Signal integrity

Signal integrity or SI is a measure of the quality of an electrical signal. In digital electronics, a stream of binary values is represented by a voltage (or current) waveform. Over short distances and at low bit rates, a simple conductor can transmit this with sufficient fidelity. However, at high bit rates and over longer distances, various effects can degrade the electrical signal to the point where errors occur, and the device fails. Signal integrity engineering is the task of analyzing and mitigating these impairments. Signal integrity engineering is an important activity at all levels of electronics packaging, from internal connections of an integrated circuit (IC) "Electronic Design Automation For Integrated Circuits Handbook", by Lavagno, Martin, and Scheffer, ISBN 0-8493-3096-3 A survey of the field of electronic design automation. Portions of IC section of this article were derived (with permission) from Vol II, Chapter 21, "Noise Considerations in Digital ICs", by Vinod Kariat.] , through the package, the printed circuit board (PCB), the backplane, and inter-system connections. ["High-Speed Digital Design: A Handbook of Black Magic", by Howard Johnson and Martin Graham, ISBN 0-13-395724-1. A book for digital PCB designers, highlighting and explaining analog circuit principles relevant to high-speed digital design.] 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, 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 digital computing devices. 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 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) 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 (or noise) 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.

On-chip signal integrity

Signal integrity problems in 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 drasticallyThe 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 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 arises not from fundamental physical sources, but from the operation of the circuit itself, primarily the switching of other signals. Higher interconnect density has led to each net having neighbors that are closer, thus leading to increased coupling capacitance between neighboring nets. As circuits shrink in accordance with Moore's law, several effects have conspired to make noise problems worse:
*To keep resistance tolerable despite decreased width, modern wire geometries are taller in proportion to their spacing. This increases the sidewall capacitance at the expense of capacitance to ground, hence increasing the induced noise voltage (expressed as a fraction of supply voltage).
*Technology scaling has led to lower threshold voltages, and has also reduced the headroom between threshold and supply voltage, thus reducing noise margins.
*Logic speeds, and clock speeds in particular, have increased significantly, thus leading to faster transition times. These faster transition times are closely linked to higher capacitive cross talk. Also, at such high speeds the inductive properties of the wires come into play especially mutual inductance.

These effects have increased the interactions between signals and decreased the noise immunity ofdigital CMOS circuits. This has led to noise being a significant problem for digital ICs that must be considered by every digital chip designer prior to tape-out. There are several concerns:
*Noise may cause a signal to assume the wrong value. This is particularly critical when the signal is about to be latched, for a wrong value can be loaded into a storage element.
*Noise may delay the settling of the signal to the correct value. This is often called "noise-on-delay".
*Noise may cause the input voltage of a gate to go below ground, or to exceed the supply voltage. This does not affect correct operation, but may reduce the lifetime of the device.

Finding IC signal integrity problems

Typically, an IC designer would take the following steps for SI verification:
*Perform a layout extraction to get the parasitics associated with the layout. Usually worst-case parasitics and best-case parasitics are extracted and used in the simulations. For ICs, unlike PCBs, physical measurement of the parasitics is almost never done, since in-situ measurements with external equipment are extremely difficult. Furthermore, any measurement would occur after the chip has been created, which is too late to fix any problems observed.
*Create a list of expected noise events, including different types of noise, such as coupling and charge sharing.
*Create a model for each noise event. It is critical that this be as accurate as possible.
*For each noise event, decide how to excite the circuit so that the noise event will occur.
*Create a SPICE (or another circuit simulator) netlist that represents the desired excitation.
*Run SPICE and record the results.
*Analyze the simulation results and decide whether any re-design is required.

Modern signal integrity tools for IC design perform all these steps automatically, producing reports that give a design a clean bill of health, or a list of problems that must be fixed.

Fixing IC signal integrity problems

Once a problem is found, it must be fixed. Typical fixes for IC on-chip problems include:
*Driver upsizing. The victim driving cell is made stronger by upsizing.
*Buffer insertion. In this approach, instead of upsizing the victim driver, a buffer is inserted at an appropriate point in the victim net.
*Aggressor downsizing. This works by increasing the transition time of the attacking net by reducing the strength of its driver.
*Wire Shielding. Shielding of critical nets or clock nets using GND and VDD shields to reduce the effect of Crosstalk. This technique may lead to routing overhead.
*Routing changes. Routing changes can be very effective in fixing noise problems mainly by reducing the most troublesome coupling capacitances.

Each of these fixes may possibly cause other problems. This type of issue must be addressed as part of design flows and design closure.

Chip-to-chip signal integrity

It is important to compare the interconnect flight time to the bit period to decide whether an impedance matched or unmatched connection is needed.

