1.1 Introduction

This book is written to provide the reader with the basic understanding that is needed to layout high speed digital circuit boards. The wiring and circuitry that interconnects components mounted on these boards has become a new field of engineering. This book treats the analog aspects of digital board design, as it relates to the hardware that is selected. The term analog applies to such topics such as rise and fall time, overshoot, transmission delay, reflections, radiation, settling time, cross talk, and energy flow. The term analog is often used to describe circuits that use a carrier signal, such as in cell phone communication. Much of the material in this book will apply to carrier signals, but the emphasis will be on digital circuit board layout. The methods described in this book work well for slower logic. These methods do not add to board cost. These methods do add to reliability and performance. Software design and selection of components that make up a logic design are not discussed.

Rather than gather a big list at the end of the book, I have placed a limited glossary at the end of each chapter. I feel that this glossary should contain the words that are most critical to an understanding of the material in the text. In an industry that is changing very rapidly, there are bound to be language problems. This is the case with the circuit board industry and digital logic. The definitions that are used must be as clear as possible or the reader will not be helped.

The reader is encouraged to read over the glossary after completing each chapter. It will serve as both a review and a test of understanding. If a phrase or expression is not found then the next step is to use the index. If I have left something out, the internet can provide some assistance.

Electrical phenomena can be explained using basic physics, but the words that are used must be carefully chosen. Some phrases have a way of changing meaning over time. The term critical length was first used by transmission line engineers in the early days of radio where the signals were sine waves. Today, the expression is applied to cross coupling between traces on a circuit board where digital logic is involved. By reapplying the term, some of the original meaning has changed.

Abbreviations are not listed or used in the glossary of terms. Only well-accepted abbreviations and acronyms are used in the text. The reader is referred to a list at the end of this book.

In describing electrical behavior, the explanations vary with frequency and with specific disciplines. At clock rates above 100 MHz, many of the concepts used in circuit analysis begin to fail. In circuit analysis, there is a time delay associated with phase shift. The steady state operation of a circuit may occur after a hundred sine wave cycles have gone by. This implies that in a circuit, there are transient effects that must totally attenuate before the steady state solution is available. In digital circuits, there is a time delay associated with signal propagation. There is no waiting for one hundred cycles while transients decay. Delays in analog circuitry must be treated quite differently than the delays caused by signals traveling on a transmission line.

In a digital circuit, every connecting trace and every component on a circuit board can be considered a transmission line. It takes time for a signal to travel the length of a transmission line, and this time is independent of clock rate. It takes time to obtain energy from a capacitor. In circuit analysis, there is no simple way to treat these delays or the propagation delay of a transmission line.

In a resonant circuit involving inductance and capacitance, the energy flows back and forth between the two components at one frequency. The analysis is called steady state when all transient effects have attenuated and the performance repeats for each cycle of the driving signal. When a wave reflects back and forth on a transmission line, there is a time of transit not found in a resonant circuit. There is a spectrum associated with every logic transmission that relates to the rise time and extends toward dc. Circuits are analyzed one frequency at a time. Logic reacts to signals that are composites of an entire spectrum of sine waves. Thus, the character of a logic signal is very different than the character of a sine wave, yet the term frequency is used for both.

Printed circuit boards (PCBs) or printed wiring boards started out as a way to avoid hand soldering the interconnection of electronic components. Early boards had traces on one or two sides of an epoxy board with few plated-through holes. As component densities and clock speeds increased, it became necessary to include conducting planes in the design. These planes are used to distribute power and ground to components and to provide a return path for signal currents. Today, many boards are manufactured with dozens of interconnected layers, with many ground and power planes using surface-mounted and embedded components. As the clock rates have risen, board designs have become more and more of an engineering problem. It is no longer an issue of simply connecting the components together. A few of the problem areas that we will discuss include traces that jump between layers, the spacing of vias, trace routing that uses stubs, energy distribution, energy dissipation, board radiation, and signal cross talk.

Today's circuit board designs require engineering, and this engineering must be based on physics. This physics controls the details of design and is the basis of this book. Effective engineering must consider price and performance, as well as topics such as radiation and susceptibility. Surprisingly, there is a lot to say.

Circuits are often thought of as a configuration of components. When the logic rise times are associated with 1 MHz clock rates, this is basically true. The leads that connect the circuit can be routed almost at random, and there will be few problems. In low signal level analog designs below 100 kHz, lead routing can make or break the product. In digital circuits operating at clock rates above 100 MHz, the routing of all leads is as important as the selection of components. This book discusses the engineering of board layout and wiring, so that logic boards can function correctly; and at the same time, the engineer can control cost and limit radiation.