PCB Signal Integrity
0. Introduction (video)
Lesson 1 Review Background Information 1 and 2
Lesson 2 Eliminate Reflections 17
Lesson 3 Minimize EMI and Crosstalk Coupling 18
Lesson 4 Design Differential Traces 17.9
Lesson 5 Solve Power Distribution Problems 19
Lesson 6 Understand High Frequency Resistance Losses 20
Lesson 7 Deal with Really Short Wavelengths 21
Lesson 8 Control Trace Temperatures 16
Lesson 9 Final Thoughts 22
00. Summary (video)
* Lessons correlate with this chapter in PCB Currents
Lessons 2 through 7 also correlate loosely with chapter 22
Lesson 1 Review Background Information
1.1 Gain Historical Perspective
1.2 Understand the Nature of Current
1.3 Understand Frequency
1.4 Visualize Electromagnetic Fields
1.5 Deal with Propagation Speed
1.6 Understand Impedance
Lesson 2 Eliminate Reflections
2.1 Understand Reflections
2.2 Implement the CI Solution
2.3 Avoid CI Pitfalls
Lesson 3 Minimize EMI and Crosstalk Coupling
3.1 Understand EMI and Crosstalk
3.2 Route for Minimum Coupling
Lesson 4 Design Differential Traces
4.1 Compare single-ended and differential traces
4.2 Understand differential impedance
4.3 Avoid differential trace pitfalls
Lesson 5 Solve Power Distribution Problems
5.1 Understand PDS Problem
5.2 Compare Traditional and Impedance Solutions
Lesson 6 Understand High Frequency Resistance Losses
6.1 Learn About Skin Effect
6.2 Learn MORE about Skin Effect
6.3 Learn About Dielectric Losses
6.4 Deal With Lossy Transmission Lines
Lesson 7 Deal with Really Short Wavelengths
7.1 Learn Some Design Techniques When Wavelengths are Really Short
Lesson 8 Control Trace Temperatures
8.1 Design Traces For Temperature
8.2 Design Traces For Melting Point
Lesson 9 Final Thoughts
Lesson 1: “Review Background Information” covers several basic background topics particularly helpful in understanding the following material. Doug first puts PCB signal integrity issues into an historical perspective, showing how the progression of the various topics relates to faster and faster rise times. He then reviews the basic topics of the nature of current, frequency, rise time, harmonics, and impedance. Two lessons emphasize the importance of some of the basic laws of electronics, and the nature of electromagnetic fields.
Lesson 2: “Eliminate Reflections” introduces the problem of reflections. This problem is solved with transmission lines, or “controlled impedance traces.” But this solution is not sufficient, since terminations also play an important role.
Lesson 3: “Minimize EMI and Crosstalk Coupling” describes these two effects and their importance to PCB designers. In explaining crosstalk Doug utilizes several creative animations. In Lesson 3.2 Doug shows how it is possible to route two very long traces very close together without any crosstalk at all at the far end of the “victim” trace, and demonstrates that with the results of simulations.
Lesson 4: “Design Differential Traces” introduces the concept of differential signals and why they might perform better than single-ended signals. But differential traces require their own set of design guidelines. Doug outlines what these are, and why he believes in the guidelines he proposes.
Lesson 5: “Solve Power Distribution Problems” discusses the problems we might have in making power available around our boards. The two most common problems are (a) making sure there is enough charge available at the time and point of switching, and (b) making sure that “ground bounce” doesn’t negatively impact the signal. The solution to these problems involves bypass capacitors. But then Doug discusses the questions of how many, what size, where to put them, why, and the role ESR plays in all of this.
Lesson 6: “Understand High Frequency Resistance Losses” introduces the idea that resistance might not be constant with frequency. At least it might look that way. Doug introduces and explains the phenomena of skin effect and dielectric losses and how these ultimately result in lossy transmission lines. He then shows what we might be able to do about that.
Lesson 7: “Deal with Really Short Wavelengths” covers what happens when rise times get so short, and critical lengths get so short, that there is no physical room for implementing the solutions discussed in the previous lessons. At this stage, even via lengths might be longer than critical lengths. This severely limits what we can do with vias and how we should design them. Doug covers all this in this lesson.
Lesson 8: “Control Trace Temperatures” covers the relationship between trace currents and temperatures. Traces must be sized carefully to carry their currents with acceptable temperature rise, and Doug covers why this is so, and then how to go about doing this. In the case where a catastrophic failure might severely overload a trace, Lesson 8.2 covers how to size a trace to last long enough to safely “shut down” the system after such a failure.
Lesson 9: “Final Thoughts” is just that, some concluding thoughts Doug wants to leave with you. These include a summary of what we have covered in the context of the historical evolution of signal integrity on PCBs, a summary of the important design rules introduced, Doug’s answer to the student who once asked “Why should we believe your design rules?”, and Doug’s response to those who forecast the coming “death” of the PCB.