Reasons and Methods of Signal Integrity Problems in High Speed PCB Design

Reasons and Methods of Signal Integrity Problems in High Speed PCB Design
p> With the continuous development of transistor technology and deep pressure micrometer technology, the switching speed of integrated circuits has increased from tens of megahertz to hundreds of megahertz, even several GHz In high-speed PCB design, engineers often encounter signal integrity problems, such as error triggering, damping oscillation, overshoot, undershoot, and crosstalk This paper will discuss the causes of their formation, how they are calculated, and how to use IBIS simulation method in Allegro to solve these problems
1.. Definition of Signal Integrity
Signal Integrity (SI) refers to the signal quality on the signal line.  The poor signal integrity is not caused by a single factor, but in the design at the board level, the combination of these factors The causes of signal integrity damage include reflection, ringing, ground bounce, crosstalk, and more With the continuous improvement of signal operating frequency, signal integrity has become the focus of rapid development of PCB engineers
2.. Reflection
2.1 Formation and calculation of reflections
Impedance discontinuities on the transmission line can cause signal reflections Differential transmission of signals solves many problems What is differential signal? In layman's terms, the driver sends two signals of equal and opposite phases, and the receiver judges the logic state "0" or "1" by comparing the difference between the two voltages A pair of wires that transmit differential signals are called differential wires How to calculate the impedance of differential line? The impedance of various differential signals is different. Such as the D+D - of USB, the differential line impedance is 90 ohms, and the differential line impedance of 1394 is 110 ohms. Check the specifications or relevant information first There are many tools for calculating impedance, such as polar's si9000. Factors affecting differential impedance include line width, differential line spacing, dielectric constant, and thickness of the medium (the thickness of the medium between the differential line and reference surface) Normally, the difference has been adjusted Line spacing and line width used to control differential impedance When making a circuit board, you also need to explain to the manufacturer which circuits need to control impedance A differential signal is a numerical value that represents the difference between two physical quantities Strictly speaking, all voltage signals are differential, because one voltage can only be relative to another voltage In some systems, the system "ground" is used as a voltage reference point When "ground" is used as the voltage measurement reference, this signal scheme is called single ended We use this term because the signal is represented by the voltage on a single conductor

One advantage of differential signals is that small signals can be easily identified because you are controlling the "reference" voltage In a ground reference system, the value of the measured signal in the single ended signal scheme depends on the consistency of the "grounding" in the system The farther away the signal source and the signal receiver are, the greater the possibility of differences between their local ground voltage values The signal value recovered from the differential signal is largely independent of the "ground" value, but within a certain range A second benefit of differential signaling is that it is highly immune to external electromagnetic interference (EMI).  The aggressors affect each end of the differential signal pair almost equally Since the pad logic voltage difference in the pad determines the signal value, any identical interference on the two wires will be ignored In addition to being less sensitive to interference, differential signals produce less EMI than single ended signals The third advantage of differential signals is timing positioning Because the switching change of differential signal is located at the intersection of two signals, it is different from the ordinary single ended signal which depends on the high and low threshold voltage for judgment. They are less affected by the process and temperature It can reduce timing error and is more suitable for circuits with low amplitude signals The currently popular LVDS (low voltage differential signaling) refers to this small-amplitude differential signaling technology.  Differential cannot consider crosstalk because their crosstalk results will be cancelled at reception In addition, the difference should be balanced, and parallelism is only part of the balance I think the actual PCB circuit still has an error of about 5% (a material, I have not done it myself) On the other hand, the differential circuit can be regarded as a self loop system, or the signals on its two signal lines are correlated If the coupling is too loose, it may cause interference different from other places For some interface circuits, the equal length of Allegro training differential pairs is an important factor in controlling line delay Therefore, I think the differential line should be closely coupled For most current high-speed PCB boards, it is beneficial to maintain good coupling, but I hope you do not mistakenly believe that coupling is a necessary condition for differential pairs, which sometimes limits design ideas When conducting high-speed design or analysis, it is not only necessary to know how most people do it, but why other people do it, and then understand and improve on the basis of other people's experience to constantly exercise their creative thinking ability Matching is necessary, but the reason for matching is not reflection, but to reduce the degree of cross winding interference If the reduction is related to the matching method, it will not have any effect if the series resistance is used, but if the grounding or power connection terminal matching method is used, the line impedance of the two conductors will be reduced due to the cross winding
