Detailed explanation of the three wiring skills of PCB high-speed signal circuit design

PCB board design is a compulsory course for electronics engineers, and trying to design a perfect PCB board is not as easy as it seems. A perfect PCB board not only needs to be properly selected and set, but also needs good signal transmission performance. This article will introduce and share the knowledge of wiring skills in PCB high-speed signal circuit design, hoping to help everyone's work.

Proper use of multi-layer boards for PCB layout

In the actual design process of the PCB, most engineers choose to use multi-layer boards to complete high-speed signal wiring. This multi-layer board is an indispensable component and an effective means to help engineers reduce circuit interference. When using multi-layer boards to complete the high-speed signal circuit design of PCB, engineers need to select the number of layers to reduce the size of the printed board, make full use of the middle layer to set the shielding, achieve near grounding, effectively reduce parasitic inductance and shorten signal transmission. Length, reduced cross-talk between signals, etc., all of these methods are very beneficial for the reliability of high-speed circuits.

In addition to the several methods mentioned above for improving the reliability of PCB signal transmission using multi-layer boards, there is also some authoritative data showing that the four-layer board is 20 dB lower than the noise of the double-layer board in the same material. The less the lead bends, the better. It is best to use full line, need to turn, use 45 degree fold line or arc turn, can reduce the high-speed signal external emission and mutual coupling, reduce signal radiation and reflection.

The shorter the lead between the pins of the high speed circuit device, the better

In the design and layout of PCB high-speed signal circuits, engineers need to shorten the leads between the pins of high-speed circuit devices as much as possible, so that the longer the leads, the larger the distributed inductance and distributed capacitance, which will result in High-speed circuit systems reflect, oscillate, and so on.

In addition to shortening the leads between the pins of high-speed circuit components as much as possible, in the process of PCB layout, the number of lead layers between the pins of each high-speed circuit device is as small as possible, which is used in the component connection process. The less the hole, the better. In general, a via can introduce a distributed capacitance of about 0.5 pF, which will result in a significant increase in circuit delay. At the same time, high-speed circuit wiring should pay attention to the "cross interference" introduced by the parallel lines of the signal lines. If parallel distribution cannot be avoided, a large area of ​​"ground" can be arranged on the reverse side of the parallel signal lines to reduce interference. In the adjacent two layers, the direction of the traces must be taken perpendicular to each other.

Grounding a signal line or local unit that is particularly important

In the process of designing the PCB board layout, engineers can use the ground line enveloping method for some very important signal lines, which can be added to the periphery while being difficult to interfere with signal lines such as clock signals and high-speed analog signals. Protect the ground wire and sandwich the signal cable to be protected. Because in the design process, various types of signal traces can not form a loop, the same ground line can not form a current loop. If a loop wiring circuit is generated, it will cause a lot of interference in the system. The wiring method of surrounding the signal line by the ground line can effectively avoid loop formation during wiring. One or several high frequency decoupling capacitors should be placed near each integrated circuit block. High-frequency turbulence links are used when analog ground lines, digital ground lines, etc. are connected to public ground lines. Some high-speed signal lines should be specially processed: differential signals are required to be on the same layer and as close as possible to parallel traces. No signal is allowed between the differential signal lines and isometric.

In addition to the several design methods mentioned above, engineers should also avoid high-speed signal wiring branches or stumps when designing PCB signal line layouts. The high-frequency signal line is easy to generate large electromagnetic radiation on the surface layer, and the high-frequency signal line is wired between the power source and the ground line. The radiation generated by the power source and the bottom layer absorbs electromagnetic waves, and the generated radiation is much reduced.

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