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MEASURED LOAD PULL CAPABILITIES - FOCUS
Focus measured load pull files can be read into AWR Design Environment. This project shows how to import the files and use measurements to view the data and compare it with simulated data. An optimization is also shown that helps design a matching network for maximum device gain.
Focus files have the *.lpd extension. In these files, bias and frequency are held constant; impedance is the independent variable. This file is imported and the measurement gamma and gain contours are graphed.
A stepped impedance matching network is shown, and a load pull interpolation measurement is used to optimize the match to get maximum gain from the device. After optimization, the matching network impedance point corresponds exactly with the center of the load pull contours for gain.
Imported Load Pull Data
To import a measured Focus load pull (or source pull) file, right-click the Data Files node and select Import Data File…. The Files of type: should be set to Text Data File (*.*). Browse to the directory where the load pull file resides and type “*.xxx” for the filename (where xxx is the file extension of the load pull file). Select the data file.
After importing the file, it can be viewed in the workspace.
This schematic shows the stepped impedance matching network that will be used on the output of the device measured in the load pull system.
The graph "Contour Plots" shows the gain contours from the load pull data, the gamma points used to generate the gain contours, the point of max gain based on the measurements, and the current load presented by the matching network.
While this circuit is being optimized, the impedance that it presents to the attached device “zeros in” on the center of the gain contour lines.
Press the Analyze button on the tool bar to see the load pull contours and the initial impedance of the matching network.
The matching circuit is distributed and, therefore, physical in nature. The layout view shows the current matching network (note that the initial length of one of the MLIN elements is “0” so this appears as a line in the layout window). While this circuit is being optimized, the layout updates real time.
The optimization goal uses the LPINT measurement, which allows load pull data to be used by the linear simulator and, thus, used as an optimization goal for linear simulation. The goal is greater than 18 dB of gain at 925 MHz, which, once achieved, should correspond to the center of the gain contours.
Choose Simulate->Optimize from the main menu, select the Simplex Optimizer method, and click Start to begin the optimization.
Contour Plots Graph
• The LPGPM measurement displays the reflection coefficient/impedance points that the data was taken at. These points can be found in columns 2 and 3 (real and imaginary, respectively) of the data file.
• The LPCM measurement displays the contours of the third measured data column. In this data file, the data in the third column is gain.
• The LPCMMAX measurement displays the reflection coefficient corresponding to the maximum value in the third measured data column. In this data file, the data in the third column is gain.
• The S(1,1) measurement displays the reflection coefficient presented to the device by the matching network.
Interpolated Pts Graph
Graph shows the gain of the load pull device if it’s output port were terminated with the schematic “Matching_Circuit.” Also shown on the plot is the optimization goal, which is to have the gain greater than 18 dB.
Load Pull Data File Detail
Reference Material - Graphically Viewing Load Pull Data
Load pull data can be viewed via measurements. These measurements are found under the Load Pull heading.
 LPGPM: This measurement allows you to view the reflection coefficient (impedance) points the data was taken at. This measurement is used with the Smith Chart. The characteristic impedance with which the reflection coefficients are normalized must be specified (defaults to 50 ohms). One common issues is that the default marker interval for this measurement will not be set to 1 by default. So it will appear that only some of the points are show. To correct this, go to the graph options, find the Traces tab, and make sure the marker interval for this trace is set to 1.
 LPCM: This measurement plots the contours of the selected column of measured data and is intended to be used with the Smith Chart. The data file, max contour level, min contour level, step size, column of data to be plotted, and characteristic impedance must be specified.
 LPCMMAX: This measurement plots the gamma value that corresponds to the maximum value in the selected column of measured data and is intended to be used with the Smith Chart. The data file, max contour level, min contour level, step size, column of data to be plotted, and characteristic impedance must be specified.
 LPINT: This measurement allows the measured data to be used with the linear simulator. The linear simulator provides an impedance looking into a port on the specified schematic/data file/EM structure to the load pull simulator. Using the “thin plate spline” two-dimensional interpolation algorithm, the load pull simulator calculates the interpolated value of the selected measured parameter at that impedance. This measurement allows the user to have more than one port that are “input” ports (ports connected to a measured device). An example of such a circuit would be for cases where multiple devices are connected via power combiners/dividers as part of the matching network. This measurement is intended to be used with a rectangular graph. A warning is given if the data point is being extrapolated (lies outside of the measured data) and not interpolated. Note: The number of the column in the data file must be specified for this measurement. For Focus files, the first three columns are ignored - the measurement considers column four as the first measurement column. For Maury files, the first two columns are ignored.
Optimizer goals can be set up for this measurement to optimize on circuit parameters in the schematic for a desired measured data level (for example, optimizing the parameters of a matching circuit to obtain maximum power added efficiency).
Other Load Pull Measurements
There are two other load pull measurements that have not yet been mentioned: LPGPT and LPCS. These measurements are generally used with the Microwave Office Load Pull Wizard. Here is a description of the measurements:
 LPGPT: This measurement allows the user to display a custom set of reflection coefficient (impedance) points. The points must be specified in a two-column, tab delimited, text data file (.txt extension). The first column specifies the real part, while the imaginary part is specified in the second column. Comment lines can be included and must be preceded by a ‘!’ character. A Load Pull Wizard can then be simulated at these specified, custom points. The measurement is used with the Smith Chart. The characteristic impedance with which the reflection coefficients are normalized (the Zo of the data file) must be specified (defaults to 50 ohms). An option to display the points at a different Zo is also provided (defaults to 50 ohms).
 LPCS: This measurement plots the contours of a row-column, tabular data file. This data file is created when running a simulation using the Load Pull Wizard. The data file, max contour level, min contour level, and step size must be specified.
 LPCSMAX: This measurement plots the gamma value that corresponds to the maximum value in the selected column of measured data and is intended to be used with the Smith Chart. The data file, max contour level, min contour level, step size, column of data to be plotted, and characteristic impedance must be specified.
A graph containing a LPCS measurement is automatically generated after running a simulation using the Load Pull Wizard.