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 software—either 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 rigorous—but practical—solution 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 decades—techniques 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).

SOFTWARE TAKES A DEFINING ROLE
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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