Signal Integrity Matters

Signal Integrity is a field of study half-way between digital design and analog circuit theory. It’s about ringing, crosstalk, ground bounce, and power supply noise. It’s all about how to build really fast digital hardware that really works. It’s about practical, real-world solutions to high-speed design problems.

Signal Integrity is not just a "nice to know" subject. It is essential to the proper operation of every high-speed digital product. Without due consideration of the basic signal integrity issues typical high-speed products will fail to operate on the bench and, worse yet, become flaky or unreliable in the field.

Signal Integrity is a deterministic, predictable field of study (a "hard science"). Signal integrity specialists make frequent use of the fact that most signal integrity problems are easily observed. A good simulation, or a good laboratory demonstration, can usually put to rest any question about the efficacy of a particular solution. This is one area in which we signal integrity specialists enjoy a natural advantage over our EMC counterparts.

Signal Integrity didn’t always matter. In the golden years of digital computing (1970-1990), gates switched so slowly that, on the whole, digital signals actually looked like ones and zeros. Analog modeling of signal propagation was not necessary. Unfortunately, those days are long gone. At today’s speeds even the simple, passive elements of a high-speed design—the wires, PC boards, connectors, and chip packages— can make up a significant part of the overall signal delay. Even worse, these elements can cause glitches, resets, logic errors, and other problems. As you push toward ever-higher operating speeds, here’s a look at the primary issues you will face:

• A greater percentage of PCB traces in new designs will require terminators. Terminators help control ringing and overshoot on transmission lines. As speeds increase, more and more PCB traces will begin to take on aspects of transmission line behavior, and thus will require terminators. Unfortunately, terminators occupy precious space on every printed circuit board, and dissipate quite a bit of power. You will want to optimize the use of terminators, placing them precisely where needed, and only where needed.
• The exact delay of individual PCB traces will become more important. Already, CAD manufacturers are beginning to incorporate features useful for matching trace lengths, and guaranteeing low clock skew. At very high speeds, these features are crucial to system operation. You will want to master the study of propagation delay in all its many forms.
• Crosstalk will overwhelm many systems. Every time the clock rate is doubled in a system, crosstalk intensifies by a factor of two. This effect will bring some systems to their knees. Some of the symptoms include flaky or data dependent logic errors, sudden system crashes, software branches to nowhere, impossible state transitions, and unexplained interrupts. You will want to compress your layout to the maximum extent possible (for cost reasons), but without compromising crosstalk on critical signals.
• Ground bounce and power supply noise will boil over. Higher-powered drivers, switching at unbelievable rates, in massive parallel bus structures, are a sure formula for a power system meltdown. Sure, adding more power and ground pins and piling on more bypass capacitors will help, but where’s the limit? These things aren’t free. You want guaranteed glitch-free operation at a minimum cost.

Signal Integrity is a rapidly-growing field. There is no one right way learn it, and no one right way to practice it. The most important thing is to maintain a healthy interest in properly balancing your signal integrity, EMC, and manufacturing cost objectives. Get some formal training, constantly keep on the lookout for new tools, and tear apart lots of other people's products to see what the competition is doing. The payoff is easy to understand: better system-level performance, a more reliable product, and an overall reduction is cost. Who could ask for more?