392 IEEE TRANSACTIONS ON ELECTROMAGNETIC COMPATIBILITY, VOL. 52, NO. 2, MAY 2010 Signal Integrity Design for High-Speed Digital Circuits: Progress and Directions Jun Fan, Senior Member, IEEE, Xiaoning Ye, Member, IEEE, Jingook Kim, Member, IEEE, Bruce Archambeault, Fellow, IEEE, and Antonio Orlandi, Fellow, IEEE (Invited Paper) Abstract—This paper reviews recent progress and future directions of signal integrity design for high-speed digital circuits, focusing on four areas: signal propagation on transmission lines, discontinuity modeling and characterization, measurement techniques, and link-path design and analysis. Index Terms—Differential signaling, discontinuities, equalization, equivalent circuit modeling, high-speed digital circuits, jitter analysis, mode conversion, modulation, signal integrity (SI), SI measurements, transmission line losses. I. INTRODUCTION IGNAL integrity (SI) is the practice of ensuring sufficient fidelity of a signal transmitted between a driver and a receiver for proper functioning of the circuit, e.g., the signals over the high-speed bus between a processor and its chipset. The integrity of a signal in real-world applications is compromised by artifacts of the layout of the circuits on a printed circuit board (PCB), the type of IC package being used, and is impacted by the circuit logic family, power delivery network, and other aspects of high-speed digital design. These nonideal, real-world effects can lead to severely distorted voltage and current waveforms and signal jitter, and result in faulty switching and logic errors. Originating from timing issues in digital circuits, SI design has undergone significant development in the past 15 years, and grown into an area covering many critical aspects in high-speed digital circuit design, including signal propagation on transmission lines (loss, termination, crosstalk, etc.), characterization of parasitics and discontinuities, power integrity, IC driver and receiver modeling, link-path and jitter analysis, interconnect design, and so on. With the continuous increase of operating speed, frequency, and circuit density, as well as the decrease of circuit dimension, system form factor, and logic level, nowadays it is increasingly critical to ensure good SI design for high-speed digital circuits [1]. Otherwise, they may not function properly. S Manuscript received September 29, 2009; revised February 17, 2010. First published April 1, 2010; current version published May 19, 2010. J. Fan and J. Kim are with the EMC Laboratory, Missouri University of Science and Technology (formerly University of Missouri-Rolla), Rolla, MO 65401 USA (e-mail: jfan@mst.edu; kimjing@mst.edu). X. Ye is with Intel Corporation, Hillsboro, OR 97124 USA (e-mail: xiaoning.ye@intel.com). B. Archambeault is with International Business Machines Corporation, Research Triangle Park, Durham, NC 27713 USA (e-mail: barch@us.ibm.com). A. Orlandi is with the Department of Electrical Engineering, University of L’Aquila, L’Aquia I-67040, Italy (e-mail: antonio.orlandi@univaq.it). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/TEMC.2010.2045381 Recent progress and future directions of SI design for highspeed digital circuits will be briefly overviewed in this paper. However, it is not meant to be thorough and comprehensive due to the length restriction and the limited knowledge of the authors. In addition, two important topics, SI simulation and power integrity, will have separate review papers, and thus, are not the focus of this paper. Signal propagation on transmission lines will be discussed first in Section II, focusing on losses and differential signaling. Then, discontinuity modeling and measurement techniques will be reviewed in Sections III and IV, respectively. Link-path design and analysis will be covered in Section V, including equalization and modulation techniques as well as jitter models. II. SIGNAL PROPAGATION ON TRANSMISSION LINES In today’s high-speed digital systems, traces in PCBs or multichip modules (MCMs) are usually not electrically short any more, and thus, need to be treated, designed, and analyzed using a multiconductor transmission line (MTL) model [2], [3]. The term MTL typically refers to a set of n + 1 electrically long parallel conductors that transmit electrical signals between two or more points, e.g., between a driver and a receiver. Although this model was originally developed in the early fifties [4], the first systematic approach oriented to electromagnetic compatibility (EMC) and SI applications was reported in [5] and [6], respectively. Since then, much advancement has been achieved on the solution of the MTL equations [7], [8], as well as on the adapting of the model to evolving PCB technology. MTL structures on a typical PCB or MCM consist of conductive traces buried in or attached to a lossy dielectric layer with one or more reference planes. Effects previously considered to be negligible and ignored in low-speed digital designs can become primary design issues with the increase of signal speed and the decrease of circuit form factor. Nonideal effects such as frequency-dependent losses, imbalance, and mode conversion are among the most important variables that can significantly affect SI in modern designs. A. Frequency-Dependent Losses Frequency-dependent losses during the propagation of digital signals on transmission lines are due to