By Richard Goering

 

Richard Goering, senior manager of technical communications, CadenceRichard Goering is senior manager of technical communications at Cadence Design Systems. As an editor for Computer Design, EE Times, and SCDsource, he has been writing about EDA and IC design for 25 years.

From the 1930s to the 1980s, comedian Henny Youngman was famous for his one-liner jokes. One of them is not just a joke for signal integrity guru Eric Bogatin, it is a cornerstone engineering principle for understanding and solving signal integrity problems:
Patient: “Doctor, it hurts when I raise my arm.”
Doctor: “Well, don’t raise your arm.”

In a standing-room-only talk at the recent DesignCon conference, Bogatin explained the “Youngman principle” along with five essential principles that all signal integrity engineers should understand.

Currently a Signal Integrity Evangelist at Teledyne LeCroy, Bogatin has been teaching and writing about signal integrity for many years. He believes, however, that much of what students are taught in school about how signals behave on interconnects is wrong. In the “speed training” event at DesignCon, Bogatin promised to show attendees the “right way” to think about signals on transmission lines. His talk was titled “The Most Important Signal Integrity Principles Every Engineer Should Know.”

PCB prescription

Bogatin started his fast-moving talk with a prescription for problem solving. First, identify the problem; next, find the root cause and understand what feature in the design causes it. Then, apply the Youngman principle and eliminate that feature. Finally, establish a design guideline that avoids the problem in the future, and develop some specific design rules.

“However corny that [doctor joke] sounds,” Bogatin said, “it is at the heart of how you turn a root cause into a design guideline. If your arm hurts when you raise it, don’t raise your arm! If you know the root cause of a problem, and you know the problem happens because your design has some feature in it, eliminate that feature! This is at the core of how we solve problems.”

Bogatin went on to present his list of five “essential” signal integrity engineering principles, as described below.

Principle #1: All interconnects are transmission lines

“Every single interconnect is a transmission line—no exceptions,” Bogatin declared. When you look at a circuit board from the top down, you are only seeing the signal paths of the lines. But every transmission line has two conductors. Most people would say the second conductor is the ground plane, but Bogatin said, “to calibrate your engineering intuition, when you think of the second conductor, always call it the return path. Forget the word ground.”

Principle #2: Signals are dynamic

A signal is the difference in voltage between the signal path and the return path. “Once a signal is launched on the transmission line,” Bogatin said, “There is absolutely nothing you can do to prevent that signal from propagating down the line.” It will move at the speed of light through the material. The speed of light in air is about 12 inches per ns, and the speed of light in most interconnects is about 6 inches per ns.

Principle #3: A signal sees the instantaneous impedance on the line
Here Bogatin asks us to set aside the textbooks and “become one with the signal.” Acknowledging that it is “kind of a Zen approach,” Bogatin said that “we’re going to jump onto that transmission line and become the signal, and ask what it sees electrically.” And the most important thing it sees is the instantaneous impedance at each step along the way.

The traditional textbook models, Bogatin said, are lumped circuit approximations. They don’t tell us what impedance the signal sees as it walks down the line. Impedance, as he reminded the audience, is V (voltage) over I (current). So the signal asks, as it goes down the line, “What’s the ratio of my voltage divided by my current?”

In a thought exercise, Bogatin asked attendees to “become” a 1V signal. At each step, we (the signal) dump enough charge to bring that step up to 1V. If we’re dumping the same amount of charge for the same amount of time, we have constant current. With constant current and constant voltage, impedance is constant as well.

If a transmission line is a uniform cross section, signals will propagate at a uniform speed, and we will see the same instantaneous impedance. From this, we can derive the characteristic impedance of the line. If there is no uniform transmission line, there is no characteristic impedance. “This concept of characteristic impedance is at the foundation of our thinking about transmission lines,” Bogatin said.

Principle #4: Current propagates as a signal-return wavefront with a direction of propagation and a direction of circulation

When an electric field changes, displacement current arises. This is as important a component of a signal as the voltage. As the voltage propagates, the displacement current between the signal and return is going to propagate as well.

Current has two directions—the direction of propagation, and the direction of circulation (which is based on whether voltage is increasing or decreasing). These directions are completely independent.

Principle #5: Reflections occur when the instantaneous impedance that the signal sees changes

When a signal encounters a change in instantaneous impedance, two things happen. First, a distortion is transmitted. Secondly, reflections will occur (although you may see the effect as “ringing”). This is the basis of many signal integrity problems, especially since real circuits typically have multiple discontinuities.

Returning to the problem-solving approach that Bogatin outlined at the start of the talk, suppose you identify a problem as “ringing.” The root cause may be an impedance difference between the driver and the receiver. Now apply the Youngman principle—”if the arm hurts, don’t raise it.” If the root cause of the ringing is an impedance change at the receiver, don’t allow the impedance that the signal sees to change.

Establish a design guideline such that the instantaneous impedance that the signal sees remains constant. From here you can create specific design rules. Solutions might include controlled impedance lines, termination strategies, or routing topologies.