With the continuous development of semiconductor technology and deep-pressure micron technology, the switching speed of IC has increased from tens of MHZ to hundreds of MHz, or even several GHz. In the design of high-speed
PCB board, engineers often encounter signal integrity problems such as false triggering, damped oscillation, overshoot, undershoot, and crosstalk. This article will discuss why they are formed, how they are calculated, and how to solve these problems using the IBIS simulation method in Allegro.
1. Definition of Signal Integrity
Signal Integrity (SI) refers to the signal quality on the signal line. Poor signal integrity is not caused by a single factor, but by a combination of factors in board-level design. Causes of damage to signal integrity include reflections, ringing, ground bounce, crosstalk, and more. With the continuous improvement of signal operating frequency, the problem of signal integrity has become the focus of high-speed PCB board engineers.
2. Reflection
2.1 Formation and calculation of reflections
Impedance discontinuities on the transmission line can cause signal reflections, and when the source and load impedances do not match, the load reflects a portion of the voltage back to the source. Differential line transmission of signals solves many problems. 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 logical state "0" or "1" by comparing the difference between the two voltages. The pair of wires that carry the differential signal is called a differential wire. How to calculate the impedance of the differential line? The impedance of various differential signals is different, such as the D+ D- of USB, the differential line impedance is 90ohm, the differential line of 1394 is 110ohm, first look at the specification or related information. There are already many tools for calculating impedance, such as polar's si9000. Factors that affect differential impedance include line width, differential line spacing, dielectric permittivity, and thickness of the medium (the thickness of the medium between the differential line and the reference surface). Generally, the difference is adjusted. Line spacing and line width to control differential impedance. When making the board, it is also necessary to explain to the manufacturer which lines need to control the 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. In some systems, system 'ground' is used as the voltage reference point. When 'ground' is used as a voltage measurement reference, this signal scheme is called single-ended. We use this term because a signal is represented by the voltage on a single conductor.
One benefit of differential signaling is that small signals can be easily identified because you are controlling the 'reference' voltage. In a system with a ground referenced, single-ended signal scheme, the value of the measured signal depends on the consistency of 'ground' within the system. The farther the signal source and the signal receiver are, the more likely it is that there will be differences between their local ground voltage values. The signal value recovered from the differential signal is largely independent of the value of 'ground', but within a certain range. A second benefit of differential signaling is that it is highly immune to external electromagnetic interference (EMI). An aggressor affects each end of a differential signal pair almost equally. Since the difference in the PADSLOGIC voltage in the PADS determines the signal value, any identical disturbances that appear on the two conductors will be ignored. In addition to being less sensitive to interference, differential signals generate less EMI than single-ended signals. The third advantage of differential signals is timing positioning. Since the switching changes of differential signals are located at the intersection of two signals, unlike ordinary single-ended signals that rely on high and low threshold voltages for judgment, they are less affected by process and temperature. It can reduce timing errors 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 can not consider crosstalk, because their crosstalk results will cancel when they are received. In addition, the differential should be balanced, and the parallel is only part of the balance. I think the coupling of the differential pair should still be needed, for single-line matching, although It is very mature in theory, but the actual PCB circuit still has an error of about 5% (a material, I have not done it myself). On the other hand, a differential line can be seen as a self-looping system, or the signals on its two signal lines are correlated. If the coupling is too loose, it may cause different interference from other places. For some interface circuits, the equal length of the Allegro training differential pair is an important factor in controlling the line delay. So, I think the differential line should be tightly coupled. For most of the current high-speed PCB boards, it is beneficial to maintain good coupling, but I hope you do not mistakenly think that coupling is a necessary condition for differential pairs, which sometimes limits the design ideas. When doing high-speed design or analysis, it is not only necessary to know how most people do it, but also why others do it, and then understand and improve on the basis of other people's experience, and constantly exercise their creative thinking ability. Matching is required, 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, if the series resistance is used, it will have no effect, but if the grounding or power-connected termination matching method is used, due to Cross-winding is reduced because the line impedance of the two wires is reduced...
For PCB LAYOUT engineers, the concern is how to ensure that these advantages of differential routing can be fully utilized in actual routing. Maybe anyone who has been in contact with Layout will understand the general requirements of differential routing, and the PCB board design is "equal length, equal distance". Equal length is to ensure that the two differential signals maintain opposite polarities at all times and reduce common mode components; equal distance is mainly to ensure that the differential impedance of the two is consistent and reduce reflection. The "as close as possible principle" is sometimes one of the requirements for differential routing. Differential traces can also be run in different signal layers, but this method is generally not recommended, because the 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, it will reduce the ability of the differential trace to resist noise, 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 at a distance of 500Mils from a differential trace 3 meters away has reached 60dB, which is enough to meet the FCC's electromagnetic radiation standards, so The designer does not have to worry too much about the electromagnetic incompatibility caused by insufficient differential line coupling. But all of these rules are not meant to be rhetorical, and many engineers don't seem to understand the nature of high-speed differential signaling. The following focuses on discussing several common misunderstandings in PCB differential signal design. It is believed that the differential traces must be very close. Keeping the differential traces close is nothing more than to enhance their coupling, which can not only improve immunity to noise, but also make full use of the opposite polarity of the magnetic field to offset electromagnetic interference to the outside world. Although this approach is very beneficial in most cases, it is not. If we can ensure that they are fully shielded from external interference, then we do not need to achieve anti-interference and anti-interference through strong coupling with each other. the purpose of suppressing EMI. How can we ensure that the differential traces have good isolation and shielding? Increasing the spacing with other signal traces is one of the basic ways. The energy of the electromagnetic field decreases with the square of the distance. Generally, the line spacing exceeds 4 times the line width. , the interference between them is extremely weak and can be basically ignored. In addition, the isolation of the ground plane can also play a good shielding role. This structure is often used in the design of high-frequency (above 10G) IC package PCB boards. It is called the CPW structure, which can ensure strict differential Impedance Control (2Z0). Think that differential signals do not need a ground plane as a return path, or think that differential traces provide return paths for each other. This misunderstanding is caused by being confused by superficial phenomena, or the understanding of the mechanism of high-speed signal transmission is not deep enough. Differential circuits are insensitive to similar bounces and other noise signals that may be present on the power and ground planes. The partial return 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 return, the mechanism of differential routing and ordinary single-ended routing is the same, that is, high-frequency signals are always Returning along the loop of the inductor, the difference is that in addition to the coupling to the ground, the differential line also has mutual coupling. Whichever coupling is strong, that one becomes the main return path. On the PCB board In 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 trace still exists in the ground plane. When the ground plane is discontinuous, in the area without reference plane, the coupling between the differential traces will provide the main return path, although the discontinuity of the reference plane has no effect on the differential traces. It is serious, but it will still reduce the quality of differential signals and increase EMI, which should be avoided as much as possible. Some designers also think that the reference plane under the differential traces can be removed to suppress part of the common-mode signal in differential transmission, but this approach is not desirable in theory. How to control the impedance? Do not provide the common-mode signal. The ground impedance loop is bound to cause EMI radiation, which does more harm than good. Think maintaining equal spacing is more important than matching line lengths. In the actual PCB board layout, the requirements of differential design are often not met at the same time. Due to factors such as pin distribution, vias, and routing space, the purpose of matching the line length must be achieved through appropriate routing, but the result must be that some areas of the differential pair cannot be parallel.
PCB board differential routing The most important rule in the design is to match the line length, and other rules can be flexibly processed according to the design requirements and practical applications.
