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Modifying Insertion Loss Values on a Cable Test Profile When Using the Advanced Cable Tester v2

The Advanced Cable Tester v2 supports testing a range of USB and video cable types. Upon testing, users can select relevant cable testing profiles that measure and test a cable against various criteria from the latest cable specifications, from pin continuity, DC resistance, E-Marker, to signal integrity values.

Advanced Cable Tester v2

The Advanced Cable Tester v2 is designed to give users a quick confirmation that a given cable sample can carry data at the advertised rate, can carry power at the rated levels, is safe to use, and wired up correctly. 

In certain cases, customers may want to test against values that are more or less stringent than the cable specification, allowing them to measure the cable per their own testing requirements. Specifically, users have expressed interest in modifying insertion loss values to change the size of the HEO/VEO mask seen on the eye diagram within the signal integrity test.

This is possible by creating a custom cable testing profile in the Advanced Cable Tester v2 GUI.

How the Eye-Diagrams are Generated for the Signal Integrity Test

The insertion loss curves within each test profile are taken directly from the cable specifications. The USB insertion loss curves are from the USB core specifications or the USB compliance specifications. DisplayPort curves come from the VESA specs, and HDMI from the HDMI specs.

USB specifications for signal integrity values are built with measurement criteria that would be captured with TDR (Time Domain Reflectometer) and VNA (Vector Network Analysis) equipment. As this equipment is expensive and time-intensive, the Advanced Cable Tester v2 uses an alternative approach that is closer in behavior to how end-user equipment would operate, which gives users an idea of a cable's capabilities more affordably, in a shorter length of time.

For our own signal integrity test, the main criteria for generating the eye-diagrams and masks is the insertion loss limit. While we don't measure this directly, we do use this as an input to our measurement approach. The Advance Cable Tester v2 test profiles contain an insertion loss limit curve, matching the relevant specifications. We use that curve as a model of "the worst-case cable." We also have loss curves for our cable tester PCB, modules, and models of the transmitter and receiver.

We use this all together to run a full software simulation of transmitting a PRBS-7 signal through the system and generating a “worst-case” eye diagram. If de-emphasis is used, this is factored in as well. This eye diagram is analyzed to determine the minimum horizontal and vertical eye opening (HEO/VEO) values as percentages of "fully open". We also generate our HEO/VEO mask from this simulation.

By transmitting an actual signal through the cable at the selected bitrate, the minimum HEO/VEO is compared to the actual observed HEO/VEO of the cable to determine a pass or fail. 

Modifying the Insertion Loss Values on a Custom Test Profile 

For customers wanting to test against marginal values, this may not be optimal using the default profiles. In such cases, customers may create a custom test profile so that all profile data can be modified or customized as needed. For complete instructions on how to create a custom test profile, see our App Note: How to Create a Custom Test Profile for the Advanced Cable Tester v2.

The best way for users to adjust the test profile to test for a worse-quality cable in terms of signal performance is to customize the insertion loss mask. This can be performed by adjusting the values in the Insertion Loss table/chart within the Signal Integrity settings. 

For Hi-Speed and SuperSpeed settings, users can adjust the Loss (in dB) relative to the Frequency (MHz) to change the shape of the eye mask within the eye-diagram, which is used to determine whether the cable passes or fails the signal integrity assessment.

 Hi-Speed Cable Insertion Loss Chart Settings



SuperSpeed Cable Insertion Loss Chart Settings

Based on the user’s inputs, either by adjusting the Freq or Loss values, the corresponding insertion loss chart and mask will adjust accordingly. This curve offers users a convenient visualization of the changes of these values.

For example, modern protocols like USB and DisplayPort can communicate two bits per Hz of bandwidth, so in the following chart, the loss with 10Gbit/s speeds would be at worst -33.

DisplayPort Signal Integrity Insertion Loss Curve

Insertion loss is typically expressed in decibels (dB) and is a negative value since it represents the reduction in signal strength. Our insertion loss chart depicts the dB value at specific frequencies. A higher dB value (more negative) means more signal loss at that frequency, while a lower dB value (less negative) corresponds to less loss.

By changing the insertion loss chart to be more permissive, the relative HEO/VEO mask on the eye diagram will shrink, and vice versa; by reducing the insertion loss values, the HEO/VEO mask, as seen in this eye diagram, will enlarge. The mask size is also affected by the bitrate being tested and the de-emphasis.

Passing eye-diagram taken at 10Gbps

The example test output below displays the signal integrity results taken from a USB Type-C to USB Type-C cable. This test examines how a cable performs when testing different data pairs against a specific data rate and de-emphasis. Each test uses an HEO/VEO mask to determine the pass or fail criteria.