CST – Computer Simulation Technology

USB 3.1 Type-C Simulation and Compliance Test

With the continued adoption of USB interface, there is a need to adapt the USB technology with the newer devices as they become smaller, thinner, lighter and faster. Several types of USB connector are currently available on the markets, including the Type-A, Type-B, Mini-A, Mini-B, Micro-A and Micro-B. All of these different types of USB connector are by design difficult to insert incorrectly when mating the USB plug and receptacle. However, it is still difficult for the user to mate the USB connector correctly since the visible side to be distinguished between plug and receptacle is not clear at the user eye height level, especially for the micro connector.
The new design of USB connector now allows the user to insert the connector on both sides. The new Type-C connector is reversible and smaller compared to the micro-B connector and will be available for the USB 3.1 generation. Several USB generations are shown in Figure 1....



Figure 1: Table of USB Generations

The 3D CAD models of the USB 3.1 Type-C plug and receptacle are shown in Figure 2. They represent the USB 3.1 Type-C full featured model, which consists four differential pairs for SuperSpeed mode and one differential pair for high speed mode, the USB 2.0. The four SuperSpeed differential pairs are TX1+/- , RX2+/- , Rx1+/-, and TX2+/-. This signals are located at the top and bottom of plug and receptacle tongue and isolated by the middle plate inside the tongue. This middle plate needs to make contact with the grounding points in order to reduce the EMC. The receptacle pin assignments (looking into the product) are shown in figure 3.



Figure 2: Connector models of USB 3.1 Type-C receptacle (top) and plug (bottom), courtesy of Simula Technology Inc


Figure 3: Pin assignment of USB 3.1 Type-C full featured

Figure 4 shows a mated connector as defined in the specification of USB 3.1 Type-C consists a receptacle and plug mounted on the PCB test fixture. In contrast to previous generations, the USB 3.1 Type-C specification of the electrical characteristics consists of two specifications: the informative and normative. The informative specification can be viewed as guidelines rather than hard requirements. Failing the informative specification doesn't mean that it will not pass the compliance testing. Furthermore, the compliance program will also not test the informative specification items. The informative specification items are: differential TDR, differential insertion and return loss, X-talks limits between SuperSpeed pairs and X-talks between D+/- and SuperSpeed pairs. The normative specification is the hard requirement and requires a design improvement if the device fails compliance testing. All the compliance tests come from the normative specification as well.



Figure 4: Mated USB 3.1 Type-C connector mounted on the pcb test fixture

The USB 3.1 Type-C cable to cable assembly is part of the normative specification and illustrated in Figure 5. The cable assembly length is 1 m and modeled using the CST CABLE STUDIO®. The cable cross section is also depicted in the Figure 5 and uses the coaxial wire for four SuperSpeed pairs and twisted cable for the high speed pair (in the center). All the wires diameter is based on the 34 AWG which is about 0.16 mm. The overall cable diameter is approximately 3.36 mm. One should note that the additional wires, Vconn, SBU and power are not modeled.

For compliance test, all of the S-parameter results are combined using circuit simulator, CST DESIGN STUDIO™ (CST DS). The mated connector is simulated using the 3D time domain solver with single ended port configuration. The total number of ports are 24. Since the test points 1 (TP1) and 2 (TP2) consist of identical connectors, only one simulation of the connector is carried out, since the results can be duplicated. The simulation of the cable assembly uses CST CABLE STUDIO® and the S-parameter results are then imported in CST DS and combined. The schematic representation of this configuration is shown in Figure 6. Please note that the high speed pairs, D1+/- and D2+/- are shorted at the cable interface. The differential mode and common mode ports are normalized to 90 ohm and 22.5 ohm respectively.



Figure 5: Illustration of test points for mated cable assembly and its cross-section view


Figure 6: Mated connector and cable assembly schematic for compliance test

Once the complete results are available inside CST DS, the compliance test tool for the USB 3.1 Type-C can be launched in order to evaluate the device performance. The tool will perform the test for all the SuperSpeed Pairs, TX1+/-, TX2+/-, RX1+/-, and RX2+/-, based on the normative specifications, which are the Insertion Loss Fit at Nyquist Frequency (ILFitatNq), Integrated Multi-reflection (IMR), Integrated Crosstalk between SuperSpeed Pairs, Integrated Return Loss, and Differential to Common Mode Conversion. The pass/fail criteria will be also evaluated as the final results. The compliance test tool is shown in Figure 7. One should also note that the connector model is still underdevelopment and certain parameters currently fail the specification.



Figure 7: USB 3.1 Type-C compliance test tool

One of the important parameter in order to estimate the passive channel performance is the Insertion Loss Fit at Nyquist Frequency (ILfitatNq). It measures the attenuation of the cable assembly without including the multi reflection. The multi reflection is represented from the ripples in the S-parameter. The insertion loss of the S-parameter results are fitted using the smooth curve function in order to obtain the ILfitatNq. The plot of the Insertion Loss Fit at Nyquist Frequency and Channel Insertion Loss curves is shown in Figure 8. For the USB 3.1 Type-C to Type-C cable assembly, the ILfitatNq must meet the requirements:

  • • -4 dB at 2.5 GHz (5 Gbps data rate, SuperSpeed Gen 1)
  • • -6 dB at 5 GHz (10 Gbps data rate, SuperSpeed Gen 2)
  • • -11 dB at 10 GHz (possible future data rate 20 Gbps)

Based on this given requirement, all the compliance tests will automatically performed, which provides a report about the channel's electrical performance.



Figure 8: ILFit at Nyquist Frequency from S1,3

Conclusion

The simulation of a USB 3.1 Type-C connector has been presented. The time domain solver has been used to estimate the electrical performance of this connector. The cable assembly performance tests are also applied by cascading the S-parameter of the connector and the cable inside the CST DESIGN STUDIO™. The additional compliance test tool is applied afterwards in order to estimate the electrical performance, especially passing the normative specification. Important parameters from normative specification such as Insertion Loss Fit at Nyquist Frequency (ILfitatNq), Integrated Multi Reflections (IMR) and integrated crosstalk between the SuperSpeed pairs are calculated by this compliance test tool.

References

[1] Universal Serial Bus Type-C Cable and Connector Specification Revision 1.0, August 11th 2014.

[2] Methodology Used to Determine SuperSpeed USB 10 Gbps (USB 3.1) – Gen2 Channel and Cable Assembly High Speed Compliance, Yun Ling and Kuan-Yu Chen, April 23rd 2013.

[3] Simula Technology Inc. (http://www.simula.com.tw)

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