Eric Bogatin's Blog: What I Learned This Month

There is an old joke told by Henny Youngman (1906 –1998) that goes something like this,

A man goes to a doctor and says to him, “Doctor, it hurts when I raise my arm, what should I do?” the Doctor says, “Don’t raise your arm.”

Surprisingly, this is an excellent strategy to follow to design signal integrity problems out of your next product. The details of this strategy are a core part of the Essential Principles of SI (EPSI) class I teach. Here’s a brief snapshot and a glimpse at how you apply the “Youngman Principle”.

The first step in designing SI problems out of your product is to identify the signal integrity problems to avoid. The next step is to identify the root cause of the problem. If you know the root cause, it often screams out at you (in pain) the way to avoid the problem.

For example, a common problem to avoid is reflection noise. The root cause is, that when the instantaneous impedance the signal sees changes, a reflection is created. Reflections that rattle around create ringing. How do you eliminate this problem? If it hurts when the instantaneous impedance changes, don’t change the instantaneous impedance. This means use controlled impedance interconnects, manage the reflections at the ends of lines with a termination strategy and use a routing topology that is linear.

Ground bounce is the most difficult problem to eliminate because it can reach very large voltages, can be long range, is poorly understood by most engineers, and involves the return path, which is often hard to trace out. The problem is voltage noise injected on signal lines when nearby signal lines switch.

The root cause of ground bounce is two fold. First, the return path of a signal needs to be screwed up from the usual wide return path. Second, the return path of one conductor needs to share this return path of another conductor.

If it hurts when the return path is not in a wide plane and when the return paths are shared, don’t screw up the return path and don’t share return paths.

If you can figure out the root cause of a problem, it screams out at you, when you do this, (raise your arm), the problem arises. The solution is, as Henny Youngman points out, “don’t raise your arm!”

To learn about the root cause of some of the common signal integrity problems, check out the articles I have on my web site.