Layout is one of the most basic work skills of PCB design engineer. The quality of wiring will directly affect the performance of the whole system, most of the high-speed design theory must be finally realized and verified by Layout, so it can be seen that wiring is crucial in high-speed PCB design. The following will be in view of the actual wiring may encounter some situations, analysis of its rationality, and give some more optimized routing strategy. Mainly from the right Angle line, difference line, snake line and so on three aspects to elaborate.

1. Rectangular go line

Right-angle wiring is generally required to avoid the situation in PCB wiring, and has almost become one of the standards to measure the quality of wiring, so how much impact will right-angle wiring have on signal transmission? In principle, right-angle wiring will change the line width of the transmission line, resulting in impedance discontinuity. In fact, not only right Angle line, ton Angle, acute Angle line may cause impedance changes.

The influence of right-angle alignment on signal is mainly reflected in three aspects: first, the corner can be equivalent to the capacitive load on the transmission line, slowing down the rise time; Second, impedance discontinuity will cause signal reflection; Third, 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)1/2/Z0

In the above formula, C refers to the equivalent capacitance at the corner (pF), W refers to the width of the line (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 (εr 4.3), the capacitance of a right Angle is about 0.0101pF, and the rise time variation 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 capacitance effect brought by right-angle wiring is extremely small.

As the line width of right-angle line increases, the impedance at this point will decrease, so there will be a certain signal reflection phenomenon. We can calculate the equivalent impedance after the line width increases according to the impedance calculation formula mentioned in the section of transmission lines, and then calculate the reflection coefficient according to the empirical formula: ρ=(Zs-Z0)/(Zs+Z0), the general right-angle wiring resulting in impedance changes between 7%-20%, so the maximum reflection coefficient is about 0.1. Moreover, as can be seen from the figure below, the transmission line impedance changes to the minimum within the length of W/2 line, and then restores to the normal impedance after W/2 time. The time for the whole impedance change is very short, usually within 10ps. Such a fast and small change is almost negligible for the general signal transmission.

Many people have such an understanding of right-angle routing, believing that the tip is easy to emit or receive electromagnetic waves and produce EMI, which has become one of the reasons why many people think right-angle routing is not possible. However, many practical test results show that right-angle line does not produce much EMI than straight line. Perhaps the current instrument performance and test level restrict the accuracy of the test, but at least it shows that the radiation of right-angle line is less than the measurement error of the instrument itself. In general, right-angle alignment is not as terrible as it might seem. At least in applications below GHz, any effects such as capacitance, reflection, EMI, etc. are almost not reflected in TDR tests. The design engineer of high-speed PCB should focus on layout, power/ground design, wiring design, perforation, etc. Although, of course, the effects of rectangular go line is not very serious, but is not to say that we can walk right Angle line, attention to detail is the essential quality for every good engineers, and, with the rapid development of digital circuits, PCB engineers processing of signal frequency also will continue to improve, to more than 10 GHZ RF design field, These small right angles can become the focus of high-speed problems.

2. Difference of

DifferenTIal Signal is used widely in high-speed circuit design. The most important Signal in a circuit is DifferenTIal Signal design. How to ensure its good performance in PCB design? With these two questions in mind, we move on to the next part of our discussion.

What is a differential signal? In plain English, the driver sends two equivalent and inverting signals, and the receiver compares the difference between the two voltages to determine whether the logical state is “0” or “1”. The pair of wires carrying differential signals is called differential wires.

Compared with ordinary single-ended signal routing, differential signal has the most obvious advantages in the following three aspects:

A. Strong anti-interference ability, because the coupling between two differential lines is very good, when there is noise interference, they are almost coupled to 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 cancelled.

B. It can effectively suppress EMI. Similarly, because two signals are of opposite polarity, the electromagnetic field radiated by them can cancel each other. The closer the coupling is, the less electromagnetic energy released to the outside world.

C. Timing positioning is accurate. Since the switching change of differential signals is located at the intersection of two signals, unlike common single-ended signals which are judged by high and low threshold voltages, it is less affected by process and temperature, which can reduce timing errors and is more suitable for circuits with low amplitude signals. LVDS (low voltage differenTIalsignaling) refers to this small amplitude differential signal technology.

For PCB engineers, the most important concern is how to ensure that these advantages of differential routing can be fully utilized in the actual routing. Perhaps as long as it is in contact with Layout people will understand the general requirements of differential routing, that is “equal length, equal distance”. Isometric is to ensure that the two differential signals always maintain opposite polarity, reduce the common mode component; Isometric is mainly to ensure the same differential impedance, reduce reflection. “As close as possible” is sometimes one of the requirements for differential routing. But none of these rules are meant to be applied mechanically, and many engineers do not seem to understand the nature of high-speed differential signalling. The following focuses on several common mistakes in PCB differential signal design.

Misconception 1: Differential signals do not need ground plane as backflow path, or think that differential lines provide backflow path for each other. The cause of this misunderstanding is confused by the surface phenomenon, or the mechanism of high-speed signal transmission is not deep enough. As can be seen from the structure of the receiving end in FIG. 1-8-15, the emitter currents of transistors Q3 and Q4 are equivalent and opposite, and their current at the junction exactly cancellations each other (I1=0). Therefore, the differential circuit is insensitive to similar ground projectials and other noise signals that may exist in the power supply and ground plane. The partial backflow cancellation of ground plane does not mean that the differential circuit does not take the reference plane as the signal return path. In fact, in signal backflow analysis, the mechanism of differential routing is the same as that of ordinary single-end routing, namely, high

The frequency signal always flows back along the circuit with the smallest inductance. The biggest difference lies in that the difference line not only has coupling to the ground, but also has coupling between each other. The strong coupling becomes the main backflow path.