both the conductors and the dielectrics that form the line geometry. The skin effect in conductors manifests itself as resistance and inductance variations due to the migration of the conduction current toward exterior surfaces. To correctly quantify this effect, three 0018-9375/$26.00 © 2010 IEEE FAN et al.: SIGNAL INTEGRITY DESIGN FOR HIGH-SPEED DIGITAL CIRCUITS: PROGRESS AND DIRECTIONS high-frequency physical phenomena need to be taken into account including the crowding of the current at the corner of conductors [9], the proximity effect between traces and/or traces and reference plane [10], and the surface roughness of conductors [11]. Although several formulas are available in the literature to calculate the skin-effect frequency-dependent resistance and inductance [12]–[14], it should be noted that the crowding and the proximity effects cause the direct calculations of the internal inductance from the high-frequency resistance using Wheeler’s rule [6] to be inaccurate. Therefore, they should be computed separately. Furthermore, the square root of frequency dependence (typical for conductors with a circular cross section) is no longer valid, and more accurate modeling should be performed. As discussed earlier, high-frequency signals experience increased series resistance at high frequencies due to the migration of the current toward the conductor outer surfaces. The formulas to account for this loss, however, are derived on the assumption of perfectly smooth metal surfaces. In reality, the metal surfaces are rough, which effectively increases the resistance when the mean surface roughness is a significant percentage of the skin depth. In recent and advanced experimental studies [15], it has been found that high-frequency signals traveling on lines with significant surface roughness exhibit losses that are higher than those calculated with the ideal formulas by as much as 10% to 50%. Because the roughness pattern is random, it is impossible to predict the skin effect loss exactly. The roughness of a conductor is usually described as a tooth structure and the magnitude of the surface variations is described as its tooth size. From a design point of view, the surface roughness begins to affect the accuracy of the ideal ac resistance equations when the magnitude of the tooth size becomes significant compared to the skin depth. The ac resistance can be measured, and an innovative and effective procedure to obtain the portion of the measured attenuation constant α due to surface roughness was illustrated in [11]. The challenge in this field is the incorporation of these effects into practical design tools such as circuit simulators. A significant attempt in this direction was presented in [16]. The frequency-dependent dielectric loss is due to the electric polarization of the molecules forming the dielectric substrate. This microscopic phenomenon is described using a macroscopic damping effect that changes with frequency. The challenges here are associated with both the experimental characterization and the modeling of dielectric materials. For experimental characterization, accurate measurement techniques and calibration procedures are needed [17]. In addition, a statistical approach is necessary to take care of the unavoidable differences among the multiple stocks from which the test substrates are built. In terms of modeling, the dielectric properties extracted from measurements need to be not only accurate but also, more importantly, passive and causal [18]. Without the fulfillment of both constraints, the model of a dielectric material is useless and when used in transmission line simulations, it can generate incorrect and instable results. A procedure for the characterization of a dielectric material using a genetic algorithm was developed in [19], which addresses both passivity and causality. Developing efficient modeling approaches that accurately account for various loss mechanisms and dielectric properties for 393 transmission lines is of great need when data rate continues to increase. B. Differential Signaling, Imbalance, and Mode Conversion The use of so-called differential signaling for high-speed signals has become very common in today’s high-speed system designs. While this signal strategy allows better data quality and SI at the receive end of long traces, there are some EMC and SI issues that are not easily apparent [20]–[22]. A significant amount of common-mode signal can be created when the differential signals have small amounts of in-pair skew, or if the rise/fall times are slightly different, or the amplitude of the two traces in the differential pair is slightly different [23]. This common-mode signal can have a significant impact on the EMC performance of the system as well as affect the differential SI by crosstalk onto nearby traces or through mode conversion, where the common-mode signal becomes a differential-mode noise. The differential signaling in today’s high-speed designs often uses a pair of complementary single-ended nets. That is, both traces are driven with respect to the PCB ground reference. For this reason, these nets should be called “pseudodifferential”. This ground reference, whether on the PCB or within the application-specific integrated circuit (ASIC) package, is often imperfect, and may be a power layer or have discontinuities. At the receive end, the ASIC is usually a true differential receiver, which only looks for the difference between the two nets, without reference to the PCB (imperfect) ground-reference plane. In this way, the complementary single-ended nets become differential at the receiver. Since the receiver is only measuring the difference between the two nets, any common-mode noise is ignored by the receiver. This provides additional signal quality and allows longer traces to be used for higher data rates than the traditional single-ended nets [24], [25]. However, potential common-mode noise due to imperfect reference, whether on PCB or on chip, could affect SI through crosstalk or mode conversion as discussed earlier. Thus, these nonideal effects need to be carefully considered in high-speed designs. 