In addition to differing gamma characteristics across the frequency range, there are a few more distinctions between passive and active load-pull:
Impedance range: Active load-pull can generate a broader range of impedances on the Smith Chart compared to passive load-pull, provided that the signal generator for α2 has sufficient power.
Power budget: Active load-pull requires sufficient power for α2 to support the targeted gamma at a specific frequency. In a typical hardware configuration, supporting a wide impedance range requires 10x more power than the DUT output power.
One of the critical aspects of wideband load-pull testing is understanding and characterizing nonlinear distortion. Regardless of the method (passive or active), the test system is required to comprehensively characterize the nonlinearity of PAs under modulated signals. This is crucial for designing amplifiers that maintain signal quality, particularly for modern communication systems.
VNA-BASED WALP METHOD IMPLEMENTATION
The VNA-based WALP method has unique technical aspects that enable accurate gamma control and nonlinearity characterization in the modulated signals.
Vector correction: Utilizes the VNA’s calibration capabilities for accurate measurements of signals at the reference plane.
Multi-tone measurement: Accurately evaluates the multi-tone spectrum (amplitude and phase) of the scattered wave from the DUT (b2) and the synthesized signal from the signal generator (α2) across the frequency band.1
Optimization algorithm: An optimization algorithm estimates the α2 required to achieve the desired impedance across the frequency range, improving measurement time.2
Nonlinearity estimation: Facilitates accurate estimation of nonlinear distortion under wideband modulated signals, supported by a modulation distortion algorithm.3
User Interface (UI) and Testing Capabilities
Figure 6 Representative measurement results display.
The VNA-based WALP method requires the integration of advanced hardware with a comprehensive software interface. This streamlines the complexity of load-pull testing. The UI benefits from the intuitive and easy adjustment of crucial test parameters, including frequency range, power levels and load impedance conditions.
Test parameter: Users configure critical test parameters, such as power levels and load impedance conditions (load grid), along with gamma search conditions.
Data displays: The data visualization capabilities of the software allow for a quick review of results. This feature provides immediate, intuitive insights into power sweep results for various load conditions to provide a sanity check of the measurement.
A representative display of parameters is shown in Figure 6.
With modern VNAs and software tools, the user can define a custom gamma at the reference plane. This facilitates power sweep measurements. This is essential for acquiring detailed insights into nonlinear distortion using the specified gamma trajectory across the frequency range. This will enhance the accuracy of the PA performance evaluation. Figure 7 shows representative results from this process. In this case, the S-parameters for a bandpass filter are given, along with the gamma trajectory and the bandpass PA/filter response for that desired gamma response is shown.
Figure 7 Representative results from a custom gamma creation and power sweep.
COMPREHENSIVE TESTING FUNCTIONS
The combination of an intuitive UI and advanced visualization features makes the VNA-based WALP method a powerful tool for PA testing. It not only streamlines the testing process but also provides deeper insights into PA performance, aiding designers to efficiently optimize their designs. Several aspects of the software features enhance testing capabilities:
Automated test sequences: Test sequences that can be automated through SCPI commands save time and reduce the potential for human error. The automation interface is especially useful for repetitive or standardized testing procedures.
Compatibility and integration: Compatibility with a wide range of testing equipment and the ability to integrate seamlessly into existing testing environments ensures that the solution can be adopted without extensive modifications to current setups.
Result export and visualization: The ability to export allows detailed measured test data to be collected and analyzed. External software, like Keysight’s ADS,4 can be used for further analysis and to create contour plots for distortion parameters like EVM and ACPR. Contour plots are created by taking a set of parameters like output power and gain for each load condition and plotting a constant value for one of the parameters. The power contour indicates the load impedances that, when presented to the output of an amplifier for a given source impedance and power combination, cause that power to be delivered to the load. This helps determine trade-offs between output power, gain or EVM for different load conditions. Figure 8 shows a representative set of plots from ADS for a PA contour plot.
Figure 8 Example of an ADS contour plot result.
CASE STUDIES AND PRACTICAL APPLICATIONS
The true value of the VNA-based WALP solution is best understood through its use in actual applications. The full details are beyond the scope of this article, but the following case studies indicate actual uses where the method has been used to address specific challenges in PA testing.
Enhancing PA performance in front-end modules: This study details the use of the VNA-based WALP solution by a leading component company to optimize the distortion performance of PAs. Similar to the results described in Figure 7, the PA was combined with an output chain filter with a specific gamma across the frequency band. The ability of the solution to evaluate nonlinearities and predict system-level performance for various gamma trajectories improved the efficiency of the design flow for the matching circuit and this helped to optimize linearity.
Improving satcom PA systems: The VNA-based WALP solution was employed to test and improve PAs used in phased array systems in satellite communication. In these applications, reliability and performance stability over a broad range of conditions are critical. The testing capabilities of the solution helped to enhance reliability in demanding conditions.
Robustness testing: The VNA-based WALP method assessed PA robustness under extreme load conditions. The insights ensured consistent performance by evaluating PA performance before and after extreme loading conditions.
Each case study shows how the solution provided insights into the sensitivity of PA performance to load impedance variations. These insights are crucial and help designers understand the trade-offs between output power, efficiency and linearity. For all the case studies, a key benefit of using the VNA-based WALP method is the measurement and calibration capabilities of the VNA. This extends the abilities of the measurement setup from WALP measurements to small-signal S-parameters, gain compression, noise figure and others, providing measurement flexibility. The VNA-based WALP system allows users to make those measurements seamlessly, without the need to reconfigure the test setup.
CONCLUSION
As this article has demonstrated, the VNA-based WALP method represents a significant advancement in the field of PA testing. The method enables users to replicate actual load conditions that a PA may see in an application. The ability to test PAs across a wide range of frequencies makes this method an important tool in the modern RF communication system design process. The landscape of RF communication is continuously evolving, with new challenges emerging as technologies advance. The VNA-based WALP method not only addresses current testing challenges but also positions itself as a tool capable of adapting to future advancements.
References
- J. Verspecht, A. Stav, T. Nielsen and S. Kusano, “The Vector Component Analyzer: A New Way to Characterize Distortions of Modulated Signals in High-Frequency Active Devices,” IEEE Microwave Magazine, Vol. 23, No. 12, pp. 86–96, Dec. 2022.
- A. M. Angelotti, G. P. Gibiino, T. S. Nielsen, D. Schreurs and A. Santarelli, “Enhanced Wideband Active Load-Pull with a Vector Network Analyzer Using Modulated Excitations and Device Output Match Compensation,” IEEE/MTT-S IMS, 2020.
- J. Verspecht, A. Stav, J. -P. Teyssier and S. Kusano, “Characterizing Amplifier Modulation Distortion Using a Vector Network Analyzer,” ARFTG, 2019.
- “Example ADS Workspace for Plotting Load Pull Contours,” Keysight Technologies, Web: https://edadocs.software.keysight.com/eesofkcads/how-to-display-contours-from-wideband-active-load-pull-measurements-817202131.html.
