Nonlinear S-Parameters And SDR Impact Test And Measurement Equipment

Jan. 26, 2010
Test and measurement makers are innovating ways to add functionality and describe the behavior of nonlinear components in response to large-signal stimuli.

Test and measurement is crucial for research and development through production. As a result, test-equipment manufacturers have had to speed the evolution of their instruments to keep up with rapidly changing wireless-communications standards. They also are relying more on softwareeither through links to electronic-design-automation (EDA) tools or via software-designed-radio (SDR) architectures. At the same time, test equipment is increasingly being tasked with performing nonlinear device characterization, which is leading companies to handle scattering parameters (S-parameters) in new and unusual ways. To provide the optimal solutions for microwave engineers working on both current and next-generation designs, test and measurement companies are doing a balancing act to respond to these trends while constantly raising the performance of their products.

For almost half a century, S-parameters have been at the roots of microwave theory and measurement. Using a vector network analyzer (VNA), engineers can easily measure S-parameters at high frequencies. At Agilent, Marketing Program Managers Jennifer Stark, Frank Palmer, and Jan Whitacre emphasize that a well-calibrated S-parameter measurement will represent the intrinsic properties of the device under test (DUT)independent of the VNA system that was used to characterize it. They note that essential DUT properties, such as gain, loss, and reflection coefficient, are familiar, intuitive, and important. As a result, S-parameters are still commonly used for nonlinear devices like transistors and amplifiers.

As the Agilent folks point out, however, S-parameters are limited in that they only describe the behavior of a nonlinear component in response to small-signal stimuli. At that point, the device can be approximated as a linear component at a static (e.g., fixed DC) operating point. This trend has created an urgent need for a rigorousbut practicalsolution for characterizing, modeling, and designing nonlinear components at high frequencies. Agilent's response has been what it calls X-parameters. These rigorous supersets of S-parameters can tackle both linear and nonlinear components, as they are excited by small- and large-signal conditions. In the small-signal limit, X-parameters reduce to S-parameters. Unlike S-parameters, however, the Agilent folks note that X-parameters contain detailed and useful information about the nonlinear behavior of a DUT. Examples include the magnitude and phase of distortion products generated by the nonlinear component in response to large-signal conditions.

X-parameters are a key capability of Agilent's PNA-X (Fig. 1). Although it is suitable for linear network analysis, this instrument can easily switch into the nonlinear-vector-network- analyzer (NVNA) mode for direct nonlinear measurements of amplifiers and other nonlinear components. The NVNA capability features a breakthrough in X-parameters that allows engineers to quickly and accurately design and develop linear components and subsystems by reducing or removing trial-and-error loops from their design process. The PNA-X family covers 10 MHz to 13.5, 26.5, 43.5, or 50 GHz. It offers both two and four ports and an internal combiner and mechanical switches with a 10.4-in. touchscreen.

The need for NVNAs also has been recognized by Anritsu Co. According to Steve Reyes, Product Marketing Manager VNAs, "Digital modulation schemes, such as PSK and QAM (used in 3G systems such as WCDMA and EV-DO) and OFDM (used in WiMAX and 4G systems such as LTE), result in a high peak-to-average ratio (PAR). Power-amplifier (PA) design engineers must take into account the consequences of amplifying a communication signal with high PAR yet still maintain linearity and acceptable error-vectormagnitude (EVM) rates. A linear VNA provides useful information regarding the performance of the PA under linear conditions. However, as the PA becomes compressed in a digitally modulated system, the VNA must provide additional information to help the design engineer optimize PA performance."

The firm has responded to this need with the VectorStar VNA. To thoroughly analyze a linear PA, Reyes points out that this instrument provides many builtin functions, such as automatic power sweep for gain compression, intermodulation- distortion (IMD) measurements, and characterization down to 70 kHz to measure memory effects. Notably, the VectorStar VNA can also be easily upgraded to NVNA status by simply including an external test set, software, and miscellaneous hardware, such as couplers and load pull tuners (see Cover Feature).

While traditional frequency-domain instruments for linear device characterization have adopted new techniques and proprietary measurement parameters to represent nonlinear behavior, Tektronix has teamed with Mesuro Ltd., a Cardiff University Venture, on a timedomain approach for nonlinear device characterization. According to Darren McCarthy, Tektronix's Microwave & RF Technical Marketing Manager, "Waveform engineering overcomes today's fragmented collection of measurement techniques, enabling the replication of S-parameter concepts within the nonlinear domain. By directly working with the time-domain stimulus and response, the Mesuro Active Harmonic Load Pull system overcomes many of the limitations of traditional systems with low frequency, high power, and direct device measurements at the impedances that matter. As part of the founding members of the OpenWave Forum, Mesuro and Tektronix are committed to the continued advancement of nonlinear technologies and measurement techniques with open data formats."

