Layout is one of the most basic job skills for PCB design engineers. The quality of the wiring will directly affect the performance of the entire system, and most high-speed design theories will eventually be realized and verified through Layout. It can be seen that wiring is crucial in high-speed PCB design. The following 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 routing, differential routing, and serpentine routing.
1. Right-angle routing
Right-angle routing is generally a situation that is required to be avoided in PCB routing, and it has almost become one of the standards for measuring the quality of wiring. So how much impact will right-angle routing have on signal transmission? In principle, right-angled traces will change the line width of the transmission line and cause impedance discontinuity. In fact, not only right-angle traces, but also sharp-angle traces may cause impedance changes.
The impact of the right-angle trace on the signal is mainly reflected in three aspects:
a. The corner can be equivalent to a capacitive load on the transmission line, slowing down the rise time;
b. Impedance discontinuity will cause signal reflection;
c. EMI generated by the right-angled tip.
The parasitic capacitance brought 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), W refers to the width of the trace (unit: inch), εr refers to the dielectric constant of the medium, and Z0 refers to the characteristic impedance of the transmission line. For example, for a 4Mils 50 ohm transmission line (εr is 4.3), the capacitance 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. It can be seen from the calculation that the capacitive effect brought by the right-angle trace is extremely small.
As the line width of the right-angle trace increases, the impedance at this place will decrease, so a certain signal reflection phenomenon will occur. We can calculate the equivalent impedance after the line width is increased according to the impedance calculation formula mentioned in the transmission line section, and then Calculate the reflection coefficient according to the empirical formula: ρ=(Zs-Z0)/(Zs+Z0).
Generally, the impedance change caused by right-angle traces is between 7% and 20%, so the maximum reflection coefficient is about 0.1. Moreover, it can be seen from the figure below that the impedance of the transmission line changes to the minimum within the time of W/2 line length, and then returns to the normal impedance after W/2 time. The entire time for impedance change is extremely short, often within 10 ps In general, such fast and small changes are almost negligible for general signal transmission.
Many people have such an understanding of right-angle routing, thinking that the tip is easy to emit or receive electromagnetic waves and generate EMI, which has become one of the reasons why many people think that right-angle routing cannot be used. However, the results of many actual tests show that right-angled traces do not produce significantly more EMI than straight-line traces. Perhaps the current instrument performance and test level restrict the accuracy of the test, but at least one problem is explained, the radiation of the right-angle trace is already smaller than the measurement error of the instrument itself.
In general, right-angle traces are not as scary as imagined. At least in applications below GHz, any effects such as capacitance, reflection, EMI, etc. can hardly be reflected in TDR testing. The focus of high-speed PCB design engineers should still be on layout, power/ground design, and wiring design. Vias and other aspects. Of course, although the impact of right-angled wiring is not very serious, it does not mean that we can all walk right-angled lines in the future. Attention to detail is the basic quality that every excellent engineer must have. Moreover, with the rapid development of digital circuits, PCB The frequency of signals processed by engineers will continue to increase. In the field of RF design above 10GHz, these small right angles may become the focus of high-speed problems.
2. Differential wiring
Differential Signal (DifferentialSignal) is more and more widely used in high-speed circuit design. The most critical signal in the circuit often adopts differential structure design. Why is it so popular? How to ensure its good performance in PCB design? With these two questions in mind, we proceed to the next part of the discussion.
What is a differential signal? In layman's terms, the driving end sends two signals of equal value and opposite phase, and the receiving end judges the logic state "0" or "1" by comparing the difference between the two voltages. The pair of traces that carry differential signals is called a differential trace.
Compared with ordinary single-ended signal routing, differential signals have the most obvious advantages in the following three aspects:
a. Strong anti-interference ability, because the coupling between the two differential lines is very good. When there is noise interference from the outside world, they are coupled to the two lines almost at the same time, and the receiving end only cares about the difference between the two signals. So external common mode noise can be completely canceled.
b. It can effectively suppress EMI. For the same reason, because the polarities of the two signals are opposite, the electromagnetic fields radiated by them can cancel each other out. The tighter the coupling, the less electromagnetic energy is released to the outside world.
c. Accurate timing positioning. Because the switching change of the differential signal is located at the intersection of the two signals, unlike ordinary single-ended signals that rely on high and low threshold voltages to judge, it is less affected by the process and temperature, and can reduce timing errors. , but also more suitable for circuits with low amplitude signals. The currently popular LVDS (lowvoltagedifferentTIalsignaling) refers to this small amplitude differential signal technology.
For PCB engineers, the most concerned thing is how to ensure that these advantages of differential routing can be fully utilized in actual routing. Perhaps anyone who has been in touch with Layout will understand the general requirements for differential routing, which is "equal length and equal distance". The equal length is to ensure that the two differential signals keep opposite polarities at all times and reduce the common mode component; the equal distance is mainly to ensure that the differential impedances of the two are consistent and reduce reflections. "As close as possible to the principle" is sometimes one of the requirements for differential routing. But all these rules are not used to apply mechanically, and many engineers seem to still not understand the nature of high-speed differential signal transmission.
The following focuses on several common misunderstandings in PCB differential signal design.
