Learning does not end when we leave school and start our professional careers. The only thing that changes is the direction of the money flow. When we are students, we pay to learn and work on projects. When we are professional engineers working in industry, we are paid to learn and work on projects.

In this respect, we are all lifelong learners. While we can passively absorb information from live courses, books, articles, or videos, there is no substitute for hands-on learning. This is where tools come in, whether simulation tools or hardware tools, accessible to lifelong learners.

Affordable Tools for Lifelong Learners

One of the activities of my group at the University of Colorado, Boulder, is curating tools for lifelong learners that are either free or affordable on a personal budget. This means they come with a price tag of $0 to $1000 while providing high value for exploring SI/PI problems and solutions.

These tools will not substitute for professional-level tools. They will never design your 224 Gbps system, but they may aid in understanding how current flows in a microstrip or what an S-parameter file looks like as a differential TDR response.

Some of these tools have been mentioned in previous columns, such as

• Low-Cost and Free Tools Fit for an Engineer’s Personal Budget
• Ansys Releases Free Electronics Desktop Student Product Download
• Sierra Circuits Releases a Free, Online, 2D Field Solver
• A Free Impedance Analyzer on your Desktop
• A Low-Cost Capacitor Characterization System

Four Animation Tools Showing the Dynamic Nature of Signals

In this column, I will reveal four animation/simulation tools available from Teledyne LeCroy. Full disclosure: I am a Fellow with Teledyne LeCroy. However, I use these tools all the time in my classes and lectures. They are completely free, with no registration required, and once downloaded, run on your PC. The links lead to a landing page on the Signal Integrity Academy that includes a brief video description of the tool and how to use it, with a link to download a zip file containing the executable.

These tools were created by Yoshi Tsuji, the Application Engineering Manager at Teledyne LeCroy-Japan. Teledyne Lecroy's value in making them freely available is the brand association, as there is a prominent Teledyne LeCroy logo on the page for these tools. This is the only commercial aspect of the tool.

The chief value of these tools to a lifelong learner is that they illustrate the dynamic nature of signals. Many of the problems in signal integrity arise because signals always propagate, and as they propagate, they interact with the impedance changes, resulting in reflections. You cannot understand signal integrity unless you understand the dynamic nature of propagating signals, which these tools illustrate well.

The four tools are:

• TDR reflection animation tool
• Current propagation animation tool
• Crosstalk animation tool
• Ripples in return loss animation tool

Reflections in Propagating Signals

The first tool is a TDR reflection animator. You can set up four different transmission line topology circuits and adjust the characteristic impedance and time delay of each transmission line. You can also adjust the source voltage, rise time and impedance, and load impedance. Figure 1 shows an example of the screen.

Figure 1. Reflections on an interconnect.

Once the geometry is set, you simulate the behavior of a signal on the interconnect and can watch the dynamic behavior of the signal in real time as it encounters changes in the impedance, keeping track of all the multiple reflections. This tool will help you develop intuition about the dynamic nature of signals, that they are always propagating, and that the propagation of the signal interacting with all the impedance changes is one source of signal distortion.

Current Flow in a Transmission Line

Signals are both a voltage change and a current loop propagating down a transmission line. Coincident with the voltage change is the current wavefront propagation between the signal and return path. This is no better illustrated than in the current animation tool, in which you can watch the current loop flowing between the signal and return conductors propagating down the transmission line.

Figure 2. Snapshot view of the signal propagating on a transmission line.

The Origin of NEXT and FEXT Crosstalk

Crosstalk on transmission lines is confusing because there is a very different signature at the two ends of the victim transmission line. This is due to two important features. One is the circulation direction of the induced current loop in the victim line. This means that capacitively coupled noise looks very different in the two directions than inductively coupled noise due to the dynamic nature of the inductively induced noise.

Figure 3. Crosstalk from capacitive and inductive coupling.

The second feature is the dynamic nature of the aggressor signal propagating on an aggressor line and the induced noise propagating on the victim line.  As the aggressor signal propagates in the forward direction, the forward coupled noise propagates coincidentally, and the noise builds up like a snowball. It is very difficult to imagine this behavior. That is where this tool excels. There is no better way to visualize the dynamic nature of signal propagation in crosstalk.

 How Return Loss Gets its Ripples

S-parameters have become the de facto standard in describing the electrical properties of interconnects. One of the most common features in the return loss from a uniform transmission line is ripples. They are fundamentally due to the interference at the source from reflections off the front and back of the transmission line. This behavior is difficult to understand without thinking about and visualizing the dynamic nature of the sine wave signals propagating back and forth through the interconnect.

Figure 4. Dynamic nature of sine waves interfering back at the source.

This is where this tool comes in. It helps visualize how a sine wave reflects from the front and back of the transmission line and how the interconnect length affects the interference of these two signals. This tool helps visualize why the ripples in return loss arise.

Conclusion

None of these tools will help you design your high-performance product. They are not a substitute for high-end professional-level tools; however, they will help you understand the results from high-end tools and accelerate the learning curve, which is the goal for all of us as lifelong learners.