The channel flight time (delay) of the interconnect is roughly 1 ns per 6 inch or 15cm of FR4 stripline (speed of light in the dielectric). Reflections of previous pulses at impedance mismatches die down after a few bounces up and down the line i.e. on the order of the flight time. At low bit rates, the echoes die down on their own, and by midpulse, they are not a concern. Impedance matching is neither necessary nor desirable.

The gentle trend to higher bit rates accelerated dramatically in 2004, with the introduction by Intel of the PCI-Express standard. Following this lead, the majority of chip-to-chip connection standards underwent an architectural shift from parallel busses to serializer/deserializer (SERDES) links called "lanes." Such serial links eliminate parallel bus clock skew and reduce the number of traces. But these advantages come at the cost of a large increase in bit rate on the lanes, and shorter bit periods.

At multigigabit/s data rates, link designers must consider reflections at impedance changes (e.g. where traces change levels at vias), noise induced by densely-packed neighboring connections (crosstalk), and high-frequency attenuation caused by the skin effect in the metal trace and dielectric loss tangent. Examples of mitigation techniques for these impairments are a redesign of the via geometry to ensure an impedance match, use of differential signaling, and preemphasis filtering, respectively. [ [http://www.home.agilent.com/upload/cmc_upload/All/GTL11.pdf "Signal Integrity: Problems and Solutions," "Eric Bogatin, Bogatin Enterprises"] ] [ [http://cp.literature.agilent.com/litweb/pdf/5988-5623EN.pdf "Eight Hints for Debugging and Validating High-Speed Buses," "Application Note 1382-10," Agilent Technologies] ] .

At these new multigigabit/s bit rates, the bit period is shorter than the flight time and echoes of previous pulses can arrive at the receiver on top of the main pulse, and corrupt it. In communication engineering this is called intersymbol interference (ISI). In signal integrity engineering it is usually called eye closure (a reference to the clutter in the center of a type of oscilloscope trace called an eye diagram). When the bit period is shorter than the flight time, elimination of reflections using classic microwave techniques like matching the electrical impedance of the transmitter to the interconnect, the sections of interconnect to each other, and the interconnect to the receiver, is crucial. Termination with a source or load is a synonym for matching at the two ends. The interconnect impedance that can be selected is constrained by the impedance of free space (~377 ohm), a geometric form factor and by the square root of the relative dielectric constant of the stripline filler (typically FR4, with a relative dielectric constant of ~4). Together, these properties determine the trace's characteristic impedance. 50 ohms is a convenient choice for single-end lines, [cite web |url=http://www.microwaves101.com/encyclopedia/why50ohms.cfm |title=Why 50 Ohms? |publisher=Microwaves101 |accessdate=2008-06-02] and 100-ohm for differential.

As a consequence of the low impedance required by matching, PCB signal traces carry much more current than their on-chip counterparts. This larger current induces crosstalk primarily in a magnetic, or inductive, mode, as opposed to a capacitive mode. To combat this crosstalk, digital PCB designers must remain acutely aware of not only the intended signal path for every signal, but also the path of returning signal current for every signal. The signal itself and its returning signal current path are equally capable of generating inductive crosstalk. Differential trace pairs help to reduce these effects.

A third difference between on-chip and chip-to-chip connection involves the cross-sectional size of the signal conductor, namely that PCB conductors are much larger (typically 100 um or more in width). Thus, PCB traces have a small series resistance (typically 0.1 ohms/cm) at DC. The high frequency component of the pulse is however attenuated by additional resistance due to the skin effect and dielectric loss tangent associated with the PCB material.

Finding signal integrity problems

* Perform a layout extraction to get the parasitics associated with the layout. Usually worst-case parasitics and best-case parasitics are extracted and used in the simulations. Because of the distributed nature of many of the impairments, electromagnetic simulation [ [http://www.agilent.com/find/signal-integrity "Hurdle the multigigabit per second barrier"] ] is used for extraction.
* If the PCB or package already exists, the designer can also measure the impairment presented by the connection using high speed instrumentation such as a vector network analyzer. For example, IEEE P802.3ap Task Force uses measured S-parameters as test cases [ [http://www.ieee802.org/3/ap/public/channel_model/index.html#Interference%20Tolerance%20Test%20Channels IEEE P802.3ap Task Force Channel Models] ] for proposed solutions to the problem of 10Gbit/s Ethernet over backplanes.
* Accurate noise modeling is a must. Create a list of expected noise events, including different types of noise, such as coupling and charge sharing. Input Output Buffer Information Specification (IBIS) or circuit models may be used to represent drivers and receivers.
*For each noise event, decide how to excite the circuit so that the noise event will occur.
*Create a SPICE (or another circuit simulator) netlist that represents the desired excitation.
*Run SPICE and record the results.
*Analyze the simulation results and decide whether any re-design is required.