1) Effects of Skew and Other Distortions: Among various imbalances that are possible in a practical differential pair, skew between the two complementary traces is a key parameter that can significantly affect the performance of the differential pair. It is found that a small amount of differential delay skew does not affect the differential-mode signal too much. However, it could result in a significant amount of common-mode noise [26], [27]. Although common-mode noise is more of a concern for EMC, it could cause SI problems as well if the noise is coupled to the adjacent traces through crosstalk or through a nonideal return path. As mentioned earlier, other differential signal distortions, such as rise/fall time mismatch and pulse amplitude mismatch can also create significant common-mode signals. While these effects are typically slightly less than the in-pair skew effects, they cannot be corrected on the PCB. 2) Mode Conversion: Whenever there is asymmetry in a differential pair, such as the “ground” pins in a connector or location of “ground” vias near a differential pair, there is some 394 IEEE TRANSACTIONS ON ELECTROMAGNETIC COMPATIBILITY, VOL. 52, NO. 2, MAY 2010 Fig. 1. Common-mode to differential-mode conversion due to asymmetric ground via locations. amount of differential-mode to common-mode conversion as well as common-mode to differential-mode conversion. The latter can affect the SI of the differential signal, since any commonmode noise, whether from any of the aforementioned causes, or from common-mode noise picked up from power distribution network (PDN) noise between the planes, can potentially increase jitter and decrease the eye opening of the intentional signal [28]. As an example, a simple four-via structure in a multilayer PCB is studied with two center vias as differential signal vias and two outer vias connected to the ground-reference planes. The right-hand side ground via has a longer distance from the signal vias than the left-hand side one. The amount of commonmode noise that is converted to the differential mode (|Scd21 |) is shown in Fig. 1. As the difference in distance increases, the amount of mode conversion increases until the ground-reference via is far away from the differential pair and is effectively no longer part of the circuit. Often, the symmetry of the ground-reference is ignored, since the designer is only interested in the differential-mode signal and spends a lot of effort on the differential-mode loss, reflections, etc. However, with the common-mode signals created by small amounts of mismatch in the in-pair skew, rise/fall time mismatch, and/or amplitude mismatch, coupled with the fact that real-world PCBs often have many via transitions (each with some amount of mode conversion), significant distortion may occur to differential signals, especially those with marginal eye openings due to dielectric and skin effect losses. Therefore, for high-speed designs, not only the differential but also the common modes need to be carefully considered. III. DEALING WITH DISCONTINUITIES In practical high-speed digital circuit designs, discontinuities are inevitable. At high frequencies, they can cause many SI problems. Reflection, loss, crosstalk, noise coupling, and mode conversion could increase significantly in the discontinuity regions [29]. There are various kinds of discontinuities in a signal propagation path at both the PCB and package levels. One of the most common discontinuities is the via transition. Signal traces are more often routed through multiple vias, when more components are integrated together and system complexity increases. This kind of routing is usually more efficient with the tradeoff between performance and cost. In a signal via transition, both the impedance and the reference of the trace can change. The effects of via transitions on SI, as well as modeling approaches, have been extensively investigated analytically and numerically [30]–[36]. Recently, equivalent circuit models for via transitions that include the distributed behaviors of plane pairs were proposed using parallel-plane impedances and lumped capacitances in [30] and [31]. A 2-D cavity model combined with the segmentation technique is used to obtain the multilayer parallelplane impedances. The lumped capacitances can be calculated from analytical formulas that were derived either based on coaxial line equations [32], or using magnetic frill currents and the Green’s function for the bounded parallel plate cavity [33]. Besides via transitions, there are many other discontinuities in a common high-speed PCB or package. As a method of reducing