Signal integrity

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),[1] through the package, the printed circuit board (PCB), the backplane, and inter-system connections.[2] 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:

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.[1] 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:

These effects have increased the interactions between signals and decreased the noise immunity of digital 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 that must be mitigated:

Finding IC signal integrity problems

Typically, an IC designer would take the following steps for SI verification:

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. However, such tools generally are not applied across an entire IC, but only selected signals of interest.

Fixing IC signal integrity problems

Once a problem is found, it must be fixed. Typical fixes for IC on-chip problems include:

Each of these fixes may possibly cause other problems. This type of issue must be addressed as part of design flows and design closure. Re-analysis after design changes is a prudent measure.

On-die termination

On-die termination or ODT is the technology where the termination resistor for impedance matching in transmission lines is located within a semiconductor chip, instead of a separate, discrete device mounted on a circuit board.

Chip-to-chip signal integrity

For wired connections, 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 15 cm (6 in) of FR-4 stripline (the propagation velocity depends on the dielectric and the geometry). 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. There are many circuit board types other than FR-4, but usually they are more costly to manufacture.

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 buses to serializer/deserializer (SERDES) links called "lanes." Such serial links eliminate parallel bus clock skew and reduce the number of traces and resultant coupling effects 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.[4][5]

At these new multigigabit/s bit rates, the bit period is shorter than the flight time; 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 Ω), a geometric form factor and by the square root of the relative dielectric constant of the stripline filler (typically FR-4, with a relative dielectric constant of ~4). Together, these properties determine the trace's characteristic impedance. 50 Ω is a convenient choice for single-end lines,[6] 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 µm or more in width). Thus, PCB traces have a small series resistance (typically 0.1 Ω/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.

The main challenge often depends on whether the project is a cost-driven consumer application or a performance-driven infrastructure application.[7] They tend to require extensive post-layout verification (using an EM simulator) and pre-layout design optimization (using SPICE and a channel simulator), respectively.

Finding signal integrity problems

There are special purpose EDA tools[10] 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.

Fixing signal integrity problems

An IC package or PCB designer removes signal integrity problems through these techniques:

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

See also

Notes

  1. 1 2 Louis Kossuth Scheffer; Luciano Lavagno; Grant Martin (eds) (2006). Electronic design automation for integrated circuits handbook. Boca Raton, Florida: CRC/Taylor & Francis. 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.
  2. ↑ Howard W. Johnson ; Martin Graham (1993). High speed digital design a handbook of black magic. Englewood Cliffs, New Jersey: Prentice Hall PTR. ISBN 0-13-395724-1. A book for digital PCB designers, highlighting and explaining analog circuit principles relevant to high-speed digital design.
  3. ↑ Ruckerbauer, Hermann. "An Eye is Born". Gives an example video of construction of an eye pattern
  4. ↑ "Signal Integrity: Problems and Solutions," Eric Bogatin, Bogatin Enterprises
  5. ↑ "Eight Hints for Debugging and Validating High-Speed Buses," Application Note 1382-10, Agilent Technologies
  6. ↑ "Why 50 Ohms?". Microwaves101. Retrieved 2008-06-02.
  7. ↑ Rako, Paul (April 23, 2009). "Voices: Signal-integrity experts speak out: Two experts discuss signal-integrity challenges and their expectations for signal integrity.". EDN. For cost-driven consumer applications ... [i]t's tempting to compact [the parallel buses], but the risk is postlayout failure ... For performance-driven applications, the pinch points [is] prelayout design-space exploration ...
  8. ↑ "Hurdle the multigigabit per second barrier"
  9. ↑ IEEE P802.3ap Task Force Channel Models
  10. ↑ Breed, Gary (August 2008). "High Speed Digital Design Benefits from Recent EDA Tools Development" (PDF). High Frequency Electronics. p. 52. Retrieved May 1, 2009. ...with continued increase in clock rates of digital circuits, the realms of RF and digital circuits are now more closely tied than ever before.
  11. 1 2 "Using Pre-Emphasis and Equalization with Stratix GX" (PDF). Altera.
  12. ↑ "Using Clock Jitter Analysis to Reduce BER in Serial Data Applications", Application Note, literature number 5989-5718EN, Agilent Technologies

References

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