Electromagnetic compatibility refers to the ability of electronic equipment to work in a variety of electromagnetic environment. Electromagnetic compatibility is designed to make the electronic equipment not only can inhibit a variety of external interference, the electronic equipment can work normally in the electromagnetic environment in particular, but also can reduce the electronic equipment itself on other electronic devices and electromagnetic interference.
1. select the reasonable width of the wire
Due to the impact of the transient current on the printed wire is mainly caused by the inductive component of the printed wiring, the inductance of the printed wire should be reduced as much as possible. The inductance of printed wiring is proportional to its length and inversely proportional to its width, so that the short and fine wire is beneficial to suppress interference. The signal lines of a clock lead, line driver, or bus driver often contain a large transient current. For the discrete component circuit, when the printed wire width is about 1.5mm, it can fully meet the requirements. For the integrated circuit, the printed wire width can be selected between 0.2 and 1.0mm.
2. the correct wiring strategy
The equal line can reduce wire inductance, mutual inductance and capacitance between conductors but increases, if the layout allows, the best use of well shaped mesh wiring structure, specifically a horizontal wiring printed board, on the other side of the vertical wiring, and then in the cross hole hole connected by metal.
3. in order to suppress the crosstalk between wires of the printed board, in the design of wiring should be avoided when long distance equal to walk the line, as far as possible between the open line and the line distance, the signal line and the ground and power line as far as possible not to cross. In a number of very sensitive to interference between the signal line to set up a grounding line, can effectively suppress crosstalk.
4. in order to avoid the high frequency signal through the printed wiring when the electromagnetic radiation generated in the printed circuit board wiring, should also pay attention to the following points:
(1) to minimize the discontinuity of printed wiring, such as the width of the wire should not be changed, the wire should be more than 90 degrees round the corner to prohibit the loop line, etc..
(2) the clock signal lead is most likely to produce electromagnetic radiation interference, the line should be close to the ground loop, the driver should be close to the connector.
(3) the bus driver should be close to the bus. For those leaving the printed circuit board, the driver should be closely connected to the connector.
(4) the wiring of the data bus should be sandwiched between two signal lines. It is best to place the circuit next to the least significant address, since the latter is often loaded with high frequency current.
(5) when the printed circuit board is equipped with high speed, medium speed and low speed logic circuits, the device shall be arranged in accordance with figure 1.
5. suppression of reflection interference
In order to suppress the interference of the printed wiring terminal, except the special needs, the length of the printed circuit and the slow circuit should be shortened as much as possible. If necessary, the terminal can be matched, that is, at the end of the transmission line, the ground and the power supply are respectively connected with a matching resistor with the same resistance value. According to the experience, the general speed of the TTL circuit, the printed lines longer than 10cm should be used when the terminal matching measures. The resistance of the matching resistor should be determined according to the output current of the integrated circuit and the maximum of the absorption current.
6. the circuit board design process using the differential signal wiring strategy
The wiring is very close to the differential signal to each other will be closely coupled, the coupling between each other can reduce EMI emission, usually (with a few exceptions) the differential signal is high speed signal, so the high-speed design rules are often applied to differential signal wires, especially the signal line transmission line design the more so. This means that we must be very careful in designing the wiring of the signal line to ensure that the characteristic impedance of the signal line is continuous along the line and keeps a constant.
In the process of the layout of the differential pair, we hope that the two PCB lines in the differential pair are exactly the same. This means that, in practice, we should try our best to make sure that the PCB line of the differential pair has exactly the same impedance and the length of the wire is also exactly the same. The differential PCB lines are always in pairs, and the distance between them is constant at any position along the direction of the pair. Typically, the layout of the differential pair is always as close as possible.