Continuing in our series of Lists to Live By, here is my list of nine SI Principles

It is impossible to consolidate all the important principles that are taught typically in a 2-semester class, or in a five-day workshop, into just nine essential principles. But “Sometimes and OK answer NOW! is better than a good answer late.”

Generally, the most important signal integrity principle is the one that causes your product to not work or miss a release date. This will vary, custom to each product, depending on its features, operating bandwidth and complexity.

From my personal experiences and working with many design teams, there are some problems which I see occur over and over again across product families and applications. They have a handful of essential principles at their underlying root cause. If we really understand these underlying principles, the root cause of the problems will be more apparent and their solutions closer to implement.

If I had to limit my list of the essential principles all SI engineers should understand, here is my list. If you have suggestions for ones I should take out or add in, please post them with your advice in the comments section.

Here is my list of the nine most essential principles of signal integrity:

1. All interconnects are always transmission lines with a signal and a return path.

2. Signals are dynamic. Once launched on a transmission line, they will propagate at the speed of light in the material.

3. Signals see an instantaneous impedance each step along their path through an interconnect.

4. Current propagates as a signal-return path loop with a direction of circulation and a direction of propagation. The return current returns between the signal and return path by displacement current.

5. Reflections occur whenever the instantaneous impedance changes.

6. Inductance is fundamentally about how efficient a conductor is in generating rings of magnetic field lines at the cost of current.

7. Current in a conductor redistributes at higher frequency driven by minimizing loop inductance. This is called the skin depth effect. This causes the series resistance of a trace to be frequency dependent, starting at above about 10 MHz for a ½ oz trace.

8. Dielectric materials absorb electric-field energy by rotating their dipoles in the electric field, causing friction, dissipating heat energy and attenuating the propagating signal.

9. Common currents in external conductors radiate and are most often the cause of EMC certification failures.