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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