Detailed analysis of 11 techniques for high-speed signal routing

When high-speed signal wiring, the transmission line theory is needed. In the wiring process, some methods are different from the conventional general signal wiring. The wiring techniques of some high-frequency signal lines are generally given below.
1. Multi-layer wiring high-speed signal wiring circuit tends to have high integration degree and high wiring density. The use of multi-layer boards is both necessary for wiring and an effective means to reduce interference. Reasonable selection of the number of layers can greatly reduce the size of the printing plate, and can fully utilize the intermediate layer to set the shielding, which can better achieve the near grounding, can effectively reduce the parasitic inductance, can effectively shorten the signal transmission length, and can greatly reduce the signal. Inter-interference, etc., all of which are beneficial to the reliable operation of high-speed circuits. According to the data, the four-layer board is 20dB lower than the noise of the double-panel. However, the higher the number of layers, the more complicated the manufacturing process and the higher the cost.
2. The less the lead bends, the better the lead bend between the pins of the high-speed circuit device is as small as possible. The lead wire of the high-speed signal wiring circuit wiring is preferably a full line, which needs to be turned, and can be folded by a 45° fold line or a circular arc (as shown in FIG. 1 ). This requirement is only used to improve the fixing strength of the steel foil in the low frequency circuit. In high-speed circuits, meeting this requirement can reduce the external transmission and mutual coupling of high-speed signals, reducing signal radiation and reflection.

Figure 1 The way the wiring is turned
3. The shorter the lead, the better the lead between the pins of the high-speed signal wiring circuit device is as short as possible. The longer the lead wire, the larger the distributed inductance and distributed capacitance value, which has a lot of influence on the passage of the high-frequency signal of the system, and also changes the characteristic impedance of the circuit, causing the system to reflect and oscillate. These are the problems we have to avoid.
4. The less the alternating between the lead layers, the better the number of lead layers between the high-speed circuit device pins is as small as possible. The so-called "the least alternating between the layers of the leads is better" means that the fewer vias used in the component connection process, the better. According to tests, a via can bring about a distributed capacitance of about 0.5pf, resulting in a significant increase in the delay of the circuit, and a reduction in the number of vias can significantly increase the speed. This will be described in detail in the high frequency characteristics of the vias at the back.
5. Pay attention to the parallel cross-interference high-speed signal wiring circuit wiring should pay attention to the "cross-interference" introduced by the parallel line of the signal line. If parallel distribution cannot be avoided, a large area "ground" can be arranged on the reverse side of the parallel signal line. Reduce interference. Parallel traces in the same layer are almost inevitable, but in the adjacent two layers, the direction of the traces must be taken perpendicular to each other.
6. The ground wire is surrounded by the ground wire, also called the ground wire isolation, and the ground wire is surrounded by a particularly important signal line or local unit. Some signal pairs are more stringent, and signals must be protected from interference, such as clock signals, high-speed analog signals, and small analog signals. In order to protect these signals from crosstalk and interference from the surrounding signal lines as much as possible, a protective ground line can be added to the periphery of these signal lines, and the signal line to be protected is sandwiched.
7. Traces avoid loops of various types of signal traces that cannot form loops, and ground loops cannot form current loops. If a loop circuit is generated, it will cause a lot of interference in the system. Protel's automatic routing principle is based on the X-direction, Y-based and Daisy Chain wiring. The use of daisy-chain wiring can effectively avoid loops during wiring, but it also causes problems that are not easy to route.
8. Arrange Decoupling Capacitors One or several high frequency decoupling capacitors should be placed in the vicinity of each integrated circuit block. The nearby high-frequency channel is provided for the transient current of the integrated chip, so that the current does not pass through the power supply line with a large loop area, thereby greatly reducing the outward radiated noise. At the same time, since each integrated chip has its own high-frequency channel, there is no common impedance between them, which suppresses the impedance coupling.
9. Use high-frequency turbulence links to simulate ground, digital ground, etc. When connecting to public ground, high-frequency turbulence is used. In the actual assembly of the high-frequency turbulence link, the high-frequency ferrite bead through which the center hole is threaded is often used, and it is generally not expressed on the schematic diagram of the high-speed signal wiring circuit, thereby forming a network table (Netlist). ) It does not contain such components, and it will be ignored when wiring. In response to this reality, it can be used as an inductor in the schematic, and a component package is defined separately in the PCB component library, and it is manually moved to a suitable position near the convergence point of the common ground line before wiring.
10. Avoid branching and stump high-speed signal routing should avoid branching or forming stubs (as shown in Figure 2). Stumps have a large effect on the impedance and can cause reflections and overshoots of the signal, so we usually avoid stumps and branches when designing. By daisy-chaining, the effect on the signal is reduced.

Figure 2 Branching, tree, daisy chain routing
11. The signal line runs as far as possible in the inner layer. The high-frequency signal line is easy to generate large electromagnetic radiation on the surface layer, and is also susceptible to interference from external electromagnetic radiation or factors. The high-frequency signal line is routed between the power source and the ground line, and the radiation generated by the power source and the bottom layer absorbs the electromagnetic wave, and the generated radiation is much reduced.
High-speed circuit design is a very complex design process. There are several factors that need to be considered when designing a high speed circuit. These factors are sometimes opposite each other. For example, when the high-speed device is placed close to each other, although the delay can be reduced, crosstalk and significant thermal effects may occur; it is also a contradiction that the high-speed signal is routed as much as possible in the inner layer and less than the via. Therefore, in the design, we must weigh the various factors and make a comprehensive compromise: to meet the design requirements and reduce the design complexity. The adoption of high-speed PCB design means the controllability of the design process. Only controllable, reliable, can be successful.