Look at the wiring of PCB high-speed signal circuit design

PCB design is a compulsory course for PCB engineers, and it is not easy to design a perfect PCB. A perfect PCB needs not only reasonable component selection and settings, but also good signal transmission performance. This paper will introduce and share wiring skills and knowledge in PCB high-speed signal circuit design in detail,

Reasonable Use of Multilayer Boards for PCB Wiring

In the actual design process of PCB board, most engineers will choose to use multilayer boards to complete high-speed signal wiring. This kind of multilayer board is not only an essential component, but also an effective means to help engineers reduce circuit interference. When using multilayer boards to complete the high-speed signal circuit design of PCB, engineers need to reasonably select the number of layers to reduce the size of the PCB, make full use of the middle layer to set shielding, and achieve nearby grounding, which can effectively reduce parasitic inductance, shorten signal transmission length, and reduce cross interference between signals. All these methods are very beneficial to the reliability of high-speed circuits.

In addition to several methods mentioned above to improve the reliability of PCB signal transmission by using multilayer boards, some authoritative data show that the noise of 4-layer boards with the same material is 20dB lower than that of double-sided boards. The less the lead bends, the better. It is better to use a full straight line, which needs turning. A 45 degree broken line or arc turning can be used to reduce the external transmission and mutual coupling of high-speed signals, and reduce the radiation and reflection of signals.

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

In the process of PCB high-speed signal circuit design and wiring, engineers need to shorten the lead between pins of high-speed circuit devices as much as possible, thinking that the longer the lead, the greater the distributed inductance and capacitance value, which will lead to reflection, vibration, etc. of high-speed circuit system.

In addition to shortening the leads between pins of high-speed circuit components as much as possible, in the PCB wiring process, the lead layer alternation between pins of high-speed circuit components should be as little as possible, that is, the fewer vias used in the component connection process, the better. Generally, a via can bring about a distributed capacitance of about 0.5pF, which will lead to a significant increase in the delay of the circuit. At the same time, the high-speed circuit wiring should pay attention to the "cross interference" introduced by the parallel routing of signal lines in short distance. If the parallel distribution cannot be avoided, a large area of "ground" can be arranged on the opposite side of the parallel signal lines to reduce the interference. In two adjacent layers, the direction of routing must be perpendicular to each other.

Enclose the signal lines or local units of special importance with ground wires

In the process of PCB wiring design, engineers can use the method of grounding wire to enclose some very important signal lines. They can add a protective grounding wire to the periphery while routing signals that are not easily interfered, such as clock signals, high-speed analog signals, and clamp the signal lines to be protected in the middle. Because in the process of design, all kinds of signal wiring cannot form a loop, and similarly, the ground wire cannot form a current loop. However, if loop wiring circuit is generated, it will cause great interference in the system. The wiring method of using the ground wire to enclose the signal line can effectively avoid the formation of a loop when wiring. One or several high-frequency decoupling capacitors should be set near each integrated circuit block. Analog ground wire, digital ground wire, etc. shall be connected to public ground wire with high-frequency choke link. Some high-speed signal lines shall be specially processed: differential signals are required to be on the same layer and close to parallel lines as much as possible. No signals are allowed to be inserted between differential signal lines, and they are required to be of equal length.

In addition to the above mentioned design methods, engineers should also try to avoid high-speed signal wiring branching or forming stubs when designing PCB signal wiring. High frequency signal lines are easy to generate large electromagnetic radiation when walking on the surface. If the high frequency signal lines are routed between the power supply and the ground wire, the radiation generated will be much reduced through the absorption of electromagnetic waves by the power supply and the bottom layer.