Layout is one of the most basic skills of PCB design engineers. The quality of wiring will directly affect the performance of the whole system. Most high-speed design theories will finally be realized and verified through layout. Therefore, wiring is very important in high-speed PCB design. The following will analyze its rationality and give some optimized routing strategies for some situations that may be encountered in the actual wiring. Mainly from the right angle routing, differential routing, serpentine line and other three aspects.
1. Right angle routing
Right angle routing is generally required to be avoided in PCB wiring, and it has almost become one of the standards to measure the quality of wiring. How much impact will right angle routing have on signal transmission? In principle, right angle routing will change the linewidth of the transmission line, resulting in discontinuity of impedance. In fact, not only right angle routing, dun angle and acute angle routing may cause impedance change.
The influence of right angle routing on the signal is mainly reflected in three aspects: first, the corner can be equivalent to the capacitive load on the transmission line to slow down the rise time; Second, the discontinuous impedance will cause the reflection of the signal; The third is the EMI generated by the right angle tip.
The parasitic capacitance caused by the right angle of the transmission line can be calculated by the following empirical formula:
C=61W(Er)1/2Z0
In the above formula, C refers to the equivalent capacitance of the corner (unit: PF), and W refers to the width of the wiring (unit: inch), ε R 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（ ε R is 4.3), the electric capacity brought by a right angle is about 0.0101pf, and then the rise time change caused by it can be estimated:
T10-90%=2.2*C*Z0/2 = 2.2*0.0101*50/2 = 0.556ps
Through calculation, it can be seen that the capacitance effect caused by right angle routing is extremely small.
As the linewidth of the right angle line increases, the impedance will decrease, and a certain signal reflection phenomenon will occur. We can calculate the equivalent impedance after the linewidth 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, as can be seen from the figure below, the impedance of the transmission line changes to the minimum within a long time of the w / 2 line, and then returns to the normal impedance after the w / 2 time. The whole time of impedance change is very short, often within 10ps. Such a fast and small change is almost negligible for general signal transmission.
Many people have this understanding of right angle routing and think that the tip is easy to transmit or receive electromagnetic waves and generate EMI, which has also become one of the reasons why many people think that right angle routing cannot be used. However, the results of many practical tests show that right angle routing does not produce obvious EMI than straight line. Perhaps the current instrument performance and test level restrict the accuracy of the test, but at least it shows a problem that the radiation of right angle routing is less than the measurement error of the instrument itself.
Generally speaking, the right angle routing is not as terrible as expected. At least in applications below GHz, any effects such as capacitance, reflection and EMI can hardly be reflected in TDR test. High speed PCB design engineers should focus on layout, power / ground design, wiring design, vias and other aspects. Of course, although the impact of right angle wiring is not very serious, it does not mean that we can take right angle wiring in the future. Paying attention to details is a necessary basic quality for every excellent engineer. Moreover, with the rapid development of digital circuits, the signal frequency processed by PCB engineers will 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 is more and more widely used in high-speed circuit design. The most critical signal in the circuit often adopts differential structure design. What makes it so popular? How can we ensure its good performance in PCB design? With these two questions, we will discuss the next part.
What is differential signal? Generally speaking, the driver sends two equivalent and inverted signals, and the receiver judges whether the logic state is "0" or "1" by comparing the difference between the two voltages. The pair of routing lines carrying differential signals is called differential routing.
Compared with ordinary single ended signal routing, the most obvious advantages of differential signal are reflected in the following three aspects:
a. The anti-interference ability is strong, because the coupling between the two differential lines is very good. When there is external noise interference, it is almost coupled to the two lines at the same time, and the receiver only cares about the difference between the two signals, so the external common mode noise can be completely offset.
b. It can effectively suppress EMI. Similarly, due to the opposite polarity of the two signals, the electromagnetic fields radiated by them can offset each other. The closer the coupling, the less electromagnetic energy released to the outside world.
c. The timing positioning is accurate. Because the switching 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, which can reduce the timing error, and is more suitable for the circuit with low amplitude signal. The current popular LVDS (low voltage differential signaling) refers to this small amplitude differential signal technology.
For PCB engineers, the most concern is how to ensure that these advantages of differential routing can be fully utilized in the actual routing. Perhaps anyone who has been in contact with layout will understand the general requirements of differential routing, that is "equal length and equal distance". Equal length is to ensure that the two differential signals always maintain opposite polarity and reduce the common mode component; Equidistance is mainly to ensure the consistency of differential impedance and reduce reflection“ The principle of "as close as possible" is sometimes one of the requirements of differential routing. But all these rules are not used to copy mechanically. Many engineers seem to not understand the essence of high-speed differential signal transmission. The following focuses on several common misunderstandings in PCB differential signal design.
Myth 1: think that the differential signal does not need the ground plane as the return path, or think that the differential routing provides the return path for each other. The reason for this misunderstanding is that they are confused by the surface phenomenon, or the understanding of the mechanism of high-speed signal transmission is not deep enough. It can be seen from the structure of the receiving end in Fig. 1-8-15 that the emitter currents of transistors Q3 and Q4 are equivalent and reverse, and their currents at the ground just cancel each other (I1 = 0). Therefore, the differential circuit is insensitive to similar ground bombs and other noise signals that may exist on the power supply and ground plane. Partial backflow cancellation in the ground plane does not mean that the differential circuit does not take the reference plane as the signal return path. In fact, in the signal backflow analysis, the mechanism of differential routing is consistent with that of ordinary single ended routing, that is, high-frequency signals always return along the circuit with the smallest inductance. The biggest difference is that the differential line is coupled to the ground, There are also mutual coupling. Whichever coupling is strong will become the main return path. Fig. 1-8-16 is the geomagnetic field distribution diagram of single ended signal and differential signal.