2.1. It is considered that the differential signal does not require a ground plane as a return path, or that the differential traces provide each other with a return path. The reason for this misunderstanding is that you are confused by superficial phenomena, or you don't have a deep enough understanding of the mechanism of high-speed signal transmission. It can be seen from the structure of the receiving end in Figure 1-8-15 that the emitter currents of transistors Q3 and Q4 are equal and opposite, and their currents at the ground just cancel each other (I1=0), so the differential circuit is for It is insensitive to ground bounce and other noise signals that may exist on the power and ground planes. The partial backflow cancellation of the ground plane does not mean that the differential circuit does not use the reference plane as the signal return path. In fact, in the analysis of signal backflow, the mechanism of differential routing and ordinary single-ended routing is consistent, that is, high-frequency signals are always The return flow is carried out along the loop with the smallest inductance. The biggest difference is that in addition to the coupling to the ground, the differential lines also have mutual coupling. Whichever type of coupling is strong will become the main return path.
In PCB circuit design, the coupling between differential traces is generally small, often only accounting for 10~20% of the coupling degree, and more is the coupling to the ground, so the main return path of the differential traces still exists on the ground plane . When the ground plane is discontinuous, the coupling between the differential traces will provide the main return path in the area without reference plane, as shown in Figure 1-8-17. Although the impact of the discontinuity of the reference plane on the differential routing is not as serious as that of 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 think that the reference plane under the differential trace can be removed to suppress part of the common-mode signal in the differential transmission, but this approach is not advisable in theory. How to control the impedance? Failure to provide a ground impedance return path for common-mode signals is bound to cause EMI radiation, which does more harm than good.
2.2. It is considered that maintaining equal spacing is more important than matching line length. In actual PCB layout, it is often impossible to meet the requirements of differential design at the same time. Due to factors such as pin distribution, vias, and wiring space, the purpose of line length matching must be achieved through proper winding, but the result must be that some areas of the differential pair cannot be parallel. What should we do at this time? What is the trade-off? Before drawing conclusions, 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 impact caused by the unequal spacing is negligible. In comparison, the impact of line length mismatch on timing is much greater (Scheme 3). From the perspective of theoretical analysis, although the inconsistent spacing will lead to changes in differential impedance, but because the coupling between differential pairs is not significant, the range of impedance changes is also very small, usually within 10%, which is only equivalent to a transition. The reflection caused by the hole will not have a significant impact on the signal transmission. Once the line lengths do not match, in addition to timing shifts, common-mode components are 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 the matching line length, and other rules can be flexibly handled according to design requirements and actual applications.
2.3. It is believed that the differential wiring must be very close. Making the differential traces close is nothing more than to enhance their coupling, which can not only improve the immunity to noise, but also make full use of the opposite polarity of the magnetic field to cancel the electromagnetic interference to the outside world. Although this approach 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 anti-interference through strong coupling with each other and the purpose of suppressing EMI too. How can we ensure that the differential traces have good isolation and shielding? Increasing the spacing with other signal traces is one of the most basic ways. The energy of the electromagnetic field decreases with the square relationship of the distance. Generally, when the spacing between lines exceeds 4 times the line width, the interference between them is extremely weak, basically Can be ignored. In addition, isolation through the ground plane can also play a very good shielding role. This structure is often used in high-frequency (above 10G) IC package PCB design. It is called CPW structure, which can ensure strict differential impedance. control (2Z0).
Differential traces can also be routed in different signal layers, but this method is generally not recommended, because differences in impedance and vias 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 tightly coupled, the ability of the differential trace to resist noise will be reduced, but if the proper spacing from the surrounding traces can be maintained, crosstalk is not a problem. At general frequencies (below GHz), EMI will not be a serious problem. Experiments show that the radiated energy attenuation of 3 meters away from differential traces 500 Mils has reached 60dB, which is enough to meet the FCC's electromagnetic radiation standards, so Designers don't have to worry too much about electromagnetic incompatibility caused by insufficient differential line coupling.
3. Serpentine line
Serpentine lines are a type of wiring method often used in Layout. Its main purpose is to adjust the delay to meet the system timing design requirements. Designers must first understand that serpentine lines will destroy signal quality and change transmission delay, and should be avoided as much as possible when wiring. However, in actual design, in order to ensure that the signals have sufficient hold time, or to reduce the time offset between the same group of signals, it is often necessary to deliberately perform winding.
So, what effect does the serpentine line have on signal transmission? What should be paid attention to when wiring? The two most critical parameters are the parallel coupling length (Lp) and the coupling distance (S). Obviously, when the signal is transmitted on the serpentine line, there will be coupling between the parallel line segments, which is in the form of differential mode. The smaller S and the larger Lp, the greater the coupling degree. It may lead to a decrease in transmission delay and greatly reduce the quality of the signal due to crosstalk. For the mechanism, please 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:
3.1. Try to increase the distance (S) of the parallel line segment, at least greater than 3H, and H refers to the distance from the signal trace to the reference plane. In layman's terms, it means routing around a large bend. As long as S is large enough, mutual coupling effects can be almost completely avoided.
3.2. Reduce the coupling length Lp. When the delay of twice Lp approaches or exceeds the signal rise time, the generated crosstalk will reach saturation.
3.3. The signal transmission delay caused by the serpentine line of Strip-Line or Embedded Micro-strip is smaller than that of Micro-strip. Theoretically, the stripline will not affect the transmission rate due to differential mode crosstalk.
3.4. For signal lines with high speed and strict timing requirements, try not to use serpentine lines, especially in a small area.
3.5. Serpentine wiring at any angle can often be used, such as the C structure in Figure 1-8-20, which can effectively reduce mutual coupling.
3.6. In 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 and has no other purpose.
3.7. Sometimes spiral routing can be considered for winding. Simulation shows that its effect is better than normal serpentine routing.
English
Chinese