Last year, the new Vectorstar microwaVe Vector network anaLyzers (Vnas) from Anritsu created a stir in the industry, with their powerful combination of high accuracy, fast measurement speed, and broad bandwidths (see Microwaves & RF, January 2009 Cover Feature). Since then, the industry's appetite for nonlinear S-parameter measurements has grown. To satisfy that need, the firm has made its MS4640A family of VNAs available as part of a system for nonlinear measurements, allowing analysis and model development of amplifiers and other active RF/microwave components tasked with processing microwave signals with complex modulation. The VectorStar Nonlinear Systems are available with a low-frequency limit of 0.5 GHz and a high-frequency limit of 18.0 GHz in various configurations.

The MS4640A family of VectorStar VNAs (Fig. 1) feature 0.1-dB compression at +10 dBm, with dynamic ranges of 123 dB to 20 GHz, 119 dB to 40 GHz, 100 dB to 110 GHz. The synthesized, error-corrected switching speed is 20 s/point with 100,000 measurement points available per channel. The analyzers can control as many as four independent signal sources for multitone measurements and include direct-access loops for source and receiver monitoring. When measuring small-signal amplifiers, the linear VectorStar VNAs provide many built-in functions to fully evaluate these active devices. Applications include built-in programmable power sweeps for gain-compression analysis at multiple frequency points, intermodulation distortion (IMD) measurements, and harmonic measurements.

As part of the nonlinear measurement systems, the VectorStar VNAs include the additional components and software required to measure how active devices operate under nonlinear, large-signal conditions. In the new system configurations, the VectorStar VNAs deliver the information needed to develop optimized nonlinear devices. To extract meaningful nonlinear data, a VNA must take into account harmonics as well as the fundamental frequencies at the output of a device under test (DUT). The VNA must also be able to view fundamental and harmonic output frequencies over a wide range of load impedances, with the aid of source-pull and load-pull impedance tuners. As with all active on-wafer devices, the performance is directly related to the source and load impedance presented to the device. To optimize performance of a nonlinear DUT, the impedance presented to the DUT must be optimized for both the fundamental and harmonic frequencies.

The VectorStar Nonlinear System stems from collaboration between the Anritsu labs and High Frequency Engineering Sagl of Switzerland (HFE). Software from HFE provides nonlinear information in a number of formats to facilitate analysis and aid in the evaluation and design process. In addition to HFE software, a VectorStar Nonlinear System also requires an HFE test set and appropriate components (Fig. 2). An owner's VectorStar VNA can be upgraded to a nonlinear system at any time by simply adding the software, test set, and components. The only requirement of the VectorStar for a nonlinear upgrade is that the external loop and receiver offset options (Options 51 and 07) are installed.

Either active or passive tuners can be used in a VectorStar Nonlinear system with load-pull measurement capability. Compared to passive tuners, active tuners provide a greater range of loads to a DUT, which is critical for nonlinear device characterization. Many nonlinear amplifier classes of operation require a reflection coefficient or gamma of 1 at harmonic frequencies for optimum performance.

The VectorStar Nonlinear System differs from traditional load-pull systems in the location of its tuners and monitoring couplers. A typical load pull system locates the coupler outside the tuner and monitors the power output of the DUT as the tuner is varied in impedance. This method requires that the tuner be pre-calibrated. The accuracy of the measurement in the typical load pull setup is determined by the repeatability of the tuner, cables, and connectors after calibration. The tuner in this traditional configuration is controlled by vendor software for both calibration and control and creates a complex relationship between the tuner software and nonlinear VNA software. Since many DUTs must be tuned over a large area of the Smith Chart, the calibration table for a traditional load pull system can be large, with calibrations often requiring hours.

The Anritsu/HFE load-pull system inserts a low-loss coupler between the DUT and tuner, achieving improved measurement accuracy of the source and load impedances at the DUT. This approach makes it possible to monitor the impedance in real time while also monitoring the performance of the DUT. The VNA provides immediate display of the DUT's performance in response to changes in impedance, allowing real-time tuning. It also means that precalibrated tuners are not needed, nor is tuner repeatability important since the impedance can be monitored while the DUT is being measured.

When configured with the Anritsu/HFE active loop tuner, gammas values to 1 at the DUT port are possible (Fig. 3). A combination of passive and active tuners can also be used for flexibility when upgrading a measurement system. With the Anritsu/HFE approach, any type of impedance tuner from any vendor can be used in the load-pull system. Vendor tuner calibration software is no longer necessary since the tuner can now be controlled by the MMSNT_LP software for reduced interface complexity. Even tuners with poor repeatability can be used to achieve accurate load-pull results.