There are special purpose EDA tools that help the engineer perform all these steps on each signal in a design, pointing out problems or verifying the design is ready for manufacture. In selecting which tool is best for a particular task, one must consider characteristics of each such as capacity (how many nodes or elements), performance (simulation speed), accuracy (how good are the models), convergence (how good is the solver), capability (non-linear versus linear, frequency dependent versus frequency independent etc.), and ease of use [ [http://signal-integrity-tips.com/2008/can-your-security-guard-solve-your-signal-integrity-problems Can your Security Guard Solve Your Signal Integrity Problems?] Accessed June 2, 2008] .

Fixing signal integrity problems

An IC package or PCB designer removes signal integrity problems through these techniques:
*Placing a solid reference plane adjacent to the signal traces to control crosstalk,
*Controlling the trace width spacing to the reference plane to create consistent trace impedance,
*Using terminations to control ringing,
*Route traces perpendicular on adjacent layers to reduce crosstalk,
*Increasing spacing between traces to reduce crosstalk,
*Providing sufficient ground (and power) connections to limit ground bounce (this subdiscipline of signal integrity is sometimes called out separately as power integrity),
*Distributing power with solid plane layers to limit power supply noise.
*Adding a preemphasis filter to the transmitter driving cell. See, for example, cite web |url=http://www.altera.com/literature/wp/wp_pre-emphasis.pdf |title=Using Pre-Emphasis and Equalization with Stratix GX |publisher=Altera] .
*Adding an equalizer to the receiving cell
*Improved clock and data recovery (CDR) circuitry with low jitter/phase noise [ [http://cp.literature.agilent.com/litweb/pdf/5989-5718EN.pdf "Using Clock Jitter Analysis to Reduce BER in Serial Data Applications", "Application Note, literature number 5989-5718EN, Agilent Technologies"] ]

Each of these fixes may possibly cause other problems. This type of issue must be addressed as part of design flows and design closure.

References

Further reading

*"A Signal Integrity Engineer's Companion", by Dr. Geoff Lawday, David Ireland, and Greg Edlund ISBN: 0-13-186006-2. Published by Prentice Hall, June 2008.
*"High-Speed Signal Propagation: Advanced Black Magic", by Howard Johnson and Martin Graham, ISBN 0-13-084408-X. Advanced-level reference text for experienced digital designers who want to press their designs to the upper limits of speed and distance.
*"Digital Systems Engineering", by William J. Dally and John W. Poulton, 1998, ISBN 0-52-159292-5, Cambridge University Press, New York, NY, USA. Textbook on the problems of building digital systems, including signal integrity.
*"Signal Integrity Issues and Printed Circuit Board Design", by Douglas Brooks, Prentice Hall, 2003, ISBN 0-13-141884-X. This book approaches electrical engineering and signal integrity principles from a basic level, assuming little prior understanding.
*"Signal Integrity Simplified", by Eric Bogatin http://bethesignal.com, ISBN 0-1306-6946-6
*"High Speed Digital Design" by Hall, Hall and Macall
*"Signal Integrity Issues and Printed Circuit Board Design" by Douglas Brooks and Jacob Bogatin
* [http://www.altera.com/technology/signal/sgl-index.html Altera Signal Integrity Center]
* [http://www.agilent.com/find/signal-integrity Agilent EEsof EDA - Signal Integrity Analysis Resources]
* [http://www.eetimes.com/news/design/showArticle.jhtml?articleID=202601693 "Design tip: Model instruments to improve signal integrity simulation", "EETimes", John Olah, 2007-October-25]
* [http://www.designcon.com/2008/abstracts.html Topics in signal integrity were discussed at DesignCon 2008] Feb 4, 2008 to Feb 7, 2008
* [http://cp.literature.agilent.com/litweb/pdf/5988-5978EN.pdf "Understanding Signal Integrity - Signal integrity is becoming a more significant problem as clock frequencies increase"] by Eric and Jacob Bogatin, GigaTest Labs, Agilent Application Note 5988-5978EN, April 2002, 8 pages, PDF, 0.9MB
* [http://cp.literature.agilent.com/litweb/pdf/5989-5763EN.pdf "Signal Integrity Analysis Series Part 1: Single-Port TDR, TDR/TDT, and 2-Port TDR"] (Agilent Application Note 5989-5763EN, February 2007, 72 pages, PDF, 5.2MB)
* [http://cp.literature.agilent.com/litweb/pdf/5989-5764EN.pdf "Signal Integrity Analysis Series Part 2: 4-Port TDR/VNA/PLTS"] (Agilent Application Note 5989-5764EN, February 2007, 56 pages, PDF, 3.6MB)
* [http://cp.literature.agilent.com/litweb/pdf/5989-5765EN.pdf "Signal Integrity Analysis Series Part 3: The ABC's of De-Embedding"] (Agilent Application Note 5989-5765EN, July 2007, 48 pages, PDF, 2.5MB)