Device characterization is required for power amplifier design, and the ideal form of the device data is a large signal model. With a model, the performance can be analyzed for varying drive and impedance conditions, so complex or multi-stage circuits can be designed. A method of formulating a large signal model is to use a measurement-based behavioral approach, as with the X-Parameter model. This is based on measurements of X-parameters, which are a superset of S-parameters for nonlinear components, and are measured using an NVNA (Non-linear Vector Network Analyzer).
Load Pull with X-Parameters
Load pull with NVNA measurements of X-parameters can be used directly by the X-Parameter model over a wide impedance range. The operator of the combined load pull NVNA system can select an impedance range of interest, possibly over the entire Smith chart. The X-Parameter model can then be used as a circuit element in a non-linear analysis with great confidence, since it is based on measurement at the actual operating conditions of the device. The load pull X-parameter measurement can include a complete sweep plan. Stimulus variables can include impedance, power drive, bias, and frequency, for example. This can extend the applicability of the X-Parameter model over a much wider range of validity - over the range of actual applications for many high-power and multi-stage PA designs.
The process has three steps:
1) The load pull system measures the X-parameters at each impedance setting, like a standard load pull, with X-parameters added to the measurement data set. When the measurements are complete at all the impedances, the measured X-parameters are saved into a single file.
2) An enhanced design kit available for use in the ADS non-linear simulator then reads the file saved by the load pull- NVNA system and creates a X-Parameter component associated with the file. This is a very quick step.
3) This component can then be dragged and dropped directly into a circuit schematic as a non-linear device, and analysis can start immediately.
Tour IVCAD Software Suite
IVCAD advanced measurement and modeling software, offered by Maury Microwave and AMCAD Engineering supports multiple load pull techniques including traditional load pull using external instrumentation, VNA-based load pull, active load pull and hybrid load pull. It performs DC-IV and pulsed-IV measurements and incorporates device modeling tools. Its modern visualization capabilities give users a greater ability to view, plot and graph measurement data in an intuitive manner.
Application Notes and Data Sheets
Load Pull + NVNA = Enhanced X-Parameters for PA Designs with High Mismatch and Technology-Independent Large-Signal Device Models
Setting Up Load Pull with X-Parameters Using the Agilent NVNA