timing error, meander delay lines have been widely adopted in critical nets to provide the desired timing delay. However, crosstalk between the adjacent lines could cause eye diagram distortion and jitter. These SI consequences were analyzed based on a time-domain even- and odd-mode analysis of a coupled transmission-line structure in [37]. For bent differential transmission lines, their SI effects were modeled and a commonmode noise reduction scheme using a shunt capacitance was proposed in [38]. Today’s highly integrated chip-package-PCB hierarchical structures also have critical discontinuities such as the ball grid array and wire-bond connections. A method to reduce the return loss of bonding wires was proposed using the comb capacitors attached to the bonding pads [39]. Electromagnetic modeling and analysis of balls with vias in 3-D packages were discussed in [40]. It was reported that 40-Gb/s signaling was achievable in the low-cost wire-bonded plastic ball grid array by employing discontinuity cancellation techniques in both the signal-current and return-current paths [41]. The corresponding design methods for bonding wires, vias, solder ball pads, and PDNs were suggested. In [42], a transition from a coplanar waveguide to a microstrip with vias, which is often used in wafer probe measurements, was investigated. It was demonstrated how the presence and placement of the vias could affect the bandwidth and alter the impedance of the transitions. Another major discontinuity category is caused by nonideal ground references. A thin slot in a ground plane is a typical case, which could cause undesired effects such as signal distortion, crosstalk, and radiation. An equivalent circuit model for this kind of geometry using a gap voltage source was developed in [43]. In [44], a generalized partial-element equivalentcircuit method was proposed for modeling a planar circuit with a thin narrow slot in the ground plane [45]. The reduction in the slot-induced ground bounce noise by using differential signaling was also investigated with a 2-D finite-difference time-domain method in [46]. In the discontinuity regions, behaviors are dominated by higher order electromagnetic modes. Thus, they are often characterized as an S-parameter network obtained either from modeling or measurements. To incorporate the discontinuity models FAN et al.: SIGNAL INTEGRITY DESIGN FOR HIGH-SPEED DIGITAL CIRCUITS: PROGRESS AND DIRECTIONS with other components for a time-domain simulation, it is often preferred to extract an equivalent simulation program with integrated circuit emphasis (SPICE) model from the S-parameter network. A popular method to do so is the pole-residue approach, which can be very effective and accurate for analyses, if the S parameters are good in quality (causal and passive). However, for designs and optimizations, a physics-based equivalent circuit model, where every circuit component is related to some geometrical dimensions, has more advantages, since it has a one-to-one relationship to geometry, and thus, could provide a direct and intuitive physical understanding. The future directions of discontinuity modeling include the development of effective approaches for model extraction, macromodeling with causality and passivity enforcement, and efficient models that can be easily integrated into circuit simulators. IV. SI MEASUREMENT TECHNIQUES Various kinds of measurements are performed in both the time and frequency domains for SI. Although digital circuits are of interest, behavior at high frequencies up to the microwave range is needed due to the high speed of the signals in the circuits. Therefore, a good portion of SI measurements involves accurate characterization of passive components in the frequency domain using a vector network analyzer (VNA). A challenge particularly critical in these SI measurements is to properly account for the effects of test fixtures, since most of the digital circuits in PCBs or packages are not coaxial and test fixtures are usually necessary. To reduce the effects of connectors, a probing station is sometimes used at both the PCB and package levels. In addition, to accurately quantify crosstalk among traces, four-port measurements with true four-port calibration are often needed. In some applications, multiport measurements with more than four ports may be needed for crosstalk between differential pairs. A lot of research work has focused on improving VNA measurement techniques. Coupled transmission lines have been experimentally characterized using four-port Sparameter measurements up to 20 GHz in [47]. An analytical methodology for characterizing electrical transitions associated with transmission line-based microwave channels was presented in [48]. Several deembedding standards and a statistical parameter estimation technique were studied in [49] to achieve more accurate measurement results. In [50], the parameters of transmission lines embedded in multilayer PCBs were measured up to 18 GHz based on a mathematical error-removal scheme using two different length transmission lines and a via–hole structure that minimizes coupling. Since transmission lines are widely used in high-speed digital circuits, quantifying transmission line parameters (propagation delay and characteristic impedance), impedance matching, and discontinuities are necessary for SI designs. In addition to the frequency