The low-speed, full-speed, and hi-speed USB 2.0 serial bus is used today for not only traditional computer/PC applications, but also for a broad range of embedded connectivity applications. For years, oscilloscopes have been the primary measurement tool used by electrical engineers to verify the signal integrity of their USB 2.0 serial bus designs. With the DSOX4USBSQ/DSOX6USBSQ signal quality test option licensed on a Keysight Technology, Inc. 4000 or 6000 X-Series oscilloscope, you can now quickly verify the analog quality of your signals generated by USB hubs, hosts, and devices based on USB-IF compliance standards.
Although USB-IF physical layer compliance testing and certification is not normally performed on embedded electronic products with USB 2.0 interfaces, for reliability purposes designers of embedded systems often need to test the physical layer of their designs based on USB-IF specified standards as a “reality check” to insure signal quality standards are met before releasing their products into production.
For USB 2.0-based products in the traditional computer/PC/peripheral industry where USB-IF physical layer testing and certification is normally performed, purchasing a complete suite of high-performance test equipment to perform full pre-compliance testing is often cost-prohibitive for smaller companies in this industry. But with Keysight’s USB 2.0 signal quality test option licensed on InfiniiVision 4000 or 6000 X-Series oscilloscopes, engineers now have a more affordable solution that can perform what many consider to be the most important series of USB 2.0 physical layer tests (signal quality) before running their final product through complete certification testing at a USB-IF designated workshop.
After running a USB 2.0 signal quality test, a complete test report with color-coded pass/fail measurement results are shown on the scope’s display with a scroll-bar to view all tests and screen images as shown in Figure 1. In addition, the complete test report can be saved as a HTML file for test documentation purposes. Figure 2 shows an example test report from a far-end, hi-speed device signal quality test. In this test, the device marginally failed the EOP bit-width test, but was granted a waiver.
Probing the USB 2.0 Differential Bus
To test USB 2.0 low- and full-speed designs, the only probes required are two 10:1 passive probes, which are shipped as standard accessories with every Keysight InfiniiVision X-Series oscilloscope.
To test USB 2.0 hi-speed designs based on pre-compliance standards with the appropriate device or host test fixture, 50-Ω SMA cables with SMA-to-BNC adapters are all that is required. For this use-model of testing, the test fixture is programmed to generate a specific test pattern. However, during the design and debug phase of product development, engineers often need to test “live traffic” in their hi-speed designs (non-compliance testing). In this case, a test fixture is not required, but a differential active probe with sufficient bandwidth is required. For this use-model of testing, Keysight recommends an InfiniiMode N2750A Series differential active probe shown in Figure 3.
The N2750A Series probe is more than just a differential probe. With the press of the InfiniiMode button on the probe, you can quickly toggle between viewing the differential signal, high-side (D+) relative to ground, low-side (D-) relative to ground, or the common-mode signal. Although ultimately it is the quality of the differential signal that really matters, if signal integrity issues do exist on the differential bus, they can often be caused by issues such as system noise coupling into just one side of the bus (or perhaps improper PC board layout and termination related to just one side of the bus).
Recommended Oscilloscope Configuration
The signal quality test option (DSOX4USBSQ or DSOX6USBSQ) is compatible with any InfiniiVision 4000 X-Series oscilloscope running on firmware version 3.10 or later, or 6000 X-Series oscilloscope. However, signal quality tests on hi-speed devices and hosts require an InfiniiVision 1.5-GHz or higher bandwidth model.
Although the USB 2.0 trigger and decode options are not required in order to run USB signal quality tests, if you plan to run these tests on “live traffic” (non-compliance testing), the USB trigger and decode options are recommended for isolating specific packets to test.
Even though one of the USB 2.0 signal quality tests is an eye-diagram mask test, the DSOX4MASK or DSOX6MASK mask test option is not required.
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