In PCB circuit design, the coupling between differential wiring is generally small, usually accounting for only 10~20% of the coupling degree, and most of the coupling is to the ground, so the main backflow path of differential wiring still exists in the ground plane. In the case of discontinuity in the local plane, the coupling between differential routes provides the main backflow path in the region without reference plane, as shown in FIG. 1-8-17. Although the impact of the discontinuity of the reference plane on differential wiring is not as serious as that of ordinary single-end wiring, it will still reduce the quality of differential signal and increase EMI, which should be avoided as far as possible. Some designers believe that the reference plane of the line of differential transmission can be removed to suppress part of the common mode signal in differential transmission, but theoretically this approach is not desirable. How to control the impedance? Without providing ground impedance loop for common-mode signal, EMI radiation is bound to be caused, which does more harm than good.

Myth 2: Maintaining equal spacing is more important than matching line length. In the actual PCB wiring, it is often unable to meet the requirements of differential design. Due to the distribution of pins, holes, and wiring space and other factors, it is necessary to achieve the purpose of line length matching through appropriate winding, but the result is inevitably part of the difference pair cannot be parallel, at this time, how to choose? Before we jump to conclusions, let’s take a look at the following simulation results. It can be seen from the above simulation results that waveforms of scheme 1 and Scheme 2 almost coincide, that is to say, the influence of unequal spacing is minimal, and the influence of line length mismatch is much greater on timing sequence (Scheme 3). From the perspective of theoretical analysis, although the inconsistent spacing will lead to the difference impedance changes, but because the coupling between the difference pair itself is not significant, so the range of impedance changes is also very small, usually within 10%, only equivalent to a reflection caused by a hole, which will not cause significant impact on signal transmission. Once the line length is mismatched, in addition to time sequence offset, common mode components are introduced into the differential signal, which reduces signal quality and increases EMI.

It can be said that the most important rule in PCB differential wiring design is to match the line length, and other rules can be flexibly handled according to the design requirements and practical applications.

Misconception three: think difference line must rely on very close. The point of keeping the difference lines close together is nothing more than to increase their coupling, both to improve their immunity to noise and to take advantage of the opposite polarity of the magnetic field to cancel out electromagnetic interference from the outside world. Although this approach is very favorable in most cases, it is not absolute. If they can be fully shielded from external interference, then we do not need to achieve the purpose of anti-interference and EMI suppression through strong coupling with each other any more. How to ensure that differential routing has good isolation and shielding? Increasing the distance between the lines and other signals is one of the most basic ways. The energy of electromagnetic field decreases with the square relation of the distance. Generally, when the distance between the lines is more than 4 times the line width, the interference between them is extremely weak and can be ignored basically. In addition, the isolation through the ground plane can also provide a good shielding effect. This structure is often used in high-frequency (above 10G) IC packaged PCB designs, known as the CPW structure, to ensure strict differential impedance control (2Z0), FIG. 1-8-19.

Differential routing can also be carried out in different signal layers, but this is generally not recommended, because differences such as impedance and through holes in different layers can destroy the differential mode transmission effect and introduce common mode noise. In addition, if the two adjacent layers are not tightly coupled, the ability of differential routing to resist noise will be reduced, but crosstalk is not a problem if proper spacing is maintained with the surrounding routing. In general frequency (below GHz), EMI will not be a serious problem. Experiments show that the radiation energy attenuation of differential lines with a distance of 500Mils beyond 3 meters has reached 60dB, which is enough to meet the ELECTROMAGNETIC radiation standard of FCC. Therefore, designers do not need to worry too much about electromagnetic incompatibility caused by insufficient coupling of differential lines.

3. serpentine

A serpentine line is often used in Layout. Its main purpose is to adjust the time delay and meet the requirements of system timing design. Designers should first understand that serpentine wire will destroy signal quality, change transmission delay, and should be avoided when wiring. However, in practical design, in order to ensure sufficient hold time of signals, or to reduce time offset between the same group of signals, winding has to be deliberately carried out.

So what does the serpentine do to signal transmission? What should I pay attention to when walking the line? The two most critical parameters are parallel coupling length (Lp) and coupling distance (S), as shown in FIG. 1-8-21. Obviously, when the signal is transmitted in serpentine line, there will be coupling between parallel line segments in the form of difference mode. The smaller S is, the larger Lp is, and the greater the coupling degree will be. This may result in reduced transmission delays and a significant reduction in signal quality due to crosstalk, as described in chapter 3 for the analysis of common mode and differential mode crosstalk.

Here are some tips for Layout engineers when dealing with serpentines:

1. Try to increase the distance (S) of the parallel line segment, which is at least greater than 3H. H refers to the distance from the signal line to the reference plane. Generally speaking, it is to take a big curve. As long as S is large enough, the coupling effect can be almost completely avoided.

2. When the coupling length Lp is reduced, the crosstalk generated will reach saturation when the delay of Lp twice approaches or exceeds the signal rise time.

3. The signal transmission delay caused by the snake-like Line of strip-line or Embedded micro-strip is smaller than that of micro-strip. Theoretically, the ribbon line does not affect the transmission rate because of differential mode crosstalk.

4. For high-speed and signal lines with strict requirements on timing, try not to walk serpentine lines, especially in a small area.

5. The serpentine routing at any Angle can be often adopted. The C structure in FIG. 1-8-20 can effectively reduce the coupling between each other.

6. In high-speed PCB design, serpentine has no so-called filtering or anti-interference ability, and can only reduce signal quality, so it is only used for timing matching and no other purpose.

7. Sometimes spiral winding can be considered. Simulation shows that its effect is better than normal serpentine winding.