domain VNA measurements, time-domain reflectometry (TDR)/time-domain transmission measurements are also widely employed, since they are simple to use and can provide straightforward information. Although the dynamic range of time-domain instruments is much smaller than frequency- 395 domain ones, they are more commonly used in manufacturing floors due to the cost consideration. A method has been developed to perform network analysis using TDR measurements including time-domain through-reflection-line (TRL) calibrations for this exact purpose [51]. Another set of SI measurements target the performance of the overall high-speed link and system interface. These measurements may involve a few different instruments including an oscilloscope, bit error rate tester, jitter analyzer, and spectrum analyzer [52]–[54]. Accurate modeling and characterization of the timing jitter introduced by the clocking network, transmitter, receiver, and the transmission media is essential, as the data rate of the chip-to-chip communication increases. Jitter can be measured and decomposed into different components that are directly related to certain noise mechanisms using an oscilloscope or signal analyzer with jitter analysis capabilities. The receiver jitter, sensitivity, and bandwidth are related, and they affect the bit error ratio (BER) of a high-speed link. Various measurements have been demonstrated for measuring the I/O link performance. A 16-Gb/s bidirectional asymmetrical memory interface was characterized, and on-chip measurements were used to complement the off-chip instrumentation [55]. Measurements of electrical on-board module-to-module links were performed with a per-channel data rate of up to 11 Gb/s utilizing a 16channel link chip with a programmable equalization in [56]. On-chip measurement circuitries were also implemented to diagnose and optimize the performance of the link. Two circuits for data jitter measurements were implemented with a 0.11 µm CMOS process and demonstrated in [57]. Improving VNA measurements continues to be a focus of the future research in this area. True differential TRL calibration techniques are of great interest when the test fixtures themselves are coupled networks. Accurate time-domain network analysis techniques are also highly desirable, especially for manufacturing floor, where TDRs are more popular than VNAs. In jitter measurements, jitter decomposition algorithms need to be further improved so that the results from different scopes can correlate better. V. LINK-PATH ANALYSIS AND DESIGN A link path starts at a transmitter on a silicon substrate, ends at a receiver on another silicon substrate, and may include siliconto-package interconnects, interconnects in packages and PCBs, package-to-PCB interconnects, as well as connectors and cables. The performance of the link path directly determines the bit error rate of the data communication between the transmitter and the receiver [58]–[61]. Hence, link-path analysis and design is a critical area in SI. Among various link path issues, equalization, modulation, and jitter analysis are focused in this section, as they are the most critical parts in today’s link-path analysis and design. A. Equalization Equalization is emerging as a critical design technique to achieve the ever-increasing data rate speeds in computer or communication systems. There are various equalization designs 396 Fig. 2. IEEE TRANSACTIONS ON ELECTROMAGNETIC COMPATIBILITY, VOL. 52, NO. 2, MAY 2010 Multitap FIR equalizer. Fig. 4. Fig. 3. IIR equalizer. available [62], [63], but the basic idea is to compensate the signal distortion caused by the link path, also called as a channel, between the transmitter and the receiver. Intersymbol interference (ISI) is one of the major contributors to the signal degradation through a channel, which is typically nonideal and its frequency response is band-limited mainly due to losses and reflections. Passing a signal through such a channel results in frequency-dependent attenuation. This filtering of the transmitted signal affects the shape of the pulse that arrives at the receiver. Equalization is the most common approach to mitigate ISI. From the frequency-domain perspective, equalization helps compensate the low-pass filtering characteristics of the channel by increasing the high-frequency components of the transmitted signal while lowering the low-frequency ones. From a time-domain perspective, equalization minimizes the amplitude of the pulse response that spreads out of its ideal position and interferes with other symbols [64], [65]. 1) Feed-Forward Versus Feed-Backward Equalization: Finite-impulse response (FIR) filters are generally used for transmitter deemphasis [62], [66], a common form of equalization. Fig. 2 illustrates a general N-tap FIR equalizer that can compensate the ISI that spreads N bits before or after the ideal response position. It should be noted that an FIR equalizer could be implemented at either the transmitter or the receiver side, as for most other equalization schemes. The FIR equalizer illustrated in Fig. 2 is a feed-forward system. This is in contrast to an infinite-impulse response (IIR) equalizer, which is a feed-backward system as shown in Fig. 3. An FIR filter is usually more preferable to an IIR, since it is inherently stable and does not require feedback. One main disadvantage of an FIR filter is that it