In PCB circuit design, the coupling between differential routing is generally small, often accounting for only 10 ~ 20% of the coupling degree, and more is the coupling to the ground. Therefore, the main return path of differential routing still exists in the ground plane. When the local plane is discontinuous, the coupling between differential routes will provide the main return path in the area without reference plane, as shown in figure 1-8-17. Although the influence of the discontinuity of the reference plane on the differential routing is not as serious as that on the ordinary single ended routing, it will still reduce the quality of the differential signal and increase EMI, which should be avoided as much as possible. Some designers also believe that the reference plane below the differential routing can be removed to suppress some common mode signals in differential transmission, but theoretically this practice is not desirable. How to control the impedance? Failure to provide ground impedance loop for common mode signal is bound to cause EMI radiation, which does more harm than good.
Myth 2: it is more important to keep the equal spacing than the length of the match line. In the actual PCB wiring, it often can not meet the requirements of differential design at the same time. Due to the existence of pin distribution, vias, routing space and other factors, the purpose of wire length matching must be achieved through appropriate winding, but the result must be that some regions of the differential pair cannot be parallel. At this time, how should we choose? Before concluding, let's take a look at the following simulation results.
From the above simulation results, the waveforms of scheme 1 and scheme 2 almost coincide, that is, the influence caused by unequal spacing is minimal. In comparison, the influence of line length mismatch on timing is much greater (scheme 3). From the theoretical analysis, although the inconsistent spacing will lead to the change of differential impedance, because the coupling between differential pairs itself is not significant, the change range of impedance is also very small, usually less than 10%, which is only equivalent to the reflection caused by a via, which will not have a significant impact on signal transmission. Once the line length does not match, in addition to the timing offset, the common mode component is introduced into the differential signal, which reduces the quality of the signal and increases EMI.
It can be said that the most important rule in the design of PCB differential routing is to match the line length. Other rules can be handled flexibly according to the design requirements and practical application.
Myth 3: I think the differential routing must be very close. Let the differential routing close to nothing more than to enhance their coupling, which can not only improve their immunity to noise, but also make full use of the opposite polarity of the magnetic field to offset the electromagnetic interference to the outside world. Although this approach is very beneficial in most cases, it is not absolute. If they can be fully shielded from external interference, we don't need to achieve the purpose of anti-interference and EMI suppression through strong coupling. How can we ensure that the differential routing has good isolation and shielding? Increasing the distance from other signal lines is one of the most basic ways. The electromagnetic field energy decreases with the distance in a square relationship. Generally, when the line spacing exceeds 4 times the linewidth, the interference between them is extremely weak and can be basically ignored. In addition, it can also play a good shielding role through the isolation of the ground plane. This structure is often used in the design of high-frequency (above 10g) IC package PCB. It is called CPW structure, which can ensure strict differential impedance control (2z0), as shown in figure 1-8-19.
Differential routing can also run in different signal layers, but this routing method is generally not recommended, because the differences such as impedance and via generated by different layers will destroy the effect of differential mode transmission and introduce common mode noise. In addition, if the two adjacent layers are not closely coupled, the ability of differential routing to resist noise will be reduced, but crosstalk is not a problem if the appropriate spacing with the surrounding routing can be maintained. At the general frequency (below GHz), EMI will not be a very serious problem. The experiment shows that the radiation energy attenuation of the differential line at a distance of 500mils has reached 60dB beyond 3M, which is enough to meet the electromagnetic radiation standard of FCC. Therefore, the designer does not have to worry too much about the electromagnetic incompatibility caused by insufficient differential line coupling
3. Serpentine line
Serpentine is a kind of routing method often used in layout. Its main purpose is to adjust the delay and meet the requirements of system timing design. Designers should first have this understanding: serpentine wire will destroy signal quality and change transmission delay. It should be avoided as much as possible when wiring. However, in the actual design, in order to ensure that the signal has sufficient holding time or reduce the time offset between the same group of signals, it is often necessary to wind the wire deliberately.
So what effect does the serpentine line have on signal transmission? What should we pay attention to when routing? The two most critical parameters are parallel coupling length (LP) and coupling distance (s), as shown in figure 1-8-21. Obviously, when the signal is transmitted on a serpentine route, the coupling between mutually parallel segments will occur in the form of differential mode. The smaller s and the larger LP, the greater the coupling degree. It may reduce the transmission delay and greatly reduce the signal quality due to crosstalk. The mechanism can refer to the analysis of common mode and differential mode crosstalk in Chapter 3.
Here are some suggestions for layout engineers when dealing with serpentine lines:
1. Try to increase the distance (s) of parallel line segments, at least greater than 3H. H refers to the distance from the signal routing to the reference plane. Generally speaking, it is to walk around a large bend. As long as s is large enough, mutual coupling effect can be almost completely avoided.
2. Reduce the coupling length LP. When the double LP delay approaches or exceeds the signal rise time, the crosstalk will reach saturation.
3. The signal transmission delay caused by the serpentine of strip line or embedded micro strip is less than that of micro strip. Theoretically, the transmission rate of stripline will not be affected by differential mode crosstalk.
4. For high-speed and signal lines with strict timing requirements, try not to take serpentine lines, especially winding lines in a small range.
5. Serpentine routing at any angle can be often used, such as C structure in figure 1-8-20, which can effectively reduce mutual coupling.
6. In the high-speed PCB design, the serpentine line has no so-called filtering or anti-interference ability, which can only reduce the signal quality, so it is only used for timing matching without other purposes.
7. Sometimes spiral routing can be considered for winding, and the simulation shows that its effect is better than normal snake routing.
