Layout is one of the most basic work skills of PCB design engineer. The quality of routing will directly affect the performance of the whole system, and most of the high-speed design theory will be realized and verified by layout. Therefore, wiring is very important in high-speed PCB design. Next, we will analyze the rationality of some situations that may be encountered in the actual wiring, and give some more optimized routing strategies. Mainly from the right angle line, differential line, snake line three aspects to elaborate.
Right angle wiring is generally 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 wiring have on signal transmission? In principle, right angle routing will change the line width of transmission line, resulting in the discontinuity of impedance. In fact, not only right angle wiring, but also sharp angle wiring 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 of the transmission line to slow down the rise time; Second, the discontinuous impedance will cause the reflection of the signal; The third is EMI produced by 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 is the equivalent capacitance of the corner (unit: PF), W is the width of the wire (unit: inch), ε R is 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 3), the electric capacity brought by a right angle is about 0. 0101pf, and then the rise time variation 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 wiring is extremely small.
As the line width of right angle transmission line increases, the impedance at this point 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 chapter of transmission line, 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% and 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 in a long time of W / 2 line, and then returns to the normal impedance after 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 such an understanding of right angle wiring. 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 is not possible. However, many practical test results show that the right angle routing does not produce obvious EMI than the straight line. Perhaps the current performance of the instrument and the test level restrict the accuracy of the test, but at least it shows a problem that the radiation of the right angle line is less than the measurement error of the instrument itself.
On the whole, right angle routing is not as terrible as imagined. At least in the applications below GHz, any effect such as capacitance, reflection, EMI can hardly be reflected in TDR test. High speed PCB design engineers should focus on layout, power / ground design, wiring design, via 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 of every excellent engineer. Moreover, with the rapid development of digital circuits, the signal frequency processed by PCB engineers will continue to increase, reaching 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 is often designed with differential structure. What makes it so popular? How to ensure its good performance in PCB design? With these two questions in mind, let's move on to the next part of the discussion.
What is differential signal? Generally speaking, the driver sends two equivalent and inverse 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 as follows:
a. Strong anti-interference ability, because the coupling between the two differential lines is very good, when there is noise interference, they are 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. In the same way, due to the opposite polarity of two signals, their external electromagnetic fields can cancel each other. The closer the coupling is, the less electromagnetic energy is released to the outside world.
c. Because the switch change of differential signal is located at the intersection of two signals, unlike ordinary single ended signal, which depends on high and low threshold voltage judgment, it is less affected by process and temperature, and can reduce the timing error. At the same time, it is more suitable for low amplitude signal circuit. The current popular LVDS (low voltage differential signaling) refers to this kind of small amplitude differential signal technology.
For PCB engineers, the most important thing 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 keep opposite polarity at all times and reduce the common mode component; Equidistant is mainly to ensure the consistency of differential impedance and reduce reflection“ "As close as possible to the principle" is sometimes one of the requirements of differential routing. But all these rules are not used to copy mechanically. Many engineers do not seem to understand the essence of high-speed differential signal transmission. The following focuses on several common mistakes in PCB differential signal design.
Error 1: the differential signal does not need the ground plane as the return path, or the differential routing provides the return path for each other. The reason for this misunderstanding is that we are confused by the surface phenomenon, or 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. The partial return cancellation of 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 analysis of the signal return, the mechanism of the differential circuit is the same as that of the ordinary single ended circuit, that is, the high frequency signal always flows back along the circuit with the smallest inductance. The biggest difference is that the differential circuit has coupling to the ground, There is also mutual coupling. Which coupling is strong will become the main return channel. Fig. 1-8-16 shows the geomagnetic field distribution of single ended signal and differential signal.
In PCB circuit design, the coupling between differential routing is generally small, which only accounts for 10 ~ 20% of the coupling, and more is the coupling to the ground, so the main return path of differential routing still exists in the ground plane. When the local plane is discontinuous, in the region without reference plane, the coupling between differential routing will provide the main return path, as shown in figure 1-8-17. Although the influence of discontinuity of reference plane on differential routing is not as serious as that on common single ended routing, it will reduce the quality of differential signal and increase EMI, which should be avoided as far as possible. Some designers think that the reference plane below the differential transmission line can be removed to suppress part of the common mode signal in differential transmission, but this method is not desirable in theory. How to control the impedance? If the common mode signal is not provided with ground impedance circuit, EMI radiation will be caused, which has more disadvantages than advantages.
