Transfer from: https://blog.csdn.net/weixin_42347882/article/details/112229754
In the PCB design process, the division of the power plane or the division of the ground plane will cause the plane to be incomplete , so that when the signal is routed , its reference plane will appear to cross from one power plane to another. This phenomenon is called cross-segmentation .
Schematic diagram of the cross-segmentation phenomenon Cross-segmentation may not be relevant to low-speed signals, but in high-speed digital signal systems, high-speed signals use the reference plane as the return path, which is the return path. When the reference plane is incomplete, the following adverse effects will occur:
a. Will cause the impedance of the trace to be discontinuous;
b. It is easy to cause crosstalk between signals;
c. Will cause reflections between signals;
d. Increase the current loop area and loop inductance to make the output waveform easy to oscillate;
e. Increase the radiation interference to space, while being vulnerable to the influence of space magnetic field;
f. Increase the possibility of magnetic field coupling with other circuits on the board;
g. The high-frequency voltage drop on the loop inductance constitutes a common-mode radiation source, and common-mode radiation is generated through an external cable.
Therefore, the PCB layout should be as close as possible to a plane and avoid cross-segmentation . If it is necessary to cross the partition or cannot be close to the power ground plane, these conditions are only allowed to exist in the low-speed signal line.
Handling of cross-segmentation in PCB design
If cross-segmentation is unavoidable in PCB design, how to deal with it? In this case, the segmentation needs to be patched to provide a shorter return path for the signal. Common processing methods include adding patching capacitors and cross-line bridging .
1. Stiching Capacitor usually place a 0402 or 0603 encapsulated ceramic capacitor at the signal span division. The capacitance value of the capacitor is 0.01uF or 0.1uF. If space permits, you can add several more such capacitors. ;At the same time, try to ensure that the signal line is within the 200mil range of the stitching capacitor, the smaller the distance, the better; and the networks at both ends of the capacitor correspond to the network of the reference plane through which the signal passes. See the network connected at both ends of the capacitor in the figure below. The two colors are high. Bright two different networks:
2. Cross -line bridging is commonly used in the signal layer to "package the ground processing" of the divided signal, or it may be the signal line of other networks. This "package ground" line should be as thick as possible. For this processing method, refer to the following Figure.
Knowledge expansion: high-speed signal wiring skills
1. Multi-layer wiring high-speed signal wiring circuits tend to be highly integrated and high in wiring density. The use of multi-layer boards is not only necessary for wiring, but also an effective means to reduce interference. Reasonable selection of the number of layers can greatly reduce the size of the printed board, and can make full use of the intermediate layer to set up the shield, which can better realize the nearby grounding, can effectively reduce the parasitic inductance, can effectively shorten the signal transmission length, and can greatly reduce the signal Cross-interference and so on.
2. The less lead bends, the better. The less lead bends between the pins of high-speed circuit devices, the better. The lead of high-speed signal wiring circuit wiring is best to adopt a full straight line, which needs to be turned, and can be turned by 45° broken line or arc. This requirement is only used to improve the fixing strength of the steel foil in low-frequency circuits, while in high-speed circuits, this requirement is met. One requirement can reduce the external emission and mutual coupling of high-speed signals, and reduce signal radiation and reflection.
3. The shorter the lead, the better. The lead between the pins of the high-speed signal wiring circuit device should be as short as possible. The longer the lead, the larger the distributed inductance and distributed capacitance, which will have a lot of influence on the passage of high-frequency signals of the system. At the same time, it will also change the characteristic impedance of the circuit, causing the system to reflect and oscillate.
4. The less alternating between lead layers, the better. The less alternating between lead layers between pins of high-speed circuit devices, the better. The so-called "the less the inter-layer alternation of the leads, the better" means that the fewer vias used in the component connection process, the better. It is measured that one via can bring about 0.5pf of distributed capacitance, leading to a significant increase in the delay of the circuit, and reducing the number of vias can significantly increase the speed.
5. Pay attention to parallel cross interference High-speed signal wiring should pay attention to the "cross interference" introduced by the signal lines in close distance and parallel wiring. If parallel distribution cannot be avoided, a large area of "ground" can be arranged on the opposite side of the parallel signal lines to greatly reduce the interference. .
6. Avoid branches and stumps. High-speed signal wiring should try to avoid branches or form stumps (Stub). Tree stumps have a great influence on impedance, which can cause signal reflection and overshoot, so we usually should avoid stumps and branches when designing. Using daisy chain wiring will reduce the impact on the signal.
7. Try to go as far as possible to the signal line in the inner layer, high-frequency signal line on the surface, it is easy to produce larger electromagnetic radiation, and it is also easy to be interfered by external electromagnetic radiation or factors. Wire the high-frequency signal line between the power supply and the ground wire, and the radiation generated by the power supply and the bottom layer will be absorbed by electromagnetic waves. (The graphic content is organized from the Internet)
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Original link: https://blog.csdn.net/weixin_42347882/article/details/112229754