Signal integrity or SI is a set of measures of the quality of an electrical signal. In digital electronics, a stream of binary values is represented by a voltage (or current) waveform. However, digital signals are fundamentally analog in nature, and all signals are subject to effects such as noise, distortion, and loss. Over short distances and at low bit rates, a simple conductor can transmit this with sufficient fidelity. At high bit rates and over longer distances or through various mediums, various effects can degrade the electrical signal to the point where errors occur and the system or device fails. Signal integrity engineering is the task of analyzing and mitigating these effects. It is an important activity at all levels of electronics packaging and assembly, from internal connections of an integrated circuit (IC), through the package, the printed circuit board (PCB), the backplane, and inter-system connections. While there are some common themes at these various levels, there are also practical considerations, in particular the interconnect flight time versus the bit period, that cause substantial differences in the approach to signal integrity for on-chip connections versus chip-to-chip connections. Some of the main issues of concern for signal integrity are ringing, crosstalk, ground bounce, distortion, signal loss, and power supply noise.
History Signal integrity primarily involves the electrical performance of the wires and other packaging structures used to move signals about within an electronic product. Such performance is a matter of basic physics and as such has remained relatively unchanged since the inception of electronic signaling. The first transatlantic telegraph cable suffered from severe signal integrity problems, and analysis of the problems yielded many of the mathematical tools still used today to analyze signal integrity problems, such as the telegrapher's equations. Products as old as the Western Electric crossbar telephone exchange (circa 1940), based on the wire-spring relay, suffered almost all the effects seen today - the ringing, crosstalk, ground bounce, and power supply noise that plague modern digital products. On printed circuit boards, signal integrity became a serious concern when the transition (rise and fall) times of signals started to become comparable to the propagation time across the board. Very roughly speaking, this typically happens when system speeds exceed a few tens of MHz. At first, only a few of the most important, or highest speed, signals needed detailed analysis or design. As speeds increased, a larger and larger fraction of signals needed SI analysis and design practices. In modern (> 100 MHz) circuit designs, essentially all signals must be designed with SI in mind. For ICs, SI analysis became necessary as an effect of reduced design rules. In the early days of the modern VLSI era, digital chip circuit design and layout were manual processes. The use of abstraction and the application of automatic synthesis techniques have since allowed designers to express their designs using high-level languages and apply an automated design process to create very complex designs, ignoring the electrical characteristics of the underlying circuits to a large degree. However, scaling trends (see Moore's law) brought electrical effects back to the forefront in recent technology nodes. With scaling of technology below 0.25 μm, the wire delays have become comparable or even greater than the gate delays. As a result, the wire delays needed to be considered to achieve timing closure. In nanometer technologies at 0.13 μm and below, unintended interactions between signals (e.g. crosstalk) became an important consideration for digital design. At these technology nodes, the performance and correctness of a design cannot be assured without considering noise effects. Most of this article is about SI in relation to modern electronic technology - notably the use integrated circuits and printed circuit board technology. Nevertheless, the principles of SI are not exclusive to the signalling technology used. SI existed long before the advent of either technology, and will do so as long as electronic communications persist.
On-chip signal integrity Signal integrity problems in modern integrated circuits (ICs) can have many drastic consequences for digital designs:
Products can fail to operate at all, or worse yet, become unreliable in the field. The design may work, but only at speeds slower than planned Yield may be lowered, sometimes drastically The cost of these failures is very high, and includes photomask costs, engineering costs and opportunity cost due to delayed product introduction. Therefore, electronic design automation (EDA) tools have been developed to analyze, prevent, and correct these problems. In integrated circuits, or ICs, the main cause of signal integrity problems is crosstalk. In CMOS technologies, this is primarily due to coupling capacitance, but in general it may be caused by mutual inductance, substrate coupling, non-ideal gate operation, and other sources. The fixes normally involve changing the sizes of drivers and/or spacing of wires. In analog circuits, designers are also concerned with noise that arise from physical sources, such as thermal noise, flicker noise, and shot noise. These noise sources on the one hand present a lower limit to the smallest signal that can be amplified, and on the other, define an upper limit to the useful amplification. In digital ICs, noise in a signal of interest arises primarily from coupling effects from switching of other signals. Increasing interconnect density has led to each wire having neighbors that are physically closer together, leading to increased crosstalk between neighboring nets. As circuits have continued to shrink in accordance with Moore's law, several effects have conspired to make noise problems worse:
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