Layout is one of the most basic work skills of PCB design engineers. The quality of the wiring will directly affect the performance of the entire system. Most high-speed design theories have to be finally implemented and verified through layout. It can be seen that wiring is very important in high-speed PCB design. The following article will analyze the rationality of some situations that may be encountered in actual wiring, and give some more optimized routing strategies. It is mainly explained from three aspects- right-angle wiring, differential wiring, and serpentine line.
1. Right-Angle Routing
Right-angle wiring is generally a situation that needs to be avoided in PCB wiring as much as possible, and it has almost become one of the standards for measuring the quality of wiring. So how much influence will the right-angle wiring have on signal transmission? In principle, right-angle wiring will make the line width of the transmission line change, causing discontinuity in impedance. In fact, not only right-angle routing, but also corners, sharp-angle routing may cause impedance changes. The influence of right-angle wiring on the signal is mainly reflected in three aspects. One is that the corner can be equivalent to a capacitive load on the transmission line, which slows down the rise time. The other is that impedance discontinuity will cause signal reflection. The third is that the right-angle tip is generated EMI.
The parasitic capacitance caused by the right angle of the transmission line can be calculated by the following empirical formula: C=61W(Er)1/2/Z0 In the above formula, C refers to the equivalent capacitance of the corner (unit: pF), and W refers to walking width of the line (unit: inch), εr refers to the dielectric constant of the medium, and Z0 is the characteristic impedance of the transmission line. For example, for a 4Mils 50 ohm transmission line (<εr=4.3), the capacitance brought by a right angle is about 0.0101pF, and then the rise time change caused by this can be estimated: T10-90%=2.2 *C*Z0/2 = 2.2*0.0101*50/2 = 0.556ps
It can be seen through calculation that the capacitance effect brought by the right-angle wiring is extremely small. As the line width of the right-angle trace increases, the impedance there will decrease, so a certain signal reflection phenomenon will occur. We can calculate the equivalent impedance after the line width increases according to the impedance calculation formula mentioned in the transmission line chapter, and then calculate the reflection coefficient according to the empirical formula: ρ=(Zs-Z0)/(Zs+Z0). Generally, the impedance change caused by right-angle wiring is between 7%-20%, so the maximum reflection coefficient is about 0.1. Moreover, it can be seen that the impedance of the transmission line changes to the minimum during the length of the W/2 line, and then returns to the normal impedance after the time of W/2. The entire impedance change time is extremely short, often within 10ps. Fast and small changes are almost negligible for general signal transmission.
Many people have this understanding of right-angle wiring, and they think that the tip is easy to transmit or receive electromagnetic waves and generate EMI. This has become one of the reasons why many people think that right-angle wiring cannot be routed. However, many actual test results show that right-angled traces will not produce obvious EMI than straight lines. Perhaps the current instrument performance and test level restrict the accuracy of the test, but at least it illustrates a problem. The radiation of the right-angled wiring is already smaller than the measurement error of the instrument itself.
In general, the right-angle routing is not as terrible as imagined. At least in applications below GHz, any effects such as capacitance, reflection, EMI, etc., are hardly reflected in the TDR test. The focus of high-speed PCB design engineers should still be on layout, power/ground design, and wiring design, via holes and other aspects. Of course, although the impact of right-angle wiring is not very serious, it does not mean that we can all use right-angle wiring in the future. Paying attention to details is the basic quality that every outstanding engineer must have. Moreover, with the rapid development of digital circuits, the signal frequency that the PCB engineer deals with will also continue to increase, to the RF design field above 10GHz, these small right angles may become the focus of high-speed problems.
2. Differential Routing
Differential signal (Differential Signal) is more and more widely used in high-speed circuit design. The most critical signal in the circuit is often designed with a differential structure. What makes it so popular? How can it be ensured in PCB design? What about the performance? With these two questions, we will proceed to the next part of the discussion. What is a differential signal? In layman’s terms, the driving end sends two equal and inverted signals, and the receiving end judges the logic state “0” or “1” by comparing the difference between the two voltages. The pair of traces carrying differential signals is called differential traces.
Compared with ordinary single-ended signal traces, differential signals have the most obvious advantages in the following three aspects:
a.Strong anti-interference ability. Because the coupling between the two differential traces is very good, when there is noise interference from the outside, they are almost coupled to the two lines at the same time, and the receiving end only cares about the difference between the two signals. Therefore, the external common mode noise can be completely eliminated.
b. It can effectively suppress EMI. For the same reason, because the two signals have opposite polarities, the electromagnetic fields radiated by them can eliminate each other out. The tighter the coupling, the less electromagnetic energy vented to the outside world.
c. The timing positioning is accurate. Because the switch change of the differential signal is located at the intersection of the two signals, unlike the ordinary single-ended signal, which depends on the high and low threshold voltages, it is less affected by the process and temperature, and can reduce the error in the timing. , But also more suitable for circuits with low amplitude signals.
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