Testing Cable Signal Integrity with Eye Diagrams using Advanced Cable Tester v2

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As data rates increase, maintaining cable signal integrity becomes more important and challenging. Even small amounts of signal degradation, including signal loss, distortion, or noise, can affect a cable’s ability to reliably transmit high-speed data. For cable manufacturers and engineers integrating cable assemblies into their designs, validating performance is essential, but detailed signal integrity measurements can require specialized equipment and time-consuming setup.

The Advanced Cable Tester v2 simplifies cable validation by transmitting high-speed differential signals through the cable and analyzing the received signal using eye diagrams. It provides a fast and cost-efficient way to evaluate high-speed cable assemblies, including USB, HDMI, and DisplayPort cables, while also performing additional electrical tests such as pin continuity, DC resistance, and E-marker validation for a complete cable testing solution.

How Cable Design and Manufacturing Affect Signal Integrity

High-speed cables, including USB, HDMI, and DisplayPort cables, are critical to maintaining reliable signal transmission between systems. As data rates increase, even small amounts of signal loss, distortion, and noise can reduce timing and voltage margins needed for a receiver to correctly interpret data.

Cable construction and manufacturing tolerances play an important role in maintaining signal integrity. Factors such as conductor geometry, shielding, insulation materials, connector quality, and overall cable design influence how signals propagate through the cable. Variations in these characteristics can introduce signal degradation, including increased attenuation, impedance mismatches, crosstalk, and jitter.

Because signal integrity depends on both cable design and manufacturing consistency, validating production cable assemblies is an important step in ensuring reliable performance. A cable design may meet the requirements of a given specification, but variations introduced during manufacturing can affect the electrical characteristics of the final assembly. Testing helps identify these variations and verifies that cables consistently maintain the signal quality required for their intended data rates and applications.

Signal Integrity Validation for High-Speed Cables

Traditional signal integrity characterization often relies on equipment such as VNAs and TDR systems, which provide detailed electrical measurements of cable characteristics such as insertion loss, impedance, and reflections. However, these measurements require specialized equipment, extensive setup, and technical expertise, making them costly and time-consuming for routine production testing.

The Advanced Cable Tester v2 provides an alternative approach for quickly verifying the signal integrity of high-speed cable assemblies, including USB, HDMI, and DisplayPort, at a more affordable cost. By automating eye diagram analysis and evaluating cable performance against predefined cable specification criteria or custom test profiles, engineers can efficiently determine whether cable assemblies meet required signal quality criteria. This enables manufacturers to incorporate signal integrity testing into production workflows and verify more cable assemblies than would typically be practical with traditional characterization methods.

How the Advanced Cable Tester v2 Performs Signal Integrity Testing

The Advanced Cable Tester v2 evaluates high-speed cable signal integrity by transmitting a PRBS9 (Pseudorandom Binary Sequence) test signal through the cable at the selected data rate. The test measures the quality of the complete differential channel by analyzing the received signal across the relevant differential pairs, providing a rapid assessment of whether the cable can reliably support its intended data transmission requirements.

During testing, the Advanced Cable Tester v2 captures a large number of signal transitions over an approximately 2.5 ms test window per channel to generate an eye diagram representation of the received signal. The Signal Integrity test evaluates differential pair performance within the cable and is configurable from 1030 MHz up to 12.8 GHz on up to 5 differential pairs, depending on the cable type and test profile.

The Advanced Cable tester V2 supports the following cable types:

• USB Type-C to USB Type-C
• USB Type-C to USB Standard-A
• USB Type-C to USB Micro-B
• USB Type-C to USB Standard-B
• USB Standard-A to USB Micro-B
• USB Standard-A to USB Standard-B
• HDMI Type-A to HDMI Type-A
• DisplayPort to DisplayPort

Evaluating Signal Integrity with Eye Diagrams

The resulting eye diagrams represent the quality of the received differential signal after traveling through the cable and test channel. By overlaying many signal transitions, the eye diagram reveals the combined effects of jitter, attenuation, noise, and other sources of signal distortion that affect signal quality. A wider eye opening indicates more available timing and voltage margin for the receiver to correctly interpret data, while a narrower eye indicates increased signal degradation and reduced communication margin.

The eye opening percentage quantifies the remaining signal margin relative to the reference signal level. Because higher frequencies experience greater insertion loss as they travel through a cable, higher-frequency measurements typically show a smaller eye opening compared to the lower-frequency reference measurement.

To determine pass/fail criteria, the Advanced Cable Tester v2 uses insertion loss requirements from the selected cable standard to simulate a worst-case channel. This simulation accounts for the expected signal behavior through the test channel and a cable operating at the allowable loss limit defined by the specification.  The simulated eye opening establishes the minimum acceptable Horizontal Eye Opening (HEO) and Vertical Eye Opening (VEO) values required for a passing result. These HEO and VEO limits are used to generate the eye mask displayed during testing.

During testing, the Advanced Cable Tester v2 compares the measured eye diagram from the cable under test against these limits. If the measured signal maintains sufficient horizontal and vertical eye opening without intersecting the mask boundary, the cable passes.

 

Accounting for Signal Loss with De-Emphasis

The Advanced Cable Tester v2 supports de-emphasis during signal integrity testing for USB, HDMI, and DisplayPort cables to account for signal loss that occurs at higher frequencies. De-emphasis is a technique that adjusts the transmitted signal to compensate for the frequency-dependent signal loss that occurs as high-speed signals travel through a cable. By reducing the amplitude of lower-frequency signal components relative to higher-frequency components, de-emphasis helps compensate for channel losses and improve the quality of the received signal.

Because different cable standards use different signaling requirements, the appropriate de-emphasis setting depends on the interface being tested. For HDMI and DisplayPort cables, the Advanced Cable Tester v2 automatically evaluates de-emphasis settings between 0 to -15 dB and selects the value that produces the widest eye opening for qualification. For USB, a fixed value of about -3.5dB is used. By default, the Advanced Cable Tester v2 applies de-emphasis for supported high-speed tests, but it can also be disabled when evaluating custom requirements.

Testing Cables Using Custom Test Profiles

While the Advanced Cable Tester v2 includes predefined test profiles for many common cable standards, some applications require customized validation criteria. Engineers developing cables with requirements that differ from standard specifications, or with stricter internal quality requirements, can create custom test profiles tailored to their specific validation needs.

For signal integrity testing, custom profiles let users adjust the tested bitrate for both High-Speed and SuperSpeed differential pairs, as well as modify the insertion loss values used to establish the eye mask requirements for each pair. Stricter insertion loss limits create more demanding HEO/VEO requirements, while more permissive limits allow testing against a wider range of cable performance levels. Users can also define custom HEO/VEO requirements based on known-good cables or internal qualification standards. This flexibility allows manufactures to adapt testing criteria for different cable designs while maintaining the automated testing workflow and repeatability offered by the Advanced Cable Tester v2.

To learn more about creating custom test profiles and defining insertion loss parameters, read our Insertion Loss application note.

Conclusion

As cable data rates continue to increase, validating signal integrity has become a critical part of developing and manufacturing reliable high-speed cable assemblies. The Advanced Cable Tester v2 simplifies cable validation by automating signal integrity testing, eye diagram generation, and evaluation of quality standards.

To learn more about the Advanced Cable Tester v2 and other Total Phase tools for SPI, I2C, USB, and CAN please contact sales@totalphase.com or request a demo.