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Load pull simulation is a very simple, yet powerful, concept in which the load or source impedance presented to an active device is swept and its performance is measured. Performance contours are then plotted on a Smith chart, which shows the designer how changing impedances impact the device’s performance.
Recently, EDA tools have been adding increased sophistication in order to deal with complex sets of load pull data.
At the center of simulated load pull is the load pull template, which provides a pre-populated schematic set-up or test bench for the active device (transistor) used in the load pull analysis.
After configuration is complete and the device under test (DUT) is swapped out, you should run the load pull simulation using the AWR “Load Pull” script. This script helps you interactively set up the parameters of the load pull simulation.
After creating a load pull template, you can activate a bias sweep by toggling the SWPVAR block. When you run the AWR “Load Pull” script, a Generalized MDIF (GMDIFD) swept load pull file is generated.
The “Load Pull” script helps you set up the following parameters of the load pull simulation:
Load or Source Pull, and which harmonics are pulled
Gamma point, including custom gamma points to specify density and distribution across a Smith Chart
Source and Load tuners and the voltage and current meters used in the simulation
Load pull data file name and number of harmonics stored in the data file
After simulation, the new swept load pull file displays under the Data Files node in the Project Browser.
DUTs can be easily changed in the load pull template by swapping a FET model for the current default device (the CURTICE FET).
Simulated load pull is performed using two HBTUNER3 elements, allowing control of terminating impedance. Both use bias tees which provide DC blocking and bias voltage to the FET.
Load pull data can be viewed on a Smith Chart via AWR “Load Pull” or “Data” category measurements such as:
Generalized Load Pull Contours (G_LPCM): Plots the contours of the selected calculated value
Max Value for Generalized Load Pull Data (G_LPCMMAX): Plots the maximum value of the selected calculated value
Plot Load Pull Gamma Points from Measured Load Pull File (G_LPGPM): Allows a view of the reflection coefficient (impedance/gamma) points at which the swept load pull data was taken
Plot Real Data from Multi-dimension MDIF File (PlotMD_R): Allows you to visualize a real-valued data from a swept load pull file. This includes calculated values, such as power-added efficiency (PAE), which can be selected from the Add/Modify Measurement dialog box.
The Cadence AWR Design Environment platform V15 offers enhanced load pull capabilities that include an updated load pull script for performing load pull analysis, an expanded harmonic balance tuner (HBTUNER3), and Rectangular - Real/Imag graph plotting of load pull data.
The AWR “Load Pull” script uses the HBTUNER3 element to control impedance at up to three additional frequencies: the 4th and 5th harmonics of the fundamental, and the “baseband” frequency; F=|F1-F2| when a 2-tone input is used. As with the fundamental, 2nd, and 3rd harmonics, these impedances can be fixed at a specified value, or swept as part of the load pull analysis. Load pull measurements also accommodate these enhancements and can be plotted on a Rectangular - Real/Imag graph, which plots the real and imaginary components of a complex measurement on a rectangular grid instead of a Smith Chart or Polar plot.
The Harmonic Balance Lossless Tuner with Bias Tee with N Harmonics (HBTUNER3) element is a frequency-dependent, lossless network which transforms the impedance Z0 seen at port 2 to a user-defined impedance seen at port 1. Specifying the magnitude and angle of the reflection coefficient, as well as the system reference impedance, sets the user-defined impedance. This model is dynamic and allows up to the 5th harmonic impedance to be specified, as well as the impedance for the baseband intermodulation products. It is assumed that port 2 of this element is loaded in the same Z0 as specified for the element. A bias voltage can be applied to port 3 to bias a device connected to port 1, eliminating the need for a BIASTEE element.
A Rectangular - Real/Imag graph is useful for plotting load pull data, allowing complex measurements which would normally be plotted on a Smith Chart or Polar plot to be plotted on a rectangular grid. Benefits include independent adjustment of the limits and step sizes on the vertical and horizontal axes, and ease of reading the real and imaginary values directly from axis labels.
To learn more about using enhanced load pull capabilities in AWR software, view the 11-minute Enhanced Load Pull Capabilities video on the Cadence YouTube Knowledge and Learning subchannel.
For more information on Cadence AWR products, visit the Cadence website.
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