In PCB design, crosstalk generated by strong interference signal lines and signal lines sensitive to interference will exist between routing lines. This adverse effect is not only related to clock or periodic signals, but also affects other important routing, data lines, address lines, control lines and IO in the system. Most of the problems come from clock and periodic signals, and the crosstalk between them will cause functional problems in other parts.
2、 Concept of 3W principle
Therefore, when multiple high-speed signal lines are routed for a long distance, such as clock routing, differential lines, video, audio, reset lines, and other key circuits of the system, in order to reduce the crosstalk between lines, it is necessary to ensure that the line spacing is large enough. When the line center spacing is not less than 3 times the line width, most electric fields can be kept from interfering with each other, which is the 3W rule. As shown in the figure below.
Meeting the 3W principle can reduce the crosstalk between signals by 70%, while meeting 10W can reduce the crosstalk between signals by nearly 98%
3、 Conditions for the establishment of 3W principle
Although the 3W principle is easy to remember, it should be emphasized that there are preconditions for the establishment of this principle.
Considering the physical significance of crosstalk causes, the spacing is related to stack height and conductor linewidth.
For the four layer board, 3W is enough for the height distance between the routing and the reference plane (5 ~ 10mils);
However, for two-layer boards, the distance between the routing and the height of the reference layer (45 ~ 55mils), 3W may not be enough for high-speed signal routing.
The 3W principle is generally established under the condition of 50 ohm characteristic impedance transmission line.
The 3W principle means that when multiple high-speed signal lines are routed for a long distance, their spacing should follow the 3W principle, such as clock line, differential line, video and audio signal line, reset signal line and other key circuits of the system, rather than all wiring on the board must comply with the 3W principle.
PCB design experience: do you know the important 3W principle in PCB design?