requires more computational power. Although a traditional IIR filter as shown in Fig. 3 is not popular in high-speed systems, an improved one with a decisionfeedback block (see Fig. 4) is very popular. This type of filter is typically called a decision-feedback equalizer (DFE). If the Typical DFE. values of the symbols previously detected are known, the ISI contributed by these symbols can be canceled out exactly at the output of the DFE filter by subtracting the past symbol values with appropriate weighting. The tap weights (K1, K2, K3, etc.) can be adjusted simultaneously to fulfill a criterion such as minimizing the mse. 2) Continuous Time-Domain Versus Discrete-Time Equalization: The equalization schemes described earlier are in the discrete time domain, while the implementation can be done in the continuous time domain as well [67]. The transfer function of this kind of passive equalizer is a high-pass filter. It is often seen in cables. In IC applications, active circuitry can be added to boost the gain of the transfer function, or add peaking of transfer frequency in the needed frequency range [68], [69]. 3) Adaptive Equalization: To account for the variations in channel performance, such as transmission line variations due to manufacturing tolerance, environmental impact, and effects of connector/socket, etc., equalization settings are preferred to be adaptive for optimal performance. Examples of equalization settings include tap coefficients in an FIR or IIR filter, or values of the circuit components used in a passive equalizer. The most widely used adaptation criterion is the mse calculated between the recovered signal and the training data in the time domain at sampling points. The LMS algorithm or its variations such as the sign–sign LMS are the most popular [70]–[72]. Another method is to obtain the error signal by sampling the equalizer output waveform, and then, analyzing its characteristics in the time domain [73], [74]. Alternatively, the error signal can also be obtained from the statistical spectrum information [63], [75]. Adaptive equalization is typically implemented at the receiver. To implement adaptive equalization at the transmitter, a feedback mechanism to the transmitter is needed that can complicate the system design, since the feedback has to go through the entire channel. B. Modulation In current state-of-the-art high-speed link design, modulation techniques as well as equalization techniques are exploited. Typical equalization techniques have to provide compensation for the frequency spectrum of the entire nonreturn to zero (NRZ) data bandwidth. As data rate increases, the roll-off of the channel frequency response is severe, and the discontinuity of the channel (such as use of vias, connectors, and sockets, etc.) may become severe in the frequency range of interest. FAN et al.: SIGNAL INTEGRITY DESIGN FOR HIGH-SPEED DIGITAL CIRCUITS: PROGRESS AND DIRECTIONS An alternative solution is to reduce the bandwidth of transmitted signal using multilevel coding or modulation. Duobinary and PAM4 signals are among the most commonly used approaches [76]–[79]. The key feature of PAM4 and duobinary signals is that their signal spectra are exactly a half of that of the NRZ data. In other words, the bandwidth requirement of the channel is narrower, which means less attenuation and distortion to the signal. However, due to multilevel signaling, the signal is more susceptible to noise, so the SNR performance has to be sacrificed. They are also harder to implement, and consumes more power compared to the standard NRZ signaling. Generally speaking, duobinary does not incur nearly as much SNR penalty as PAM4, and it is easier to implement [78]. More research is needed to further understand the performance difference in real silicon design using different signaling techniques. In [79], design and comparison of the three techniques, NRZ, PAM4, and duobinary, were reported for a 20-Gb/s backplane link and it was shown that NRZ signaling has the best BER performance. C. Jitter Analysis Jitter is the deviation of the significant edges in a sequence of data bits from their ideal timing positions. It can cause the receiver to fail in decoding the signal correctly. Jitter is a significant, and usually undesired, factor in the design of almost all communications links. Jitter is often characterized by two generalized types, deterministic jitter (DJ) and random jitter (RJ). DJ is caused by nonGaussian events. It is always bounded in amplitude and has specific causes. Examples of DJ include duty cycle distortion, data-dependent jitter, jitter caused by power supply noise, coupling between two signals, spread spectrum clocking, etc. RJ is the component that is Gaussian in nature. One example of RJ is the thermal noise in a circuit [80], [81]. 1) Peak Distortion Analysis (PDA): In recent years, PDA has become a popular method to quickly determine the worst case eye diagram of the link [82], [83]. The worst case voltage magnitude at any given sampling point can be directly calculated from the unit pulse response. In other words, the worst case eye opening due to ISI can be almost instantaneously calculated for the unit pulse response without having to run any additional simulation. Although PDA analysis is a computationally efficient approach to include ISI and crosstalk impact, it does not include the transmitter and receiver jitter. In the full-link PDA analysis, silicon jitter and channel ISI/crosstalk need to be calculated separately, and meet their individual design budgets. 