Henny Youngman’s advice to PCB designers

By Richard Goering | No Comments | Posted: March 6, 2014Topics/Categories: PCB - Design Integrity | Tags: DesignCon 2014, PCB place and route,signal integrity | Organizations: Bogatin Enterprises

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

From the 1930s to the 1980s, comedian Henny Youngman was famous for his one-liner jokes. Oneof them is not just a joke for signal integrity guru Eric Bogatin, it is a cornerstone engineeringprinciple 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 shouldunderstand.

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

PCB prescription

Bogatin started his fast-moving talk with a prescription for problem solving. First, identify theproblem; next, find the root cause and understand what feature in the design causes it. Then, applythe Youngman principle and eliminate that feature. Finally, establish a design guideline that avoidsthe 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 rootcause into a design guideline. If your arm hurts when you raise it, don't raise your arm! If youknow the root cause of a problem, and you know the problem happens because your design hassome 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, asdescribed below.

Principle #1: All interconnects are transmission lines

"Every single interconnect is a transmission line—no exceptions," Bogatin declared. When youlook at a circuit board from the top down, you are only seeing the signal paths of the lines. Butevery transmission line has two conductors. Most people would say the second conductor is theground plane, but Bogatin said, "to calibrate your engineering intuition, when you think of thesecond 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 islaunched on the transmission line," Bogatin said, "There is absolutely nothing you can do toprevent that signal from propagating down the line." It will move at the speed of light through thematerial. The speed of light in air is about 12 inches per ns, and the speed of light in mostinterconnects 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." Acknowledgingthat it is "kind of a Zen approach," Bogatin said that "we're going to jump onto that transmissionline and become the signal, and ask what it sees electrically." And the most important thing it seesis the instantaneous impedance at each step along the way.

The traditional textbook models, Bogatin said, are lumped circuit approximations. They don't tellus what impedance the signal sees as it walks down the line. Impedance, as he reminded theaudience, is V (voltage) over I (current). So the signal asks, as it goes down the line, "What's theratio of my voltage divided by my current?"

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

If a transmission line is a uniform cross section, signals will propagate at a uniform speed, and wewill see the same instantaneous impedance. From this, we can derive the characteristic impedanceof the line. If there is no uniform transmission line, there is no characteristic impedance. "Thisconcept 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 adirection of circulation

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

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

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, adistortion 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 circuitstypically have multiple discontinuities.

Returning to the problem-solving approach that Bogatin outlined at the start of the talk, supposeyou identify a problem as "ringing." The root cause may be an impedance difference between thedriver and the receiver. Now apply the Youngman principle—"if the arm hurts, don't raise it." Ifthe root cause of the ringing is an impedance change at the receiver, don't allow the impedancethat the signal sees to change.

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

Want to know more? A copy of Bogatin's presentation is available at www.bethesignal.com.