What are the Guidelines for Testing Ground and Shield Resistance of USB Cables?

Question from the Customer:

I am using the new Advanced Cable Tester v2 to test our USB 3.2 Standard-A to Type-C cables and am running into some problems after creating a custom profile. So far, I have not been able to modify the setting for the ground resistance to fit the specification. Here is where the test report shows failure:

ACT test report GND test results ACT Test Report

How do I adjust the ground resistance settings for GND and GND+Shield A-Side Link?

Response from Technical Support:

Thanks for your questions!  There are some steps to follow when you modify a profile, including loading in the new parameters and restarting the test profile. In your case, there are measurements to consider. We’ll start with updating the test profile, how cables and plugs (shells) are typically designed, then move into measurement guidelines, and how to address any issues that may occur during measurements.

Updating the Customized Cable Test Profile

After modifying the test profile with the new parameters, the test process much be restarted.

  1. If needed, clone the existing profile.
  2. Enter the new parameters.
    ... DCR: GND Wire Resistance
    … DCR: GND+Shield A-Side Link (true/false, as explained below)
  3. Click the Save button
  4. On the main screen of the Advanced Cable Tester v2, click the Change button and select the new profile. Note: Even if the profile seems to be in-process, you still need to press the Change button for the Advanced Cable Tester v2 to load the updated profile.

 

Analyzing the GND+Shell Link

According to the recent USB Type-C specification, Standard-A to Type-C cables must tie GND+Shield together within each plug so that the shield braid can carry some of the return current. It can also help control electro-magnetic interference (EMI) by ensuring the shield braid is well grounded. Typically, the shell contact is not optimal on USB Standard-A cables.

  • The classic USB Standard-A plug mechanical designs typically have separate GND and Shield connections. This design makes it difficult to physically tie them together within the Standard-A plug. The wires can be soldered directly to the lead of the plug, but that makes it difficult to cleanly tie GND+Shield.
  • The GND and Shield wires must also be tied together within the Type-C plug.
  • The Type-C plug has a paddleboard, a printed circuit board (PCB). This is easier and cleaner because GND and Shield are tied together on the PCB.

With the physical challenges of the USB Standard-A plug wiring, many cables have only the Type-C plug paddleboard GND+Shield tied together.

Guidelines to Differentiate GND+Shield Methods

It can be difficult to differentiate the two cases: simple continuity checks would show "continuous" if either spot had the GND+Shield connection. Therefore, it is necessary to confirm that the USB-A side actually has a connection. This check is performed with the GND+Shield A-Side Link test.

Measuring Resistance

The GND+Shield A-Side Link test is unique from other DC resistance measurements. The upper limit is calculated at each test run. It is important to note, this calculation will not work with an extremely high Shell DCR; in that case, the calculated values would not be correct. The report that you provided shows 5.115 ohms, which is significantly high.

Here are two measurements to take:

Measurement 1: Standard-A Shell to Standard-A GND: the current path through the A-side link.

Measurement 2: Standard-A Shell to Type-C GND & Shell) + (Type-C GND & Shell to Standard-A GND: the roundtrip path the current would take through the entire cable.

Here is what the measurement results may indicate:

  • If Measurement 1 measures less than half of Measurement 2, it probably indicates that the current path is only through the Standard-A plug
  • If Measurement 1 measures more than half of Measurement 2, it probably indicates that the current path goes through to the Type-C plug and back.
  • Note: The measurements are divided in half to move the decision point. This provides some margin for noise or poor connections.

This evaluation is not effective when the shell contact resistances are very high relative to the roundtrip wire DCR. As the test results indicated your shell contacts were several ohms, this algorithm will not work. We recommend a physical examination based on these test results. The resistance of the shell may be reduced by cleaning the Standard-A plug.

Evaluating Physical Design

Although a high resistance shell contact is not ideal, it probably causes no harm in actual use. However, this information may be useful should a performance issue occur. To confirm a design, you can also review the design specifications, and/or disassemble the connector for physical analysis.

We hope this answers your questions. Additional resources that you may find helpful include the following:

If you want more information, feel free to contact us with your questions, or request a demo that applies to your application.

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