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1. Right-angle cabling
Right-angle wiring is generally required to avoid the situation in PCB board 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 routing on signals is mainly reflected in three aspects.
2. Differential cabling
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 Differential Signaling) is a popular small amplitude differential signal technology.
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. Ground plane offset part of the return does not represent a differential circuit is not returned as reference plane as a signal path, actually on the signal flow analysis, differential line and common single-ended walk line is consistent, the mechanism of the high frequency signal is always along the circuit of the minimum inductance for reflow, the biggest difference lies in the difference line besides there are coupled to the ground, there is coupling between each other, Whichever is strongly coupled 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.
Misconception 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? 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 3: 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 play a good shielding effect, this structure is often used in high-frequency (above 10G)IC package PCB design, known as CPW structure, can ensure strict differential impedance control (2Z0).
3. The 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. Increase the distance (S) of the parallel line segment as far as possible. H refers to the distance between the signal line and 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. Reduce the coupling length Lp. When the delay of Lp twice approaches or exceeds the signal rise time, the crosstalk generated will reach saturation.
3. The signal transmission delay caused by the snake-like wire of a strip-line or Embedded micro-strip is smaller than that of a 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 run snakelike lines, especially in small areas.
5. Can often use any Angle of the serpentine, 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 routing can be considered for winding. Simulation shows that its effect is better than normal serpentine routing.