2) Statistical BER Analysis: As mentioned earlier, PDA analysis does not take into account the interaction of the transmitter/receiver jitter with the channel ISI. Another critical disadvantage of PDA is that the BER of the link is not available. If the channel memory is long, the worst case bit pattern can consist of an excessive number of bits, and the probability of sending the exact sequence of the worst case bits is close to zero. Designing for the worst case eye often results in an overdesign. 397 A general framework of statistical BER analysis was introduced in [78], where, given the pulse response from an accurate channel model including crosstalk, transmitter and receiver sampling distribution, and receiver sensitivity, BER can be determined analytically. The key to this statistical BER analysis is to represent all interference sources with probability density functions, and then, convolute them. Further progress on the statistical BER analysis was achieved in [84] and [85]. In [86], the methodology was enhanced to include the transmitter jitter amplification through the channel. In other words, the transmitter jitter distribution cannot be convoluted directly with the channel ISI and the receiver jitter distributions, since the transmitter jitter can be distorted or amplified after passing through the channel. A rigorous method was developed in [86] to take this phenomenon into account. Furthermore, in [87] and [88], new general formulations to model the white and the colored bounded jitter were introduced. 3) Time-Domain Jitter and BER Analysis: One disadvantage of the statistical jitter and BER analysis is that timedomain waveforms are not available, since the eye diagram is constructed based on the probability density functions only. Furthermore, since all the interference sources are characterized with probability density functions, the correlation of each source is not accounted for. For example, sinusoidal jitter or duty cycle distortion cannot be accurately captured, because the cycle-to-cycle relationship of the jitter sequence is totally lost when their behavior is represented by their probability density functions [89]. Time-domain empirical jitter analysis overcomes the disadvantages of the statistical analysis, by analytically constructing the time-domain waveforms through convolution of superposition. In [90], a flexible and efficient method was proposed for BER simulation using efficient time-domain interpolation and superposition. It uses the step response of the channel, and directly converts the bit sequence to the data stream in the time domain and extracts the jitter distribution from the actual reference-crossing points. The method is flexible with coding schemes and capable of adding any correlated or uncorrelated transmitter or receiver jitter. The disadvantage of this approach is that low BER performance of the link has to be extrapolated, since it is computationally intensive to construct 10e12 (or more) bits of waveforms. However, there exist various types of BER extrapolation methodologies. A good summary of the extrapolation techniques can be found in [91]. 4) Special Note on Nonlinearity and Jitter Analysis: The jitter analysis approaches mentioned earlier can accurately simulate system jitter performance only when the system is linear time invariant (LTI) or can be well approximated as an LTI system. For a nonLTI system, one cannot construct waveforms based on impulse or pulse response, and the aforementioned fast jitter analysis approaches will result in errors. There are techniques to resolve some specific nonLTI issues. In [92], a novel algorithm was introduced using the superposition of multiple edge responses to construct waveforms for different data patterns, which can account for nonlinearities that extend beyond one bit as well as asymmetric rising and falling edges. With this 398 IEEE TRANSACTIONS ON ELECTROMAGNETIC COMPATIBILITY, VOL. 52, NO. 2, MAY 2010 new algorithm, the jitter analysis approaches become applicable again. A problem that remains in link-path analysis and design is the lack of a meaningful parameter that can directly relate the linkpath behaviors to signal performance quantitatively. A lot of research work is targeting this issue. Including nonlinear effects and various noise-coupling mechanisms into consideration is also a popular direction. Further improvements in jitter and BER analysis methods are desirable as well. VI. CONCLUSION Recent progress and future directions in SI design for highspeed digital circuits have been reviewed in this paper. As technology evolves, SI, as an integral part of the design process for high-speed digital circuits, will continue being developed and to grow into new territories. In addition, it will become increasingly critical to codesign SI, power integrity, and EMC, in order to achieve the design objectives of high quality, low emission, cost reduction, as well as reduced time to market. 