Mistake 2: it is more important to keep the equal distance than to keep the match line long. In the actual PCB wiring, it often can not meet the requirements of differential design at the same time. Due to the pin distribution, vias, and routing space, the wire length matching can only be achieved through proper winding, but the result is that some regions of the differential pair cannot be parallel. How do we choose? Before we come to a conclusion, let's take a look at the following simulation results.
From the above simulation results, the waveforms of scheme 1 and scheme 2 are almost coincident, that is to say, the influence caused by the unequal spacing is negligible. In comparison, the influence of line length mismatch on timing is much greater (scheme 3). According to the theoretical analysis, although the gap inconsistency will lead to the change of differential impedance, because the coupling between differential pairs itself is not significant, the impedance change range is also very small, usually less than 10%, which is only equivalent to the reflection caused by a via, which will not have an obvious impact on signal transmission. Once the line length does not match, in addition to timing offset, the differential signal introduces the common mode component, which reduces the signal quality 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.
Error 3: think that differential routing must rely on very close. It is no more than to enhance their coupling to make the differential routing close, which can not only improve the 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 method is very beneficial in most cases, it is not absolute. If we can ensure that they are fully shielded from external interference, then we do not need to achieve the purpose of anti-interference and EMI suppression through mutual strong coupling. How to ensure that the differential routing has good isolation and shielding? One of the most basic ways is to increase the distance between lines and other signals. The electromagnetic field energy decreases with the square of the distance. Generally, when the distance between lines exceeds 4 times the line width, the interference between them is very weak and can be ignored. In addition, through the isolation of the ground plane, it can also play a good shielding role. This structure is often used in high-frequency (above 10g) IC packaging PCB design, known as 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 it is generally not recommended, because the difference of impedance and via caused by different layers will destroy the effect of differential mode transmission and introduce common mode noise. In addition, if the coupling between two adjacent layers is not close enough, the ability of differential routing to resist noise will be reduced. However, crosstalk is not a problem if the proper spacing between the two adjacent layers and the surrounding routing can be maintained. In general frequency (below GHz), EMI will not be a serious problem. The experiment shows that the radiation energy attenuation of 500 mils differential transmission line beyond 3 meters has reached 60 dB, which is enough to meet the FCC electromagnetic radiation standard. Therefore, designers need not worry too much about the electromagnetic incompatibility caused by insufficient coupling of differential transmission line
3. Serpentine
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 such understanding: serpentine will damage the signal quality, change the transmission delay, and avoid using it when wiring. But in the actual design, in order to ensure that the signal has enough holding time, or reduce the time offset between the same group of signals, it is often necessary to wind the wire intentionally.
So, what is the effect of Serpentine on signal transmission? What should we pay attention to when routing? The two key parameters are parallel coupling length (LP) and coupling distance (s), as shown in figure 1-8-21. Obviously, when the signal is transmitted on the serpentine line, the coupling between the parallel lines will take place in the form of differential mode. The smaller the S is, the greater the LP is, the greater the coupling degree is. It may lead to the decrease of transmission delay and the decrease of 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 line to the reference plane. Generally speaking, as long as s is large enough, the coupling effect can be almost completely avoided.
2. By reducing the coupling length LP, the crosstalk will reach saturation when the double LP delay approaches or exceeds the signal rise time.
3. The signal transmission delay caused by serpentine of strip line or embedded micro strip is less than that caused by micro strip. Theoretically, the transmission rate of stripline will not be affected by differential mode crosstalk.
4. For high-speed signal lines and signal lines with strict timing requirements, try not to take a serpentine line, especially in a small range.
5. Serpentine routing at any angle can be often used, such as the C structure in figure 1-8-20, which can effectively reduce the coupling between each other.
6. In the design of high-speed PCB, serpentine 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 we can consider the way of spiral winding. The simulation shows that the effect is better than the normal snake winding.
