Electronic Design

Tough Serial, Wireless Requirements Propel Test & Measurement Innovation

Today’s state-of-the-art serial communication links typically operate from 6 to 8 Gbits/s, with the highest data rates running to more than 10 Gbits/s in specialized backplanes. Such high-speed serial technologies bring an increasing level of complexity to the game for designers.

With this greater complexity comes a growing need for analysis and compliance applications with built-in domain expertise and easily reproducible test-equipment configurations. Fortunately, test and measurement (T&M) providers will answer the call with a new generation of high-end measurement hardware and software that meets the challenges.

High-end system designers must deliver higher data rates while producing cost-effective designs. These high data rates and capacities bring complexities in terms of handling channel distortions generated at the transmitter end of the link.

One resulting trend is that technologies for compliance test now require reference channel models (see the figure). Such models can typically be downloaded from a standardsbody Web site and inserted into an emulated test path. Or, if the verification team is using a custom-designed in-house test fixture, another option is to import an S-parameter file into the oscilloscope and account for the impact of the T&M hardware on the path.

For example, the recently announced specification for USB 3.0, or so-called SuperSpeed USB, promises a tenfold gain in data-transfer rates from 480 Mbits/s to 4.8 Gbits/s. To verify compliance for new USB 3.0 design as well as for other highspeed serial links, such as Serial Attached SCSI (SAS), expect to see greater adoption of reference channel models.

In high-speed serial-link design, problems can arise when data rates rise but the cabling and connectors used to construct the channel stay the same. Moving to higher-quality materials adds cost and can impact the adoption rates of high-speed links.

Running at 6 to 8 Gbits/s over low-cost materials can produce significant distortion, though. Thus, designs are now beginning to use signal-equalization techniques commonly found in the communications space, adding even more complexity to compliance validation.

In particular, designers will be forced to pay stricter attention than ever to jitter and its various sources. Forcing higher data rates through limited-bandwidth communication channels can result in severe attenuation.

Designers will need a means of determining the sources of jitter and how to clear it up. A relatively esoteric endeavor until three or four years ago, jitter decomposition enables designers to come to grips with, and ultimately sidestep, jitter that causes errors in data transmission.

Test-and-measurement providers will continue to deliver bleeding-edge instruments that are able to handle jitter decomposition. Jitter can be generated by numerous sources, ranging from random effects such as thermal noise to more deterministic causes, including crosstalk from adjacent communication channels.

Eye-pattern analysis and jitter testing are critical for understanding bit error rates, yet often the raw signals as measured on a bus present “closed eyes” due to equalization issues on the channel. To see what the receiver sees, the engineer must recreate the proper equalizations and apply them to the measured signal.

WHERE WIRELESS TEST IS HEADING • In the wireless realm, a different set of evolving standards is pushing T&M suppliers to keep up with them. These companies must work toward flexible platforms, coupled with a software infrastructure that develops as the standards are fleshed out. With the Long Term Evolution (LTE) and Mobile

WiMAX standards still maturing and subject to change and interpretation until they’re codified, T&M suppliers have delivered early availability of conformance and protocol tests for these emerging standards. These tests help alleviate interoperability issues and provide basic testing.

Rigorous test solutions also now exist for protocol development. Wireless libraries with signal-processing models and preconfigured simulation setups are available to create waveforms used to produce real-world physical test signals. However, a successful commercial launch of either LTE or Mobile WiMAX technology will require comprehensive functional testing and real-world verification of LTE and Mobile WiMAX products.

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Another challenge relates to the 802.16 standard on which Mobile WiMAX is based—it specifies a tight error vector magnitude (EVM) requirement (–31 dB, based on a 1% packet error rate). Meeting this target requires all system blocks to be more linear and phase noise to be considerably better than in an 802.11 design. Power amplifiers must also be more linear and boast higher efficiency.

The key to developing linear active devices for LTE-based or Mobile WiMAX-based systems is to first characterize nonlinear behaviors—those that don’t have a linear input/ output relation and thus contribute significantly to information interference and reduced effective bandwidth.

Look for new nonlinear vector network analysis capabilities featuring a breakthrough in X-parameters (new, nonlinear scattering parameters). These will enable engineers to quickly and accurately design and develop linear components and subsystems by removing the trial-and-error loops.

Multiple-input, multiple-output (MIMO) technology is designed to bring about a two- to four-fold increase in wireless networking throughput with no additional frequency spectrum required. While most engineers work on Nx2 (e.g., 2x2 and 4x2) MIMO configurations, the wireless local-area network (WLAN) market uses 4x4 MIMO configurations.

Due to the complexity of MIMO technology, obtaining optimal operation requires engineers to accurately test the MIMO receiver—a challenging task given the large amount of variables that must be tested in a MIMO configuration.

A critical part of testing a MIMO receiver involves replicating real-world conditions and channels and performing real-time fading of MIMO signals. A recent advance in MIMO receiver testing marries the signal source, noise source, and fader together in a fully integrated solution to successfully accomplish these tasks.

Rather than offering signal generation only, as is the case with most current solutions, this measurement approach offers an exceptionally versatile platform for testing LTE and WiMAX receivers. It not only allows the engineer to replicate real-world MIMO conditions and channels, but also to generate realistic fading scenarios including path and channel correlations.

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