Application Note HL-101: Testing USB Connectors and Cables with

Application Note HL-101: Testing USB Connectors and Cables with
Application Note HL-101: Testing USB Connectors and Cables with a High-speed TDR
In this Note, we will discuss the merits of testing USB connectors and cables using a highspeed differential time domain reflectometer (TDR) and a test point adapter (TPA).
We will also demonstrate how to perform these tests and read the results using the
HL2200USB, a USB-controlled TDR, and the HL9801, a USB-specific TPA, both from
HYPERLABS, Inc.
Required Hardware and Software
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HL2200USB Differential USB-controlled TDR
HL9801 USB 2.0 Test Point Adapter
Antistatic wrist strap*
Output software*
Computer running Windows XP or 32-bit
version of Windows Vista
* Included with the HL2200USB
Using a TDR and TPA for Testing
Figure 1: HL2200USB Differential
USB-controlled TDR
Signal integrity in USB cables depends on two major factors: the transition regions between
the connectors and the cable, and the impedance of the cable itself.
A high-speed TDR such as the HL2200USB (35 ps risetime) provides the resolution needed
to test the transition regions in the near-end connector. Discontinuities in these regions are
too close together to be properly measured by slower (200 ps risetime) TDRs.
Using a USB-specific TPA like the HL9801 simplifies the test process by allowing the
operator to quickly and directly plug in USB cables for testing.
Overview of the Testing Enviornment
The HL2200USB from HYPERLABS, Inc. is a high-speed
differential USB-controlled TDR. It is powered and controlled
via a single USB cable. The TDR connects to the device under
test (DUT) using the HL9801.
A PC running Windows XP or 32-bit Windows Vista is required
to perform the test and read the results. On 64-bit operating
systems, a virtual machine setup can be used.
To avoid static discharge, equip the antistatic wrist strap and
plug it into the HL2200USB before plugging the instrument into
the USB port on your computer.
Figure 2: Sample Test
Environment
Application Note HL-101 – Revision 1.0.1 / © 2012 - HYPERLABS, Inc. / www.hyperlabsinc.com
Basics of the Graphical Waveform
The HL2200USB software provides graphical and numerical output of its readings. Here,
we will focus primarily on the graphical output: the waveform.
Put simply, the waveform shown in the
HL2200USB software is a visual representation of
the electrical impedance within the DUT.
Figure 3 is the normalized waveform display of the
HL2200USB and the attached HL9801. This is the
view we will see in the software before plugging in
a cable to test.
The long, flat portion in the center of the waveform
shows the internal cabling of the test instrument. To
the right, impedance approaches infinity because the connection is not terminated.
Figure 3: Software Startup Screen
The software can also provide a significant amount of numerical data, including impedance
readings at any point on the waveform and measurements of cable or connector length. For
more information on the available numerical data, please see the product’s User Manual.
Overview of a USB 2.0 Cable Waveform
To create Figure 4, we acquired the waveform from a 1 meter USB 2.0 cable with an open
termination. In this plot, the X Axis is distance in meters and the Y Axis is the electrical
impedance in ohms.
To make it easier to identify the
parts of the waveform, we added
a colored bar below it.
The green band corresponds with
the instrument and adapter, with
an internal impedance of 50
ohms.
In the red band, we can see the
transition regions around the nearend connector, which we will
discuss in-depth on the next page.
Figure 4: Representation of Regions of a USB 2.0 Cable
The blue band follows the length of the cable. The impedance remains close to 45 ohms, or
90 ohms when measured differentially. This indicates that the cable does not have any
short circuits or major discontinuities.
Because the cable has an open termination, the wave form spikes upwards when it reaches
the end. In a properly terminated cable, the wave form would remain around 50 ohms. In a
cable with a short circuit in the termination, the waveform would drop back to zero.
Application Note HL-101 – Revision 1.0.1 / © 2012 - HYPERLABS, Inc. / www.hyperlabsinc.com
Testing the Transition Region of the Near-end Connector
Figure 5 zooms in on the transition regions around the cable’s near-
end connector. Connectors are more susceptible to manufacturing
defects and wear-and-tear than other parts of the cable, so it is
important to test them correctly.
For testing purposes, the transition region begins immediately
following the open edge of the TPA, found in this image at roughly
the 0.44 mark. This edge is also known as the reference point, from
which time or distance within the cable can be measured.
Here, we can actually see two distinct transition regions. The first,
from 0.44 to 0.47 in the waveform, is the transition between the
coaxial cable of the TPA and the USB connector of the TPA. The
second, from 0.47 to 0.52, is the transition between the USB
connector on the cable and the USB cable itself.
Figure 5: Close-up of
the Transition Region
This resolution is only available with high-speed TDRs such as the HL2200USB, and allows
cable manufacturers to more easily fine-tune their transition regions. With slower (i.e. lowerresolution) TDRs, the transition regions are lumped together and are harder to troubleshoot.
Within the transition region, there are several inductive (above 50 ohm) and capacitive
(below 50 ohm) discontinuities. These are due to imperfections within the USB connector
and/or the layout of the wire harness linking the connector to the cable.
After the transition region, at roughly the 0.53 mark, the waveform returns to values
between 45 and 50 ohms as the signal enters the length of the cable.
These discontinuities don’t necessarily indicate a faulty USB cable. Specifications for all
USB versions allow for maximum and minimum levels of discontinuities, which also depend
on the risetime of the test instrument being used. Please refer to the most current
specifications (available at www.usb.org) for details.
Conclusions and Additional Information
We have now finished testing a USB cable, including its connectors. To test another cable,
simply unplug the first cable from the HL9801 TPA, plug in a new one, and repeat. No
calibrations or resets are required to test the next cable.
There are other advantages to using the HL2200USB and HL9801 in tandem. Together,
they weigh less than a pound a fit in the palm of your hand. Plus, the HL9801 can be
inexpensively replaced if it wears out due to very high-volume testing (i.e. many thousands
of cables), without the need to replace the TDR instrument as well.
More about these products can be found on our website, www.hyperlabsinc.com.
Application Note HL-101 – Revision 1.0.1 / © 2012 - HYPERLABS, Inc. / www.hyperlabsinc.com
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