Discover the eye diagram, a very useful visual tool to analyze the quality of your connections in digital and high frequency applications.
As the frequency of your signal increases, the effect of connectors, wires, and contacts is critical because those elements cannot be considered “transparent” for your signal. This topic can be applied to many electronic applications and fields, but it is especially important for digital designers.
How can you be sure your digital signal is being transmitted in a good way through a set of traces, wires, connectors, or junctions? How can you be sure the signal will not be distorted creating a reliability problem (too high Bit Error Rate – BER)? Note that additionally to signal distortion, we will increase the emissions and reduce immunity of your system from an EMI point of view.
The tool to check that situation is the “eye diagram.” You can plot the eye diagram of your signal overlaying hundreds of transitions of that signal with your scope in PERSISTENCE mode. In that way, hundreds of transitions of the signal going from 1 to 0 and 0 to 1 can be analyzed in a (usually) colored display. Allow me to explain a generic eye diagram as shown in Figure 1 (left).
You see a sequence of transitions from the data signal happening every clock cycle. Note the figure looks like an open eye. The open eye shows the safe time for sampling. The amplitudes of the eye give you the voltages to be sure you have good data. A clean, wide open eye means you have a good configuration.
With the eye diagram, you can check the jitter (changes in switching times), inter-symbol interference (ISI), asymmetries in the waveforms, and more. And, as a designer, you can use “MASKS” shown in Figure 1 (right) so violations of forbidden areas can be identified automatically. Scopes such as the new RTO family from R&S have really incredible capabilities for this application with special triggering options to detect the problem.
I will use the eye diagram in a simple example for you.
Consider you have a digital system with a 20MHz clock and a DATA signal. The signal represents the communication between two PC boards, Board #1 and Board #2, shown in Figure 2, that are separated by some distance.
How can you check the quality of the connection? What about length limitations? What about quality of cable and connectors?
The first step is to estimate or measure the maximum frequency of your signal. For my digital data, I measured rise and fall times looking for the smaller of those values. Using my 200MHz bandwidth Picoscope 5444B, I measured 7ns for rise time and 5ns for fall time.
With that information I will consider bandwidth (maximum frequency) for the signal around 65MHz (wavelength λ = 4.7 meters). As a general rule, we can consider that distances of more than λ/20 = 23.5cm could create problems. Let see how we can test that situation.
In Figure 3 (left), you can see the 20MHz clock (top trace) used for triggering. Trace in red color is our data signal. In the scope screen that signal is continuously changing and you need to stop acquisition to observe the trace. That situation was created connecting boards with a very short connection (5mm). No signal distortion is present. In Figure 3 (right), we switch to persistence mode. Note that the clock signal is clearly stable and a really good open eye is present for data trace. Signal integrity is guaranteed.
Now, we will use a longer connection with two wires that are 25cm in length. We are in the limit where problems can arise. In Figure 4, you can see how the data signal (left) includes some distortion. The eye diagram (right) is not as clean as in previous example but we can consider this as an acceptable situation.
Finally, an additional 8cm cable with a small connector is inserted, increasing the total length of our connection to 33cm and including the imperfections of the connector. In Figure 5 (left) the data signal is clearly distorted and the eye diagram (right) is proof that the connection will create problems in the final application. In this case, we are using a very long connection for the transitions of our digital technology.
For applications where switching times are in the nanosecond range (or less than 1ns), maximum distances are limited to some centimeters and distortion will be related with the layout of your traces (you do not need to use cables to suffer the problem). As usual, a visual tool is a great way to understand our designs.
So, my last piece of advice: keep your eyes open!
Arturo Mediano received his M.Sc. (1990) and his Ph. D. (1997) in Electrical Engineering from University of Zaragoza (Spain), where he has held a teaching professorship in EMI/EMC/RF/SI from 1992. From 1990, he has been involved in R&D projects in EMI/EMC/SI/RF fields for communications, industry and scientific/medical applications with a solid experience in training, consultancy and troubleshooting for companies in Spain, USA, Switzerland, France, UK, Italy, Belgium, Germany, Canada and The Netherlands. He is the founder of The HF-Magic Lab®, a specialized laboratory for design, diagnostic, troubleshooting, and training in the EMI/EMC/SI and RF fields at I3A (University of Zaragoza), and from 2011, he is instructor for Besser Associates (CA, USA) offering public and on site courses in EMI/EMC/SI/RF subjects through the USA, especially in Silicon Valley/San Francisco Bay Area. He is Senior Member of the IEEE, active member from 1999 (Chair 2013-2016) of the MTT-17 (HF/VHF/UHF) Technical Committee of the Microwave Theory and Techniques Society and member of the Electromagnetic Compatibility Society. Arturo can be reached at firstname.lastname@example.org. See more at www.cartoontronics.com.