The design of PCB board is a compulsory lesson for electronic engineer, and design a perfect PCB board is not so easy as it seems. A perfect PCB board is not only needs to be component selected and set up properly, but also needs good signal transmission performance. This article will be on the PCB high-speed signal circuit design knowledge of wiring techniques, expand the detailed introduction and sharing, hoping to be helpful to everyone's work.
Rational use of multi-layer PCB layout
In the actual design of the PCB board, most engineers will choose to use multi-layer board to complete the high-speed signal routing work, this multilayer board is an integral part of helping engineers to reduce circuit interference also is an effective means. The use of multi-layer PCB to complete the high-speed signal circuit design, engineers need to rationalize the selection of layers to reduce the size of the PCB, the middle layer to make full use of shielding to achieve the nearest ground, can effectively reduce the parasitic inductance and shorten the signal transmission Length, reducing crosstalk between signals, etc. All of these methods are very beneficial for the reliability of high-speed circuits.
In addition to the above mentioned reliable methods of using multi-layer PCB to enhance the reliability of PCB signal transmission, there are some authoritative data shows that the same material of four-layer board than the double-sided decrease noise of 20dB. Lead bending is as less as possible, the best cjoice is all-straight, need for turning, available 45-degree polylines or arcs can reduce the high-speed signal transmission and the external coupling between each other to reduce the signal radiation and reflection.
The pin between high-speed circuit device pins is as short as possible
During 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 greater the distributed inductance and distributed capacitance, which will lead to High-speed circuit system reflection, oscillation and so on.
In addition to shortening the leads between the pins of the high-speed circuit elements as much as possible, the fewer and the better the lead layers between the pins of the respective high-speed circuit devices during the PCB layout are the better, that is the fewer holes the better. In general, a via can bring about 0.5PF of distributed capacitance, which results in a significant increase in circuit delay. At the same time, high-speed circuit wiring should pay attention to the signal line close parallel lines introduced by the "cross interference", if parallel distribution can not be avoided, parallel lines can be arranged in the opposite large area "to" reduce interference. In the two adjacent layers, the direction of alignment must be taken perpendicular to each other.
Enclosures are ground lines that are particularly for important signal lines or local cells
In PCB layout design process, engineers can ground some of the very important signal lines surrounded by ways, such as clock signals, high-speed analog signals, such as these are not subject to interference signal traces at the same time in the periphery Protected ground wire, signal cable to be protected in the middle. Because in the design process, all kinds of signal routing can not form a loop, the same ground can not form a current loop. If the loop wiring circuit will produce a lot of interference in the system. The use of ground wiring surrounded by signal lines, can effectively avoid the formation of the loop wiring. One or several high frequency decoupling capacitors should be placed near each IC block. Analog ground, digital ground, etc. When connected to the public ground using high-frequency choke link. Some high-speed signal lines should be special treatment: differential signals require the same layer and as close as possible parallel lines, the differential signal lines are not allowed to insert any signal, and the required length.
In addition to the several design methods mentioned above, engineers should also try to avoid high-speed signal wiring branches or stumps during PCB signal line layout design. High-frequency signal line walking in the surface prone to large electromagnetic radiation, the high-frequency signal line wiring between the power and ground, through the power and the bottom of the electromagnetic wave absorption, the radiation will be reduced a lot.