Nonlinear measurement results for a DUT can be used to design an optimum impedance matching network or exported to an electronic-design-automation (EDA) program, such as AWR's Microwave Office, for model development. Unlike some nonlinear VNA measurement solutions, the Anritsu/HFE system does not port the data to a "black box" representation within the EDA tool. Rather, data are available directly from the MMSNT_LP software.

Because of the large quantity of data provided during nonlinear measurements, the data should be formatted in a way that is easy to store, convenient to open and view, organized in a manner that is flexible and pliable, and easy to share with colleagues. In support of these goals, the OpenWave Forum (OWF) is an alliance of RF and microwave firms formed to collaborate, create, and promote a unified and transparent data exchange format for large-signal nonlinear simulations, measurements, and models. An open standard ensures that data files will be compatible and transportable to an EDA environment that recognizes the standard. As a founding member of the forum, Anritsu/HFE is participating in defining the open standard and is incorporating the format into the MMSNT_LP software.

The MMSNT_LP software is structured to acquire a large number of data files while still providing a user-friendly environment for analysis. The software is structured as a multidocument and multiview application. Each document represents a particular measurement task: an S-parameter measurement is associated with one document, and a load-pull session is held in another document. The result is measurement software that can simultaneously display power sweep curves, time domain waveforms, load pull contour maps, eye diagrams, and I/V parameters on the same screen. To simplify the user interface, each document controls the presentation of the main application menu and toolbars; the respective measurement is triggered and updated by the corresponding window.

Five different documents reside within the MMSNT_LP software:

the Test Set document (*.tst) handles the calibration, multiport system port assignments, and the control of the bias and the tuners; the S-parameter document (*.spx) performs the S-parameter measurements, stores and retrieves the S-parameter data, and computes the differential parameters if appropriate; the Macro document (*.mcx) programs and runs Visual Basic for Application (VBA) macros; the CalKit document (*.std) creates, stores, and retrieves calibration standard descriptions, using the auxiliary Winkit Manager application; and the Load Pull document (*.lpx) creates, stores, and retrieves load-pull and time-domain waveform load-pull data.

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A document can be split into multiple views, enabling the visualization of different parameters or graphs in multiple windows (Fig. 4). This multiple-view approach is particularly useful when performing tuning and optimization on nonlinear active devices, to see the effects of changes in impedance on amplifier efficiency and gain, for example.

The MMSNT_LP software sets up and controls DUT current-voltage (I-V) parameters via GPIB-controlled power supplies, controls the load-pull tuners for fundamental and harmonic impedances, drives the power source for power swept conditions to the DUT, and controls and switches the VNA waveforms for real-time display of measurement results including time-domain and eye-diagram waveforms. The flexible software also provides storage and organization of the data files for future analysis within the MMSNT_LP software and companion EDA software, such as AWR's Microwave Office.

The MMSNT_LP software also coordinates calibration of the nonlinear VNA system. The dynamic calibration is based on a computer analysis of the measurement system's topology and operating conditions. From this analysis, an optimized standard sequence is generated to lead a user through the calibration procedure. The software offers a wide range of calibration options to achieve the best possible accuracy for a given connector scenario. Available calibrations include a number of techniques based on the use of short (S), open (O), load (L), reflect, and thru standards, including ESOLT, QSOLT, SOLR, TRL/LRL/LRM/LSM, and MTRL. An extra calibration step is needed to perform time-domain waveform measurements to ensure absolute phase alignment. This part of the calibration is performed across a bandwidth of interest in fixed steps (e.g., 10 MHz), and one of the calibrated frequencies must fall on the desired fundamental frequency of the DUT in order to calibrate the VNA for harmonics as well as the fundamental frequency.

The VectorStar's use of active loop modules makes it possible to configure the test system for load-pull measurements of differential devices. Since it is possible to actively monitor the real-time impedance presented to the device, it is also possible to actively and independently tune the input of the device to specific phase positions. In the case of a differential device, this means by adding a second leg to the device, it is now possible to actively control the differential input and tune the device under varying source and load conditions. Upgrading to the differential configuration does not require additional modifications or options to the VectorStar VNA.

The Anritsu/HFE nonlinear VNA system offers unique nonlinear load-pull measurements in a flexible, powerful, yet low-cost approach. It can be used to optimize the efficiency of power amplifiers for emerging wireless communications systems and offers an upgrade path that can be traveled in increments, ultimately allowing multiharmonic active loop nonlinear load pull measurement systems including true balanced differential measurements. Anritsu Company, Americas Sales Region Headquarters, 1155 East Vollins Boulevard, Suite 100, Richardson, TX 75081; (972) 644-1777, FAX: (972) 671-1877, Internet: www.us.anritsu.com/.