This article is the following article "Summary of RF PCB Layout and Louting", mainly describes the skills and methods of high-frequency PCB design, electromagnetic compatibility design, and the use of differential signal line routing strategy in the circuit board design process.
Electromagnetic compatibility refers to the ability of electronic equipment to work in a coordinated and effective manner in various electromagnetic environments. The purpose of electromagnetic compatibility design is to enable electronic equipment to suppress all kinds of external interference so that the electronic equipment can work normally in a specific electromagnetic environment, and at the same time to reduce the electromagnetic interference of the electronic equipment itself to other electronic equipment.
Since the impact interference generated by the transient current on the printed lines is mainly caused by the inductance of the printed wires, the inductance of the printed wires should be minimized. The inductance of the printed wire is proportional to its length and inversely proportional to its width, so short and precise wires are beneficial to suppress interference. The signal lines of clock leads, row drivers, or bus drivers often carry large transient currents, and the printed wires should be as short as possible. For discrete component circuits, the printed wire width is about 1.5mm, which can fully meet the requirements; for integrated circuits, the printed wire width can be selected between 0.2mm and 1.0mm.
The use of equal routing can reduce the wire inductance, but the mutual inductance and distributed capacitance between the wires increase. If the layout allows, it is best to use a grid-shaped wiring structure. The specific method is to wire one side of the printed board horizontally and the other side to wire vertically. Then connect with metalized holes at the cross holes.
To suppress the crosstalk between the conductors of the printed circuit board, when designing the wiring, try to avoid long-distance equal routing, and extend the distance between the lines as much as possible, and the signal line, the ground line, and the power line Do not cross. Setting a grounded printed line between some signal lines that are very sensitive to interference can effectively suppress crosstalk.
To avoid electromagnetic radiation generated when high-frequency signals pass through printed wires, the following points should also be noted when wiring the printed circuit board:
To suppress the reflection interference that appears at the terminal of the printed line, in addition to special needs, the length of the printed line should be as short as possible and a slow circuit should be used. Terminal matching can be added when necessary, that is, a matching resistor of the same resistance is added to the end of the transmission line to the ground and the power terminal. According to experience, for general faster TTL circuits, terminal matching measures should be adopted when the printed lines are longer than 10cm. The resistance value of the matching resistor should be determined according to the maximum value of the output drive current and the absorption current of the integrated circuit.
Differential signal pairs with very close wiring will also be tightly coupled to each other. This mutual coupling will reduce EMI emissions. Usually (of course there are some exceptions) differential signals are also high-speed signals, so high-speed design rules usually apply. This is especially true for the routing of differential signals, especially when designing signal lines for transmission lines. This means that we must carefully design the wiring of the signal line to ensure that the characteristic impedance of the signal line is continuous and constant along the signal line. In the layout and routing process of the differential pair, we hope that the two PCB lines in the differential pair are the same. This means that in practical applications, the greatest effort should be made to ensure that the PCB lines in the differential pair have the same impedance and the length of the wiring is the same. Differential PCB lines are usually routed in pairs, and the distance between them is kept constant at any position along the line pair direction. Under normal circumstances, the placement and routing of differential pairs are always as close as possible.