Based on Tektronix's AWG7122B arbitrary waveform generator (AWG) and DSA8200 sampling oscilloscope, the Mesuro MB 20 open-loop, activeharmonic load-pull systems enable the characterization of devices and power amplifiers for any signal and impedance environment to 150 W. At the heart of these systems is a waveform-engineering technique that enables the replication of S-parameter concepts within the nonlinear domain. Thanks to this capability, the systems can test applications that are still in development over wider harmonics to arrive at reference designs with better impedance-matching efficiencies. In addition, on-wafer measurements enable the device manufacturer to efficiently characterize RF power devices before sorting and packing begins. For device and PA manufacturers, the Mesuro active-loadpull product provides an opportunity to fully characterize devices within an accelerated amplifier design cycle. The systems can measure RF waveforms, power spectrum, S-parameters, and direct-current current-voltage (DCIV) data.

The move to nonlinear S-parameters will certainly provide increasingly advantageous capabilities going forward. Yet it is important to note that many engineers are focused on the immense amount of passive products that are essential to microwave designs. According to Justin Panzer, Manager, Product Marketing for Rohde & Schwarz North America, "Many of our VNA customers are involved with passive-device development (like filters and couplers), where nonlinear S-parameters have no advantages. And many amplifier developers are still content to use the techniques they've been using for several decadestechniques that have been honed to provide consistent, timely results that instill confidence for their customers. Of course, for very high-performance applications, nonlinear S-parameters may provide keys to improved product design. But it's still not proven to justify the incremental cost and complexity."

According to Panzer, Rohde & Schwarz offers a nonlinear S-parameter solution using its ZVA VNA and the "ZVA Plus" hardware and software package developed by NMDG. It provides nonlinear results that may be exported into modeling tools, such as AWR's Virtual System Simulator (VSS) software or Agilent's Advanced Design System (ADS) suite of software programs. The latest member of the ZVA family is the R&S ZVA67, which spans 10 MHz to 67 GHz (Fig. 2). The R&S ZVA67 boasts a dynamic range of 110 dB at 67 GHz with measurement time of just 3.5 s for each test point. With +6 dBm output power at 67 GHz and a power sweep range of more than 40 dB, this VNA can characterize small and large signal behavior on active components. In addition to S-parameters, it analyzes harmonics, compression, intermodulation, and noise parameters. This VNA has garnered a lot of attention for its ability to measure the relative and absolute group delay on frequency-converting components like mixers even when the local oscillator is not accessible (see November Cover Story).

Test and measurement manufacturers are increasingly turning to softwareprogrammable approaches so that they can provide increased functionality while meeting shortened time lines. With SDR at the heart of an instrument, for example, firms are able to make test equipment available just as a standard is emerging and fine-tune it when the standard is finalized. As noted by Tektronix's Darren McCarthy, "With the re-farming of digital broadcast channels and with the proliferation of multiple technologies vying for the same RF spectrum, the use of adaptive and cognitive radio technologies will increase the importance of testing digital technologies. This will continue to put an emphasis on time-correlated measurements across all domains of the test environment: digital, analog, and RF."

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One example of software-programmable platforms can be seen in the Aeroflex PXI 3000 series (Fig. 3). The product range includes a broad choice of PXI chassis and modular PXI instruments for wide-bandwidth RF signal generation, analysis, and conditioning for signals to 6 GHz. The series is supported by the PXI Studio application software for the waveform generation and vector signal analysis of complex wireless communications systems. According to Bill Burrows, Business Development Manager for Aeroflex International, "The Aeroflex PXI 3000 series addresses wireless parametric measurements for all of the existing cellular and wireless broadband technologies on a single platform. By building a wideband, generic signal source and signal analyzer, we are able to characterize its performance to the customer requirement by supplying a software application."

The 3020 series compact, 3U-high precision PXI modular RF signal generators come with an integrated dualchannel arbitrary waveform generator (AWG). They serve RF test systems for design verification and manufacturing to 6 GHz. The signal generators provide RF output-power control ranging from 121 dBm to +17 dBm with modulation bandwidths to 90 MHz. They boast level accuracy of 0.3 dB.

