Testing next generation wireless-LAN RFICs takes some doing

Testing next generation wireless-LAN RFICs takes some doing
By Larry Dibattista, EE Times
January 21, 2003 (3:41 p.m. EST)
URL: http://www.eetimes.com/story/OEG20030121S0037

Advanced process technologies permit semiconductor companies in the wireless local-area network (WLAN) space to integrate RF, analog and digital circuits on a single die. This makes possible complex RFICs with interference-resistant signaling methods like direct-sequence spread-spectrum (DSSS) and complex modulation protocols like orthogonal frequency division multiplexing (OFDM) and n-QAM (quadrature amplitude modulation).

Serving as the foundation of such key WLAN standards as 802.11a, HiperLAN2, 802.11b and 802.11g, signaling methods like DSSS, OFDM and n-QAM offer greater data throughput at lower transmission rates. With their wide bandwidth and high dynamic range, however, those approaches present a significant challenge to companies looking to be first to market with more-sophisticated WLAN devices.

Not the least of the challenges is test. Traditional test methods and test equipment created for narrowband (less than 100-kHz) single-carri er designs lack the performance characteristics needed to deal with emerging WLAN devices for 2.4-GHz and 5-GHz bands.

Furthermore, traditional test methods using sinusoidal stimuli are ineffective for devices using the kind of complex modulations prescribed in WLAN standards. Because active devices like power amplifiers behave differently under sinusoidal and modulated stimuli, traditional test methods that rely on simple sinusoidal measurements provide misleading results. If tuned and tested with sinusoidal waveforms, those designs will perform differently with the modulated signals present in actual applications, risking field failures in devices that seem to pass performance tests in the factory.

As a result, RF designers increase guardbands, trading yield — and ultimately profit — for reduced risk of device failures.

To deal with that, RFIC companies are replacing traditional sinusoidal measurements taken with vector network analyzers with more-relevant modulated signals measured using modulated vector network analysis methods. With those modulated methods, RF designers can apply the same underlying model for interpretation of results used in earlier approaches. Because the device under test is stimulated with modulated signals, however, the results more faithfully reflect real-world performance. In turn, designers can use the more-accurate results to reduce guardbands, resulting in improved yield and higher profit.

Despite increasing test complexity, WLAN RFIC manufacturers face continuing challenges for high through-put and cost-effective test. Test equipment must provide the ability to compress RF test times and maximize throughput, handling multiple WLAN characterizations, often on several devices in parallel. Using distributed parallel-processing subsystems, advanced RFIC test systems are capable of acquiring data streams with high precision and at high sample rates — from single or multisite test setups — and complete multiple independent RF measurements from a single acquisition.

At the same time, semiconductor manufacturers must be able to quickly adapt equipment to changing requirements, easily creating RF, analog and digital test configurations to address specific test needs of varying product mixes. The emergence of scalable test system architectures promises to provide a cost-effective path for adapting and upgrading equipment as new RF, analog and digital test requirements emerge. The inherent flexibility of that approach will enable companies to adapt WLAN test equipment to the exact configuration needed to take on a specific RFIC test challenge.

As WLAN standards evolve — and demand increases for wider bandwidth, higher bit rates and more-complex signal modulations — that same adaptability will prove vital for manufacturers looking to preserve their investment in test equipment, test programs and engineering experience.

For RFIC companies, leading-edge WLAN devices present increased test challenges that demand improved test methods a nd test architectures. By providing a measurement environment well matched to real-world applications, more sophisticated test methods and advanced RFIC test systems will be able to deliver the high levels of accuracy and throughput needed to meet emerging test challenges. And by providing a flexible, configurable test capability, emerging RFIC test platforms also provide cost-effective test solutions that retain their value in the face of evolving WLAN requirements.

Larry Dibattista is a vice president and general manager of Credence Systems Corp. (Fremont, Calif.).