The Weak Link in USB-CTM: Why does inter-op still fail when I have passed TX and RX compliance certification?

Blog Post created by jitlim Employee on Apr 10, 2017

In this blog, we will address the issue of failing inter-op after getting certification for USB Type-C TX and RX compliance testing. This is particularly painful after the product is certified and shipped, and you have numerous customer returns and field failures.


The first step is to get the product back for retest and very often it passes in your lab.

The next, more complex step, is to get the link partner back. Very often it too passes.


Now what?


Take a look at a typical USB 3.1 link in Figure 1 below. In addition to the Host and Device link partners, there is a cable and connector that is part of the link. In the vast majority of cases, the cable/connector is not returned and tested as part of the root cause failure analysis.

Typical USB Link Components

Figure 1. Typical USB Link Components 


At 10Gbps for USB 3.1 Gen 2, the Type-C cable is a critical and arguably the weakest component of the link. One reason is that customers will often purchase or use whatever cables are available around them. And very often these cables are non-compliant.


A compliant cable is not just a wire but goes through a very rigorous regimen of tests before being certified. Let us 1) review these tests 2) understand why these tests are required and 3) how a failure can cause inter-op problems.



Reflections from impedance mismatches will cause signal degradation at the receiver and constraint the link capability and BER. Detailed impedance analysis will give a clue to the cables impedance, particularly where the cable mates with the connector.


Intra-Pair Skew / Differential to Common Mode Conversion

A tightly coupled differential signal has noise immunity, minimal common mode conversion, and excellent EMI susceptibility and emissions. Figure 2 illustrates a typical intra-pair skew measurement.

Intra-Pair Skew Measurement

Figure 2 Intra-Pair Skew Measurement


Propagation Delay

Link partners have latency limits before timing out and dropping the connection. The propagation delay spec ensures the maximum allowable latency time in the cable/connector path.



A receiver has a sensitivity spec and can recover a signal up to a certain amount of amplitude loss. The attenuation measurement ensures that the cable can contribute a specified amount of low frequency, amplitude loss.


Insertion Loss, Multi-Reflection, Crosstalk, Return Loss, and Shielding Effectiveness

High-frequency effects like IL and RL degrade the signal quality as it traverses the Type-C cable. Additionally, there are multiple very high-frequency signals running on the 4 TX/RX pairs in a Type-C cable that effects crosstalk and reflections. (Figure 3)

Frequency Domain Channel Metrics

Figure 3 Frequency Domain Channel Metrics


NEXT, FEXT, Coupling between Vbus, D+/D-, CC1/CC2, and SSP

Although the Vbus, D+/D-, and CC1/2 lines are very low speed compared to the TX/RX signals, they are often unshielded and susceptible to cross-talk and coupling affects. Analysis of these low-speed lines is crucial for proper inter-op and also proper negotiation of the USB-PD contract.


The following tips and techniques for Type-C cable/connector testing can help you with characterizing this link:

  • At 10Gbps, removing the test fixtures effects and test equipment calibration is critical to achieving the most accurate test results (Figure 4)
  • A Type-C cable also requires USB-PD testing for their E-Marker capability
  • A 12 port, multi-port VNA can significantly improve throughput with a single measurement
  • Switch-based automation of the VNA testing can also reduce your test time (Figure 5)


Calibration and De-embedding effects

Figure 4 Calibration and De-embedding effects


Automated calibration and analysis with switch-based solution

Figure 5 Automated calibration and analysis with switch-based solution


I hope this blog was helpful in illustrating the following:

  1. TX and RX electrical compliance is insufficient to assure inter-op.
  2. Testing the USB Type-C cable/connector performance is critical to ensure this portion of the link is also compliant.