Testing DisplayPort 2.0 vs. USB4 Over USB Type-C Connectors

Figure 1: The pin out of the USB Type-C connector.

DisplayPort™ 2.0 (DP 2.0) is a high-resolution video interface and USB4® is a high-speed data interface; what they have in common is the USB Type-C® (USB-C) connector. While DP 2.0 can also be deployed on the standard DisplayPort as well as mini-DisplayPort connectors, it is the USB-C connector that really excites electronics manufacturers because now they can use a single connector for high-speed data, high-resolution video and even power distribution.

Figure 1 shows the pin assignments for a USB-C connector, which is a mechanically reversible connector that includes four, high-speed differential data lines: TX1, TX2, RX1 and RX2.

In USB4 operations, the four data lines TX1, TX2, RX1 and RX2 form a dual-lane, duplex signal path, supporting 10 and 20 Gb/s transfers on each line. When operating in USB4 Alt mode, up-to-four of these buses can be reassigned to become four DP 2.0 video lanes, which operate at 10, 13.5 or 20 Gb/s.

Testing for both interfaces over the USB-C connector is similar, but there are some notable differences.

1.Terminology

USB4 uses the terms transmitter (TX) and receiver (RX), while DP 2.0 uses the terms Source and Sink, respectively, to refer to the same parts of the link.

2. Test Equipment

Figure 2: DPC-TPA-P plug test fixture adapts
Source port to SMA coaxial connectors. Similar
USB4 fixtures use SMP coaxial connectors.

For the most part, testing for both interfaces requires the same equipment: a wide-band oscilloscope, a BERT and a high-speed interconnect analyzer or VNA. Teledyne LeCroy oscilloscopes that meet the bandwidth requirements for speeds up to 20 Gb/s are LabMaster 10Zi-A (best for multi-lane testing) and WaveMaster®/SDA 8 Zi-B.

However, because DP 2.0 uses the USB4 Alt mode, it also requires an AUX-mode controller for initiating test patterns in Source testing and Sink BER testing.

Test fixtures and cables also differ. Figure 2 shows the DisplayPort DPC-TPA-P Source plug test fixture, which breaks out the signal to special, low-loss “golden” coaxial cables for direct input to the oscilloscope. Similar USB4 fixtures use SMP coaxial connectors.

3. Tests

DP 2.0 testing is done on four lanes (rather than two), including three lanes of crosstalk measurement on the Sink, which differs from USB4 testing.

In addition, DP 2.0 has to test the extra 13.5 Gb/s clock rate, which is not specified for USB4. This requires a 25 GHz oscilloscope to meet the bandwidth requirement.

Some test limits differ. For instance, the DP 2.0 Source jitter is defined for a BER level of e^-9, while USB4 TX jitter is defined for a BER level of e^-13.

DP 2.0 also requires de-embedding the test fixtures. The required S-parameters can be obtained with a VNA, or the WavePulser 40iX High-speed Interconnect Analyzer.

While there is a great similarity in the physical-layer testing of USB4 and DP 2.0 over the USB-C connector—after all, it is the same physical layer—these subtle differences benefit from the addition of QualiPHY compliance test software to automate the test process.

Teledyne LeCroy offers the QPHY-USB4-TX-RX option for testing USB4 over the Type-C connector, and the QPHY-DP2.0-SOURCE and QPHY-DP2.0-SINK options for testing DisplayPort 2.0 over the Type-C connector.

Note: Although the DisplayPort 2.0/2.1 CTS is still under development, QualiPHY software is updated continually to meet the changing requirements.

Watch Mike Engbretson explain the tests in our on-demand webinar, "DisplayPort 2.0 Compliance Test Overview, Pt. 2, PHY Testing."

Test Happens.

06 December 2021