 Use classified ground wires

The classification of ground wires is to set ground wires according to different power supply voltages, digital and analog, high speed and low speed, and large current and small current. The purpose of classification is to prevent ground wire impedance coupling interference. Double-sided printed circuit boards use tracks as ground wires. Even if the tracks are wide, the inductance cannot be ignored. There is still a considerable voltage drop when high-frequency current passes through, so the grounding method is generally used. The so-called grounding is separated during wiring, and finally all gathered at a point on the DC power supply.

2.2 Use mesh structure ground wire

An effective method to improve the electromagnetic compatibility effect for similar unit circuits (or components) is to use a mesh structure ground wire as shown in Figure 4. The solid line in the figure is the front track line, and the dotted line is the back track line. The solid line and the dotted line are perpendicular to each other, and the intersection is connected by a metallized hole. In this way, the current can flow back nearby. The vertical ground wire in Figure 4 may cause certain difficulties for the front wiring. It can be replaced by a small busbar and connected to the power supply line as shown in Figure 5. The vertical wide lines in the figure represent small current busbars, which can be loaded and unloaded for easy scheduling.

2.3 The power supply line should be arranged in coordination with the ground line

We should start from two aspects: first, reduce the characteristic impedance of the power supply line as much as possible; second, reduce the area of ​​the power supply loop. [page]

The power supply line of the double-sided printed circuit board is composed of tracks. In order to reduce the characteristic impedance of the power supply track pair, the power supply track and the ground track should be as wide as possible, and use the front and back sides to make them parallel and close to each other. If possible, place the corresponding surfaces to each other to reduce the power supply loop area to the minimum. Different power supply loops should not overlap each other to reduce electromagnetic interference.

2.4 Configuration of decoupling capacitors

The double-sided board uses track pairs for power supply. In addition to paying attention to the routing of the track pairs, a high-frequency decoupling capacitor with a capacity of 0.01 to 0.10 uF should be added next to each integrated circuit. A high-frequency decoupling capacitor and a low-frequency filter capacitor with a capacity of 1 to 10 uF should also be added to the introduction of the power track pair connected to the printed circuit board to further improve the low-frequency characteristics of the power decoupling filter.

3. Signal line layout

3.1 Incompatible signals should be isolated from each other

High frequency and low frequency, large current and small current, digital and analog signals are incompatible. After considering the location of incompatible components, attention should still be paid to the isolation between them in the layout of signal lines to avoid coupling interference between them. Generally, the following measures can be taken:

(1) Incompatible signal lines should be kept away from each other and not parallel; signal lines on the front and back sides should be perpendicular to each other to reduce the coupling interference of electric and magnetic fields between the lines.

(2) High-speed signal lines, especially clock lines, should be as short as possible. If necessary, isolated ground lines can be added on both sides of the high-speed signal lines.

(3) The signal lines that serve as input and output of the unit circuit should be arranged in their respective areas and should not cross each other.

3.2 Minimize the area of ​​the signal loop

Reducing the signal loop area and loop overlap is particularly important for large current loop crosstalk resistance. On a double-sided board, the signal line and its return line should be arranged close together. It is best for each signal line to have its own return line, especially for DC amplifiers, otherwise it is easy to cause interference to the circuit.

4 Other electromagnetic compatibility measures

4.1 The routing shape should not have tangles, branches or hard corners

Because that may destroy the consistency of the characteristic impedance of the wire or cause reflection and harmonics or local high voltage to cause discharge. The printed wire shapes that are generally preferred and avoided are shown in Figure 6.

4.2 Use ground and ring protection at the terminal of sensitive components and the frame of the printed circuit board, as shown in Figure 7. Note that the protection ring cannot serve as a current return line and can only be grounded at a single point.

4.3 Do not leave blank copper layers on the printed circuit board. Because they may act as both transmitting antennas and receiving antennas, they must be grounded.

4.4 Place the I/O driver circuits as close to the edge of the printed circuit board as possible and move them away from the printed circuit board as quickly as possible.

4.5 The input and output of unused gate circuits should not be left floating; the non-inverting input of unused operational amplifiers should be grounded, and the inverting input should be connected to its output.

5. Use Autorouting Selectively

Most printed circuit board wiring uses wiring software for automatic wiring, which is the main reason for the decline in the electromagnetic compatibility of printed circuit boards. Automatic wiring software performs wiring according to artificially specified methods in advance. Most of its wiring principles are to make full use of the area resources of the printed circuit board. At present, there is no automatic wiring software that can judge and identify the compatibility of adjacent parts or lines. Due to the limited area resources available for double-sided printed circuit boards, designers should use automatic wiring with caution and personally participate in part of the wiring work. General manual operations are:

Segmentation of printed circuit board area (layout of components);

Arrangement of ground wire and power supply wire;

Arrangement of high-speed signal lines (automatic routing of the first batch is possible);

Line and line end protection of sensitive devices, etc.

6 Conclusion

This article discusses the electromagnetic compatibility technology in the design of double-sided printed circuits from a practical perspective. Based on our many years of experience in the application and development of single-chip microcomputer systems, we strive to provide some corresponding anti-interference measures in the design of double-sided printed circuits from a practical perspective.