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Jun Fan (S’97–M’00–SM’06) received the B.S. and M.S. degrees from Tsinghua University, Beijing, China, in 1994 and 1997, respectively, and the Ph.D. degree from the University of Missouri-Rolla, Rolla, in 2000, all in electrical engineering. During 2000–2007, he was a Consultant Engineer at NCR Corporation, San Diego, CA. In July 2007, he joined the Missouri University of Science and Technology (formerly University of MissouriRolla), where he is currently an Assistant Professor in electromagnetic compatibility (EMC) Laboratory, Missouri University of Science and Technology, Rolla. His research interests include signal integrity and electromagnetic interference (EMI) designs in highspeed digital systems, dc power-bus modeling, intrasystem EMI and RF interference, printed circuit board noise reduction, differential signaling, and cable/connector designs. Dr. Fan was the Chair of the IEEE EMC Society TC-9 Computational Electromagnetics Committee during 2006–2008, and a Distinguished Lecturer of the IEEE EMC Society during 2007–2008. He is currently the Vice Chair of the Technical Advisory Committee of the IEEE EMC Society. He received an IEEE EMC Society Technical Achievement Award in August 2009. Xiaoning Ye (S’97–M’01) received the Bachelor’s and Master’s degrees from Tsinghua University, Beijing, China, in 1995 and 1997, respectively, and the Ph.D. degree from the University of MissouriRolla (currently Missouri University of Science and Technology), Rolla, in 2000, all in electrical engineering. He has been a Senior Hardware Engineer at Intel Corporation, Hillsboro, OR, since his graduation, where he is involved in ensuring signal integrity of high-speed differential signaling in server platforms, providing platform design guidelines, and research and development of next generation multi-Gb/s bus interfaces, etc. He was also engaged in research and development of electromagnetic compatibility solutions for enterprise platforms. He holds three patents and several patent applications in the areas of high-speed signaling. Jingook Kim (M’09) received the B.S., M.S., and Ph.D. degrees in electrical engineering from Korea Advanced Institute of Science and Technology, Daejeon, Korea, in 2000, 2002, and 2006, respectively. He was involved in power/signal integrity, package modeling in gigahertz systems, as well as electromagnetic interference/electromagnetic compatibility (EMC) design. From 2006 to 2008, he was a Senior Engineer at Memory Division, Samsung Electronics, where he was engaged in high-speed I/O circuit designs, focusing on power and signal integrity. Currently, he is at Missouri University of Science and Technology, Rolla, where he joined the EMC Laboratory in 2009 as a Postdoctoral Research Fellow. His research interests include power distribution network modeling and design at the chip/package/printed circuit board levels, IC EMC, and RF interference. Bruce Archambeault (M’85–SM’99–F’05) received the B.S.E.E degree from the University of New Hampshire, Durham, in 1977, the M.S.E.E degree from Northeastern University, Boston, MA, in 1981, and the Ph.D. degree from the University of New Hampshire, in 1997. His doctoral research involved the computational electromagnetics applied to real-world EMC problems. Currently he is an International Business Machines Corporation (IBM) Distinguished Engineer at IBM, Research Triangle Park, Durham, NC. He has authored or coauthored a number of papers in computational electromagnetics, mostly applied to real-world EMC applications, and the books PCB Design for Real-World EMI Control and EMI/EMC Computational Modeling Handbook. Dr. Archambeault is currently the Chair of IEEE EMC Society Technical Activities. He was a Member of the Board of Directors for the IEEE EMC Society and a Member of the Board of Directors for the Applied Computational Electromagnetics Society. He was a IEEE/EMCS Distinguished Lecturer.. Antonio Orlandi (M’90–SM’97–F’07) was born in Milan, Italy, in 1963. He received the Laurea degree in electrical engineering from the Sapienza University of Rome, Rome, Italy, in 1988. From 1988 to 1990, He was with the Department of Electrical Engineering, Sapienza University of Rome. Since 1990, he has been with the Department of Electrical Engineering, University of L’Aquila, L’Aquia, Italy, where he is currently a Full Professor at the UAq EMC Laboratory. He has authored or coauthored more than 190 technical papers in the field of electromagnetic compatibility in lightning protection systems and power drive systems published in various international journals and conferences. His Current research interests include numerical methods and modeling techniques to approach signal/power integrity, electromagnetic compatibility (EMC)/electromagnetic interference (EMI) issues in high-speed digital systems. Dr. Orlandi received the IEEE TRANSACTIONS ON ELECTROMAGNETIC COMPATIBILITY Best Paper Award in 1997, the IEEE EMC Society Technical Achievement Award in 2003, the IBM Shared University Research Award (2004, 2005, and 2006, respectively), the Computer Simulation Technology (CST) University Award in 2004, and the IEEE EMC Symposium Best Paper Award in 2009. He is a Member of the Education, TC-9 Computational Electromagnetics and the Chairman of the TC-10 Signal Integrity Committees of the IEEE EMC Society, the Chairman of the “EMC and Signal Integrity at PCB level” Technical Committee of the International Zurich Symposium and Technical Exhibition on EMC. He has been an Associate Editor of the IEEE TRANSACTIONS ON ELECTROMAGNETIC COMPATIBILITY and the IEEE TRANSACTIONS ON MOBILE COMPUTING.