Real-world Certified Wireless USB testing

May 1, 2008 OpenSystems Media

As wireless USB makes its debut, designers are questioning if development and production testing align with realistic cost expectations. Using a test system built to capture and analyze wireless USB signals provides comprehensive test results that meet development and production needs within budget and time constraints.

The stars are now aligning for Certified Wireless USB. Chipset makers are finally providing silicon for band group 6 (the only band group universally approved), and more reference designs are starting to appear.

Itís only a matter of time before Certified Wireless USB will show up in real-world applications. Soon, users will simply plop their digital cameras down next to their desktops or laptops and transfer photos with the press of a button. Printers will turn a document or spreadsheet into a hard copy with nary a wire between laptop and printer. All the ubiquity that wired USB now offers will be further enhanced by wireless connectivity.

But before designers break out the champagne, they must weigh the realities of wireless USB pricing versus the costs of calibration and testing. As an offshoot of WiMedia Ultra-Wideband (UWB), Certified Wireless USB is being asked to do a lot of things: reflect the utility of wired USB, have low power consumption, coexist with other wireless connectivity modalities, and be low cost. At the same time, Certified Wireless USB must meet a challenging set of parametric specifications, which is where calibration and testing enter the equation.

A closer look at the technology

First, designers should take a brief step back and determine which of the two incompatible versions of wireless USB - WirelessUSB and Certified Wireless USB - they will use.

Backed by Cypress Semiconductor, WirelessUSB is a protocol that uses the 2.4 GHz band with a maximum data rate of 1 Mbps at a distance of 10 m and 62.5 Kbps at 50 m. This article will focus instead on Certified Wireless USB, sponsored by the USB Implementers Forum.

Certified Wireless USB spans 3.1-10.6 GHz, which is divided into 14 contiguous 528 MHz bands. Signals "hop" across three adjacent bands (called a band group, covering a spectrum of 1,584 MHz), characterized by fast hopping and short symbol times (242.5 nanoseconds). This technology uses Multiband Orthogonal Frequency Division Multiplexing (MB-OFDM), and depending on the intended data rate, uses Quadrature Phase Shift Keying (QPSK) or Dual Carrier Modulation (DCM), as shown in Table 1. Certified Wireless USB can pump out 480 Mbps at up to 3 m and 110 Mbps at 10 m.

Figure 4

The band groups of three 528 MHz bands have different regional regulatory constraints (see Figure 1), and only band group 6 is globally authorized. Thus, the newest chipsets are likely to converge around group 6, whereas the first chipset generation was group 1 oriented because of its allowance in the United States.

Figure 1

The UWB common radio platform supports a variety of band-hopping sequences, called Time Frequency Code (TFC), allowing designers to use only one band or hop between two or three bands (see example in Table 2).

Figure 5

Development and production testing

A fundamental difference exists between testing for development and testing for production. In development, designers are creating a capability that meets all aspects of a standard using components whose worst-case tolerance contributions produce an end result that still fits within specifications. Once that design is verified, however, designers no longer have to test all the same design aspects as before. Now, they are essentially trying to find production errors or faulty components.

It can be argued that for development, it makes no difference whether designers use a bunch of general-purpose instruments hooked together to accurately measure the necessary parameters or a purpose-built system designed specifically for testing wireless USB.

However, in todayís commoditized, hypercompetitive electronics market, time is of the essence both for development and production. Designs must be production ready within weeks rather than months, and high-volume production capabilities must be in place as soon as the design is verified.

Under these circumstances, designers donít have the luxury of using ad hoc instrumentation clusters for production testing. But it also can be argued that waiting for a purpose-built test system could slow development.

Development test requirements

A Certified Wireless USB device transmits and receives, and both functions need to be tested. On the transmit side, tests are needed for power level and setting accuracy, Error Vector Magnitude (EVM), power spectral density (against a spectral mask), and frequency offset. But thatís just the beginning. Designers also will want to test symbol clock offset, phase noise, compression, amplitude and phase balance, amplitude variation over time, and local oscillator leakage.

