The evolving consumer demand for flexible, high-bandwidth applications is fueling a race for new, high-performance, wireless-local-area-network (WLAN) products. While drawing on the latest international standards for WLAN communications, semiconductor companies are rushing to be the first to market with increasingly complex integrated circuits (ICs). These ICs combine radio-frequency (RF), analog, and digital circuits into more highly integrated chips and chip sets. Yet evolving WLAN standards, such as IEEE 802.11a, 802.11b, and 802.11g, call for communications protocols with complex signal modulations over wider bandwidths. They also want a dynamic range that is greater than anything that has ever been seen in commercial communications products.

For the companies seeking to get to market with these complex devices, the new wide-bandwidth, high-performance WLAN devices impose severe demands on test equipment and methods. Originally, both the test equipment and the methods were created to address traditional narrow-bandwidth devices. These devices operate at information bandwidths under 100 kHz. Now, WLAN test methods and test architectures must provide real-world signal environments with flexible test systems. These systems must be able to deliver the high bandwidth, accuracy, and speed required for WLAN RFIC test. In addition, the methods and architectures have to bring forth the high throughput, lower cost of test, and capital preservation required for financial success in the competitive WLAN marketplace.

Consumers are looking for the simplified wireless connectivity of personal, workgroup, and home-entertainment appliances. Meanwhile, semiconductor companies are quickly moving to deliver higher-bandwidth products in the unlicensed 2.4-GHz and 5-GHz bands. The IEEE 802.11b standard introduced high-performance wireless products that were able to take advantage of interference-resistant, direct-sequence-spread-spectrum (DSSS) communications protocols. These products could deliver 11-Mbps single-channel data rates. By spreading the signal across several frequencies, DSSS devices use pseudo-noise-modulated signals. They can therefore achieve higher data rates with greater reliability compared to earlier-generation approaches.

Using even more complex modulation protocols, including orthogonal frequency division multiplexing (OFDM) and up to 64-quadrature amplitude modulation (64QAM), wireless vendors can achieve 54-Mbps data rates. Thanks to these protocols, WLAN devices are rapidly approaching wired rates in an emerging class of wireless equipment for bandwidth-intensive applications. These applications are as divergent as voice over IP—with its low-latency requirements—and high-definition streaming-video delivery, which has high-throughput requirements. The IEEE 802.11a and European Telecom-munications Standards Institute Broad-band Radio Access Networks (ETSI-BRAN) Hiperlan2 standards both use OFDM and 64QAM in the 5-GHz band. Yet another standard, IEEE 802.11g, uses OFDM to provide 54 Mbps in the 2.4-GHz band.

At the heart of these high-speed wireless standards, the OFDM protocol uses a unique multi-carrier modulation scheme. This scheme splits data into several parallel streams. Each of these streams modulates mutually independent or orthogonal subcarriers within a 20-MHz nominal channel. Because of the parallel data transmission, OFDM is able to achieve 54-Mbps throughput with significantly lower modulation rates.

In addition, the standards include sophisticated forward-error-correction (FEC) mechanisms. These mechanisms help to ensure data integrity even if interference or multipath problems affect the subcarriers. In turn, this combination of modulated signals, increased data rates, and complex encoding mechanisms compounds the requirements for WLAN test equipment. In the course of testing device functionality, that equipment must now be able to extract data at megabit rates from complex, high-frequency modulated signals.

As signal complexity continues to grow, semiconductor manufacturers are migrating to a new design and technology base. The latest process technologies are enabling greater integration and performance. They are continuing the progression from pure radio-frequency devices to integrated chip sets and multi-chip modules (MCMs).

Today, RFIC manufacturers are exploiting advances in a variety of process technologies. These technologies include SiGe, BiCMOS, SiGe BiCMOS, gallium arsenide (GaAs), and even ultra-deep-submicron CMOS technologies at 0.15 µm and below. In particular, SiGe/BiCMOS processes are emerging as an important technology for high-performance wireless applications.

These processes flaunt low power consumption, temperature stability, and low-noise performance. In addition, available submicron SiGe/BiCMOS processes offer extremely high bandwidth performance with device transit time frequencies (f_t) in the tens and even hundreds of gigahertz. To reduce on-chip interference, these processes are built on enhanced CMOS base processes with deep trench isolation. Advanced SiGe/BiCMOS processes can therefore accommodate digital logic circuitry along with high-performance RF and analog elements.

At the circuit level, RF designers are replacing the traditional superheterodyne receiver with zero-IF designs. These designs reduce the number of mixer stages previously needed to convert the carrier signal to and from an intermediate frequency (IF). Instead, zero-IF designs convert the signal directly to and from the baseband frequency.

At the component level, zero-IF designs also result in more compact systems. The reduction in mixer stages allows designers to eliminate associated external components, such as surface-acoustic-wave (SAW) filters. Through the exploitation of advanced process technologies, RFIC designers can now integrate the required direct-conversion elements like low-noise amplifiers (LNAs), oscillators, and baseband filters in single-chip WLAN devices. For test engineers, however, the move to zero-IF receiver design creates a need for improved test-system linearity and more effective test methods. Traditional parametric tests are no longer possible for these designs.