Electromagnetic compatibility is the ability of electronic equipment to be able to work effectively and effectively in various electromagnetic environments. Electromagnetic compatibility design aims to make electronic equipment can not only inhibit a variety of external interference, so that electronic equipment in a specific electromagnetic environment can work properly, while reducing the electronic equipment itself on other electronic equipment, electromagnetic interference.
1, select a reasonable wire width
As the transient current on the printed line generated by the impact of interference is mainly caused by the inductance of the printed wire, and therefore should minimize the inductance of the printed wire. The inductance of the printed wire is proportional to its length, inversely proportional to its width, so short and fine wires are advantageous for suppressing interference. Clock lines, line drivers, or bus drivers' signal lines often carry large transients, and the printed leads are as short as possible. For the discrete component circuit, the printed wire width of 1.5mm or so, you can fully meet the requirements; for integrated circuits, printed wire width can be selected between 0.2 ~ 1.0mm.
2, using the correct routing strategy
The use of equal alignment can reduce the wire inductance, but the wire between the mutual inductance and distributed capacitance increases, if the layout allows, it is best to use well-shaped mesh fabric structure, the specific approach is printed on the side of the horizontal wiring, the other side of vertical wiring, And then connected at the cross hole with a metal hole.
3, in order to suppress the crosstalk between the printed circuit board wiring, in the design of wiring should try to avoid long distance equal alignment, as far as possible to pull the line between the line and the distance between the signal line and the ground and power lines as far as possible cross. It is possible to effectively suppress crosstalk by setting a grounded printed line between some signal lines that are sensitive to interference.
4, in order to avoid high-frequency signal through the printed wire when the electromagnetic radiation generated in the printed circuit board wiring, should also note the following:
(1) to minimize the continuity of the printed wire, such as wire width without mutation, the corner of the wire should be greater than 90 degrees forbidden ring alignment.
(2) the clock signal lead most likely to produce electromagnetic radiation interference, the alignment should be close to the ground circuit, the drive should be next to the connector.
(3) The bus driver should be close to the bus to be driven. For leads that leave the printed circuit board, the drive should be next to the connector.
(4) The wiring of the data bus should be sandwiched between two signal lines. It is advisable to place the ground circuit next to the least important address pins, since the latter often carries high frequency currents.
(5) When arranging high-speed, medium-speed and low-speed logic circuits on printed circuit boards, the devices should be arranged in the manner shown in Fig.
5, inhibition of reflection interference
In order to suppress the reflection interference that appears at the end of the printed line, the length of the printed wire and the slow circuit should be used as much as possible, except for special needs. If necessary, can be added terminal matching, that is, at the end of the transmission line to the ground and the power side of the same resistance to the same matching resistance. According to experience, the general speed of the TTL circuit, the printed line longer than 10cm should be used when the terminal matching measures. The resistance of the matching resistor should be determined according to the output drive current of the integrated circuit and the maximum value of the absorption current.
6, the circuit board design process using differential signal line routing strategy
Wiring is very close to the differential signal pairs will be closely coupled with each other, this coupling between each other will reduce the EMI emission, usually (of course there are some exceptions) differential signal is high-speed signal, so high-speed design rules are usually applicable This is especially true for wiring of differential signals, especially when designing signal lines for transmission lines. This means that we must design the wiring of the signal lines with great care to ensure that the characteristic impedance of the signal lines is continuous and maintains a constant along the signal lines.
In the process of routing and routing of differential pairs, we hope that the two pairs of PCB lines in the differential line are exactly the same. This means that the best effort should be made in practical applications to ensure that the PCB lines in the differential line pair have exactly the same impedance and that the length of the wiring is exactly the same. Differential PCB lines are usually always in pairs, and the distance between them is kept constant at any position along the direction of the pair. In general, the layout and routing of differential pairs is always as close as possible.