The reason for more exhaustive testing is to make sure a device meets all specifications. For example, it is very possible for a device to work functionally in all aspects - range, speeds, and error rates - yet produce interfering radiation above a prescribed level.

On the receive side, designers will want to test minimum power sensitivity at a specified Packet Error Rate (PER) and maximum power sensitivity at a specified PER. This will reveal if the deviceís dynamic range is within specification. But, here again, more testing is needed for the receiverís adjacent channel rejection, alternate channel rejection, and maximum tolerable signal to make sure it is not susceptible to interference at signal levels in other channels within specification.

Using general-purpose instruments for this level of development testing usually involves a lengthy process. These instruments must be capable of accurately measuring signals spanning over 7.5 GHz and responding quickly enough to measure fast hopping and short symbols.

Most general-purpose instruments available today fall short of the 7.5 GHz bandwidth requirement. Those that have sufficient bandwidth are typically high-end systems with prices to match. On the other hand, single box testers with software-controlled vector signal analyzers can be designed for capturing and analyzing wireless USB signals and providing comprehensive test results that meet development needs. Developing such a tester that meets Certified Wireless USBís bandwidth and speed requirements has challenges of its own.

Production test requirements

A test system built to handle wireless USB development testing would unquestionably be capable of production testing as well. In fact, the total number of tests would be significantly relaxed since designers would not be trying to verify the design but rather trying to catch production problems.

In this case, transmitter testing would involve calibrating the output power, measuring the EVM, checking that power spectral density falls within the mask, and (optionally) checking the frequency offset.

On the receiver side, designers must check minimum power sensitivity at a specified PER and maximum power sensitivity at a specified PER. These tests would expose any production shortcomings.

A prototype wireless USB tester

At this time, there are no single box EVM testers designed for wireless USB development and production testing. However, that is about to change.

At the Consumer Electronics Show earlier this year in Las Vegas, LitePoint demonstrated a prototype wireless USB test system paired with the latest Certified Wireless USB chipset, as shown in Figure 2. This prototype captures continuous packets transmitted by the chipsetís reference design and displays a rich set of data plus informative graphics. The Unit Under Test (UUT) was controlled via its own driver software, allowing band group, TFC, and continuous packet output selection.

Figure 2

The test system is controlled by its GUI software, providing a user-selectable array of graphic displays and data listings. Designers can select the band group and capture mode (single signal or continuous). In single signal mode, the tester captures one packet and displays the instantaneous readings. In continuous capture, a succession of packet "snapshots" shows if the readings are relatively stable or punctuated by large, random deviations.

An important test is power spectral density (shown in Figure 3), ensuring that the transmitted signal is well within the spectrum mask limits. Signals that exceed spectral mask limits are more likely to interfere with other signals.

Figure 3

All the development and production tests alluded to earlier can be accomplished within a short timeframe. The setup remains the same, and users simply select different parameters and graphical views. Signal capture, subsequent analysis, and display each take a few seconds to complete, with a single click to produce all the analytical results.

In the near future, at least one single box EVM-based test system - the LitePoint IQultra - will be finished with beta testing and available for rapid Certified Wireless USB-based device testing.

Onno Harm is product manager for WiMAX Test Solutions at LitePoint Corporation in Sunnyvale, California. Onno has more than 15 years of experience in wireless systems, ICs, and volume manufacturing and has developed products for voice and data applications involving several popular wireless standards. Onno holds an MS in Electrical Engineering from Twente University in The Netherlands.

Rob Brownstei is VP of market relations and communications at LitePoint. He began his career as a test engineer more than 30 years ago and worked as a journalist for three electronic trade media. He holds a BS in Physics from Queens College, City University of New York, and performed graduate work in Electrical Engineering at San Diego State University.

LitePoint Corporation

Onno Harms (LitePoint Corporation)
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