Those generators are complemented by RF conditioning modules and the 3030 series of RF digitizers. The digitizers are used with a 3010 series synthesizer module to provide precision conversion of RF signals into digital IF or I and Q data. When used with PXI Studio application software, the 3030 series RF digitizer family can perform vector signal analysis of RF signals for manufacturing and design verification. The digitizers span 250 kHz or 330 MHz to 3 GHz and 6 GHz. They offer a digitized bandwidth that is 36 or 90 MHz wide (1 dB) with 13- or 14-b analog-to-digital-converter (ADC) resolution. The digitizers sample at rates to 200 MSamples/s. They boast 75 dB spurious and intermodulationfree dynamic range, level accuracy that is typically 0.3 dB, and noise spectral density below -145 dBm/Hz.

Of course, not all aspects of an instrument can be software programmable. Mike Barrick, Anritsu's Business Development and Global Account Manager, states, "All modern instruments are software programmable, with some instruments providing the user with access to manipulation of data once a measurement has been made. An example of this is the MS269X VSA, in which the user can install PC-based analysis tools that may implement user-specific functions ranging from filtering to custom data displays. Lower levels in an instrument, such as RF/baseband hardware and fundamental measurement algorithms, are related to underlying measurement accuracy, which must be specified by the instrument manufacturer. If user-implemented changes to the basic measurement algorithms were allowed by an instrument manufacturer, it is doubtful that the resulting data would be useful."

In the excitement over new capabilities, it is easy to overlook the roadblocks that are inherent to the traditional RF test arena. Rohde & Schwarz's Justin Panzer notes that high-performance requirements necessitate the careful selection of RF, IF, and LO frequencies, power budgets, and data-processing algorithms. Within this hardwaredefined context, however, he points out that instruments like the Rohde & Schwarz FSQ and FSV spectrum analyzers use SDR algorithms to take downconverted I/Q samples and demodulate many popular waveforms from LTE and WiMAX to Bluetooth and CDMA.

Of course, nonlinear S-parameters and the increasing reliance on SDR approaches are not the only developments forcing the evolution of test and measurement equipment. The test industry has to keep up with ongoing trends, such as the increasing integration of hardware and software and the trend toward millimeter-wave frequencies. Development also is largely being driven by the needs of the fourth-generation Long Term Evolution (LTE) standard. For example, test equipment must now incorporate aspects like multi-channel capability and new fading scenarios to simulate multiple-input multiple-output (MIMO) performance. Due to the great number of services that are now provided by digital broadcasters, mobile network operators, and other providers, the potential for interference between them is heightened. As a result, Panzer has seen impressive growth in the areas of interference hunting and spectrummonitoring applications.

Modern communications is creating a growing need for real-world testing as well. According to Graham Celine, Senior Director of Marketing at Azimuth Systems, "While statistical modeling of wireless conditions has been the norm for many years, the proliferation of the technology drives vendors to want to find methods to capture field conditions and recreate them in the lab. This is a complex and challenging operation."

Thanks largely to communications, microwave testing also has to adapt as digital technologies become more critical to designs. For example, faster DSPs are causing ADCs and digital-to-analog converters (DACs) to be placed closer to the antenna. This proximity makes it critical for the system engineer to be able to diagnose and troubleshoot potential software errors. Converters also are being put on the same board as the RF front end. Because most RF front-end suppliers are not the same as those supplying the baseband receivers, Panzer notes that they need testing solutions that allow them to easily work together on product development. The increasing use of digital modulation also comes into play here, as it will increasingly create a need to accurately measure modulated signals.

Tektronix's Darren McCarthy has seen a profound increase in the importance and advancements of wideband technologies supporting the spectrum efficiency and linearity of modern radars: "The ability to create spectrally efficient radar pulses is important as the NTIA and FCC work on the coexistence of commercial wireless frequencies and those frequency bands required for national infrastructure (aircraft landing and weather radars). The linearity of the chirp radars helps to improve the effectiveness of the technology." Tektronix's IPR measurement, impulse response, measures the time-side lobe response of chirp radar pulses and can detect distortions and nonlinearities due to impedance, amplitude, and phase distortions. It has replaced the use of component testing of constituent parts of the radar to give the true performance of the triple returns and other components of error within the radar transmit chain.

Clearly, test and measurement companies will continue to have to quickly adapt their equipment to meet the needs of future applications. Although the major drivers of tomorrow's innovations may be hard to predict, it is very likely that today's breakthroughs will spawn the next wave of developments. As stated by Aeroflex's Bill Burrows, "The increasing use of software-defined instruments will blur the boundaries of our current instrument definitions. Such products as spectrum analyzers, power meters, and vector analyzers will merge into generic test platforms supporting all of the expected capabilities of these individual products. This will be aided by the increasing use of digital technology and, as the speed of ADCs continues to increase, these will get closer to direct connection to the RF domain. The result of this will be increased accuracy and analysis capability, allowing signal complexity to grow to provide the increased throughput that our information-fueled lives demand."

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