Generating Representational Eye Diagrams with a VNA

March 22, 2016

As data rates and design complexity continue to increase, a host of variables can affect the integrity of signals, including transmission-line effects, impedance mismatches, signal routing, termination schemes, and grounding schemes. One tool used to monitor signal integrity quality is the eye diagram, which lets engineers quickly evaluate system performance and gain insight into the nature of channel imperfections that can lead to errors when a receiver tries to interpret the value of a transmitted data bit.

The VectorStar® Vector Network Analyzer (VNA) (figure 1) from Anritsu has the ability to generate an eye diagram representation of how the measured RF performance can have an impact on the digital data stream. This is particularly valuable in monitoring the data stream signal integrity issues that could occur with a given transmission path.

Figure 1

Figure 2 illustrates a generic eye diagram plot with key attributes. VectorStar generates a “heat map” type of eye diagram plot with the color tending to blue for low occurrences and working up through the rainbow to red for very high occurrences. VectorStar updates the eye diagram display via a trace-based process rather than a file-based method, eliminating the need to transfer SnP files and meeting the market need for a more efficient method to verify high-speed designs.

Figure 2

The ability of a VNA to generate accurate eye diagrams combined with time domain displays gives signal integrity engineers the ability to monitor the transmission quality of digitally modulated signals while simultaneously monitoring possible impedance mismatch problem area locations. This important display allows engineers to observe the likelihood of errors due to level compression, jitter, slew and edge distortion, as well as other effects. When a VNA is able to provide a trace-based rather than a file-based display, the VNA can provide the capability to show all key parameters, such as eye diagram, time domain/TDR, and S-parameters, on the same channel while continuously sweeping.

Eye Diagram Recommendations

To achieve these benefits and achieve a higher level of analyzing digitally modulated signals, signal integrity engineers should follow these five rules:

1. As with many time domain-based VNA measurements, using the widest frequency domain sweep range possible where the device-under-test (DUT) still has a response and is not radiating severely will lead to the most complete time domain response.
2. In terms of the calibration quality, cable/connector issues most strongly affect the higher frequency ranges. Paying greater attention to those aspects of the setup and calibration process provides the opportunity to accurately characterize the true RF performance of the DUT at high frequencies. It also properly delivers the intrinsic rise/fall time that starts to dominate the eye shape.
3. If only an eye diagram is needed, and depending on the level of mismatch involved, a transmission frequency response calibration (or a simple trace memory normalization) may be an adequate level of calibration. A full port calibration will reduce uncertainty and may be required for reasonable eye diagrams if the mismatch levels are high. In addition, de-embedding tools may be invoked prior to the eye diagram calculation.
4. When using jitter and rise/fall time entries, keep in mind that the limits on these scale inversely with the bit rate (as they tend to do in practice).
5. The eye diagram results are, effectively, simulations based on frequency domain data of the connected DUT. As such, they may not represent an entire system and they rely on relevant entries for noise, jitter, rise/fall times, and amplitudes.

Engineers interested in learning more about how to generate eye diagrams using a VNA can download a new Anritsu application note.