Service Manuals, User Guides, Schematic Diagrams or docs for : Agilent English _ 2014-03-03 _ PDF 1.52 MB 5991-1148EN c20140829 [12]

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Keysight Technologies
Characterizing Hi-Speed
USB 2.0 Serial Buses
In Embedded Designs



        Application Note
Introduction

     The hi-speed USB 2.0 serial bus is used today in a broad range of computer as well as embedded
     designs. One good example of an embedded design is the oscilloscope itself, which often includes
     USB interfaces for connectivity, mouse operation, and external data storage. Most other types of
     electronic products, such as medical equipment or industrial control systems, include USB
     interfaces as well. The USB 2.0 serial interface has been rapidly replacing older RS-232 serial
     interfaces in embedded designs.

     Although USB-IF physical layer compliance certification is typically required by computer OEMs for
     suppliers of USB devices and silicon chip-sets, compliance certification is typically not a requirement
     for embedded products. Nonetheless, R&D testing and verification of physical layer characteristics of
     embedded designs with integrated USB interfaces is extremely important to ensure reliable operation
     of end-products. Simply selecting USB components, integrating them into an embedded design, and
     then hoping that everything functions is not good enough. Even if the system appears to function, how
     much margin does it have? Or how does it perform under various environmental conditions such as
     temperature or humidity?

     When debugging and verifying the performance of hi-speed USB 2.0 designs, the Keysight
     Technologies, Inc. InfiniiVision 4000 and 6000 X-Series oscilloscopes offer several advantages
     over many higher performance oscilloscopes that are typically used for full compliance testing.
     One obvious advantage is the lower price of the 4000 and 6000 X-Series oscilloscopes. But the
     advantages of the scope go beyond just price. Although many higher performance Windows-based
     scopes have been optimized for advanced waveform analysis, Keysight's InfiniiVision 4000 and 6000
     X-Series oscilloscopes have been optimized for signal visualization and debug.

     This application note begins with a discussion of probing the hi-speed USB 2.0 serial bus using
     Keysight's N2750A InfiniiMode Series differential active probe. We then show some of the unique
     debugging tools and capabilities of the Keysight 4000 and 6000 X-Series portable bench-top scopes
     that can help you get your embedded designs to market faster.
03 | Keysight | Characterizing Hi-Speed USB 2.0 Serial Buses In Embedded Designs - Data Sheet



       Probing the hi-speed USB 2.0 differential bus


       Since the hi-speed USB 2.0 bus is differential, a differential
       active probe must be used to capture and analyze signals.
       Keysight recommends using the N2750A InfiniiMode
       Series of differential active probes shown in Figure 1. This
       family of differential active probes comes in three different
       bandwidth models ranging from 1.5 GHz to 6 GHz. Even if
       using the lowest-priced 1.5-GHz model (N2750A), which
       actually has a typical bandwidth of 2.0 GHz, the combined
       system bandwidth (including a 1.5-GHz bandwidth
       scope) is typically 1.5 GHz. This is the USB-IF's minimum
       recommended oscilloscope bandwidth for hi-speed
       USB applications.

       The N2750A Series probe is more than just a differential
       probe. With the press of a button on the probe, you can
       quickly toggle between viewing the differential signal, high-                    Figure 1: Keysight's InfiniiMode N2750A Series differential active probe.
       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/or improper terminations related to just one
       side of the bus.

       Figure 2 shows an example of viewing the hi-speed USB
       bus differentially. Note the significant level of noise on
       this differential signal. Although it can't be shown is this
       document, the noise was intermittent; sometimes the
       captured packets were relatively noise free, and sometimes
       they contained a significant level of noise as shown in this
       particular screen image.

       With the press of a button on the Keysight InfiniiMode
       probe, we can view just the high-side (D+) of the USB                            Figure 2: Capturing the hi-speed USB differential signal.
       bus as shown in Figure 3. In this example, the noise level
       on the D+ side of the bus was acceptable and measured
       significantly less than the original differential bus
       measurement.




                                                                                        Figure 3: Capturing just the USB high-side (D+) of the differential bus.
04 | Keysight | Characterizing Hi-Speed USB 2.0 Serial Buses In Embedded Designs - Data Sheet



       Probing the hi-speed USB 2.0 differential bus


       If we press the InfiniiMode probe button once again, we
       can then view just the low-side (D-) of the USB bus as
       shown in Figure 4. The intermittent noise reappeared when
       monitoring this side of the bus. Noise is evidently coupling
       into just one side of our differential USB bus.

       Lastly, we can also view the common-mode signal of this
       hi-speed USB bus as shown in Figure 5. This screen image
       shows the common-mode DC offset of each packet, along
       with common-mode noise and signal coupling.

       The next step in this debugging example was to attempt to
       discover the source of the noise and also try to determine
       why it coupled into just one side of the bus. After further
       troubleshooting, it was determined that the source of the
       noise was coming from the embedded system's switching                             Figure 4: Capturing just the USB low-side (D-) of the differential bus.
       power supply. Figure 6 shows a second channel of the
       oscilloscope used to simultaneously capture the output
       of the switching power supply. We can now see a clear
       correlation between the noise on captured USB packets
       (yellow trace) with the power supply's switching noise/
       ripple (pink trace). Note that the upper half of the scope's
       display shows waveforms using a wider timebase setting
       ( 1 

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