In addition to USB, Type-C connectors support alternate implementation

How will USB-CTM develop next? Currently only as an optional/alternating mode of Thunderbolt 3, it is likely to become the trend in the future.

In addition to USB, the Type-C connector also supports the implementation of alternate technologies such as DisplayPort, Thunderbolt and HDMI.

The recent announcement by Intel will revolutionize future generations of Type-C implementations. In this article we will focus on two key points in the announced plan and their impact on you:

> Integrated Thunderbolt CPU

> Thunderbolt Open Specification

The adoption of Thunderbolt 3 encountered some resistance, including its high cost, proprietary technology, and severe signal integrity (SI) issues at 20 Gbps. Intel recently announced that it will eliminate the first two barriers to promoting Thunderbolt 3 by reducing costs and exempting license fees. However, the inherent signal integrity issues inherent in high-speed 20Gbps links using passive cables remain unresolved.

In order to take the lead in Thunderbolt 3 development, you should first register as a member and obtain the latest Specification and Compliance Test Specification (CTS) by visiting the following website ( https://thunderbolttechnology.net/ ). After you have obtained membership, you can also participate in the plug-in test conference (Plugfest) three times a year in addition to the specification documents, in which you can register the products you will test and get the latest updates of the technology.

Since the specification has not yet been announced, we will learn about Thunderbolt 3 from the following perspectives: how to transmit over an 80Gbs port, transmit on a passive cable, and receive 20Gbps at the other end in the presence of crosstalk. This is essentially the challenge of implementing Thunderbolt 3.

Previous Thunderbolt/Thunderbolt 2 technical tests were fairly straightforward because they used active cables and the eye diagrams on the transmitter and receiver sides were turned on. Thunderbolt3 uses passive cables, which makes testing extremely complex.

Now let's take a look at the methods and tests needed to successfully implement Thunderbolt 3.

Like PCIe Gen 3 and USB 3.1 Gen 2, Thunderbolt 3 requires transmitter equalization (TXEQ) to compensate for channel loss. First, a series of Thunderbolt 3 transmitter equalization presets must be characterized to ensure they accurately output the correct forward and de-emphasis for each particular preset combination. The next step is to perform a preset combination calibration/optimization and select a specific preset combination as the best preset combination for a particular transmitter implementation.

We need to apply a range of CTLE DC gains to determine the best performance. After determining the best CTLE, you also need to apply the best DFE to determine the best RXEQ for your particular channel. When implementing passive cable use cases, not optimizing this parameter will result in signal transmission failure.

Thunderbolt 3 supports interoperability with previous generation Thunderbolt2 products that are connected using an adapter cable. As a result, you need to test both Thunderbolt at TBT3-20.625 Gbps and Thunderbolt 2 at 10.3125 Gbps.

Thunderbolt 3 can only be run on a Type-C connector. Because the Type-C connector has CC1 and CC2, you need to test both the TX1/RX1 pair and the TX2/RX pair—four total differential pairs. Depending on the single port, dual port or quad port implementation, you will also need to test additional Type-C ports.

The TP1 test case is related to the use of passive cables. The TP3EQ use case comes in handy when your customers use passive cables. If your implementation does not support direct access to TP1, you must de-embed the TP1 channel to avoid extreme signal loss at 20 Gbps. Similarly, when you test a 10G or 20G use case on a TP3EQ, you must implement or embed a consistent cable model.

At first glance, many Thunderbolt 3 measurements seem familiar – UI, SSC, rise/fall time, jitter, and eye diagrams. However, the basic measurement methods for SSC phase slope, CDR order, phase-locked loop bandwidth and damping, continuous-time linear equalizer (CTLE), decision feedback equalization (DFE), and uncorrelated jitter are unique. It is described in detail in the Conformance Test Specification (CTS).

As shown in Figure 1, Thunderbolt 3 delivers up to 100W. This is a USB-PD related capability. Thunderbolt 3 is an alternating mode that runs on the USB Type-C connector. Therefore, it must meet the rules and testing requirements associated with the USB Type-C and USB-PD specifications. Details of these two specifications can be found on the USB-IF website: (Figure 1 also shows that Thunderbolt requires USB and DP implementation. For more information on USB testing, please visit: For more information on DP, please visit:)

Figure 1. Thunderbolt 3 technology

(Image courtesy of Thunderbolt Technology Community)

This blog post gives you a brief overview of how to prepare for the Thunderbolt 3 implementation and hopes to help you. Once the Thunderbolt 3 specification is announced, we will share more test details.

The Keysight V (90000X Plus) series of high-end oscilloscopes are based on Keysight's proprietary Indium Phosphide (InP) process technology developed with low noise floor and low jitter, and are therefore in the Thunderbolt CTS (Compliance Test Specification) specification. The TX test uniquely specifies the recommended product.

The Keysight M8020A Series BER tester is highly recommended for RX testing with its highly flexible expandability and built-in CDR and various jitter injection and adjustable ISI features.

In order to perform accurate and efficient preset combination calibration for the signals sent by the M8020A Series BER tester, the complete Keysight TX and RX test solutions are recommended:

USB Type-CTM and USB-CTM are trademarks of the USB Application Manufacturers Forum

ThunderboltTM is a trademark of the Thunderbolt Technology Community

Finally, Xiaobian will serve you again.

A section of ThunderBolt measured video

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