PCB layout engineer, the problem is how to ensure that these advantages of differential routing are fully utilized in actual routing Maybe anyone who has been exposed to the layout will understand the general requirements of the difference distribution line, and the PCB design is "equal length" and "equal distance" The equal length is to ensure that the two differential signals always maintain opposite polarity and reduce the common mode component; Equal distance is mainly used to ensure that the differential impedance of the two is consistent and reduce reflection The principle of "as close as possible" is sometimes one of the requirements of differential routing The differential trace can also operate in different signal layers, but this method is generally not recommended, because the difference of impedance and through-hole 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, this will reduce the noise immunity of the differential trajectory. However, if the proper spacing with the surrounding traces can be maintained, crosstalk is not a problem At general frequencies (below GHz), EMI will not be a serious problem The experiment shows that the radiation energy attenuation reaches 60dB at 500Mils 3 meters away from the differential track, which is sufficient to meet the electromagnetic radiation standard of FCC. In this regard, designers need not worry too much about the electromagnetic incompatibility caused by insufficient coupling of differential lines But all these rules are not for rhetoric, and many engineers do not seem to understand the nature of high-speed differential signals The following focuses on several common misconceptions in PCB differential signal design It is believed that the differential trajectories must be very close Keeping the difference trajectories close is only 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 counteract the electromagnetic interference to the outside world Although this approach is very useful in most cases, it is not If we can ensure that they completely mask external interference, then we do not need to achieve anti-interference and anti-jamming through strong coupling between them Purpose of EMI suppression How can we ensure that the differential trace has good isolation and mask? One of the basic methods is to increase the distance from other signal traces The energy of the electromagnetic field decreases with the square of the distance Generally, the line spacing is more than 4 times of the line width, The interference between them is very weak and can be basically ignored In addition, the isolation of the ground plane can also play a good role in shielding This structure is frequently 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) It is considered that the differential signal does not need the ground plane as the return path, or the differential path provides a return path for each other This misunderstanding is caused by surface phenomena, or the understanding of high-speed signal transmission mechanism is not deep enough The differential circuit is insensitive to similar rebound and other noise signals that may exist on the power and ground planes Partial echo cancellation in the ground plane does not mean that the differential circuit does not use the reference plane as the signal echo path In fact, in the signal return analysis, the mechanism of differential routing is the same as that of common single ended routing. That is, high-frequency signals always return along the loop of the inductor. The difference is that in addition to ground coupling, differential lines also have mutual coupling No matter which coupling is strong, this becomes the main return path In the upper PCB circuit design, the coupling between differential trajectories is usually small, usually accounting for only 10-20% of the coupling degree. More importantly, the coupling with the ground. In addition, the main return path of differential trajectories still exists in the ground plane When the local plane is discontinuous, in the area without reference plane, the coupling between differential trajectories will provide the main loop, although the discontinuity of the reference plane has no effect on the differential trajectories This is serious, but it will still reduce the quality of differential signal and add EMI, which should be avoided as far as possible Some designers also believe that the reference plane under the differential trace can be removed to suppress some common mode signals in differential transmission, but this method is not desirable in theory How to control impedance? No common mode signal is provided The grounding impedance loop will inevitably cause EMI radiation, which will do more harm than good Do you think it is more important to keep equal spacing than to match the line length In fact, the requirements of PCB layout and differential design cannot be met at the same time Due to factors such as pin distribution, through-hole, and routing space, the purpose of matching the line length must be achieved through proper wiring, but the result is that some areas of the differential pair cannot be parallel PCB difference distribution line: the most important rule in design is to match the line length, and other rules can be flexibly handled according to design requirements and practical applications