Extract U-HF model

How to extract Unilateral High Frequency memory (U-HF) model for High Power Amplifier (HPA). The HPA-U-HF model is the simplest nonlinear amplifier model. Extracting this model requires little information about the component, i.e. 1-tone CW measurements, performed on a nominal load impedance. It therefore takes into account only the dispersive effects present in the amplifier band and ignores other memory effects such as those due to polarization or heating. Hence, it is suitable for applications processing signals with quasi-constant envelope (frequency modulation), radar (frequency translation or slow amplitude variation, low duty cycle). This model does not take into account load impedance changes.

1. Create a new HPA device
   In an opened project, you can create a device from Applications window or Workspace window.

   • From Applications window, right-click on Device modeler and click on Create device. You can also right-click on HPA and click on Create HPA device.
   • From Workspace window, click on Device modeler button, select HPA and click on Open button then New button.
   The Create a new device dialog box is displayed.

   
   

   

   
   

   1. In Type field, select HPA.
   2. In Model field, select HPA-U-HF.
   3. In Name field, edit the name of your device. Here, we will name it "HPA_example1".
   Click on Create button to display the new device in the tree of Applications window and the settings of the extraction in Workspace window.

   
   

   

   
   

2. Choose your data file
   
   

   

   
   In the Extraction Settings section, fill in the Data file field with the absolute or relative path of your measurement or simulation file with the extension .dat. Click on Browser button to open the file browser and select your file in the local file system. The file browser opens directly to the data directory specified when creating the project.
3. Tune power and frequency approximation order parameters
   In Power approximation order and Frequency approximation order fields, start to put low orders and checks results graphically after extraction.
   Nota Bene:

   The power approximation order can not be greater than the number of power points included in the data file.

   The frequency approximation order can not be greater than the number of frequency points included in the data file. If exceeded, VISION will send a message in the Output Console window and automatically truncate the order of approximation to the maximum number allowed.

   Also, you must take care to consider the frequency approximation order as low as possible (do not seek a perfect fit of the frequency characteristics by pushing the order of approximation to the maximum).

   The Technological dispersions option allows to specify a distribution law of the gain (module) and phase shift characteristics of the amplifier. Two laws of dispersion are possible (Uniform or Gaussian law). The dispersion is characterized by two parameters: the standard deviation Module, given in % of the nominal value for the gain, and the standard deviation Phase in degrees for the phase shift.

4. Extract behavioral model and check with output graphs
   Click on Extract button to start the extraction process of the model. The output console is displayed:
   

   

   
   
   The message Model Fit Error is showing the normalized mean square error (NMSE) between data and model. Close the window to see in the Applications window the number of the newly created extraction, here, 001. The results are saved and can visualized at any time by designating in the tree the associated extraction. Click on the Output graphs tab to see comparisons between data and model.

   
   

   

   
   

   Various graphs are available to check the quality of the model according to two dimensions: power and frequency. To examine the quality of the approximation on the gain, select Volterra Model HPA-U-HF [1Tone CW Gain] in Figures section and choose graphs you want to display in Graphs section:

   • Tick dB[CW Gain] [par=Pin] to display, for different input power, the modulus of gain in dB as a function of dFreq, the offset between the central frequency of the device characterization band and the frequency of the CW signal.
   • Tick phase[CW Gain] [par=Pin] to display, for different input power, the phase of gain in dB as a function of dFreq, the offset between the center frequency of the device characterization band and the frequency of the CW signal.
   • Tick dB[CW Gain] [par=Freq] to display, for different frequencies, the modulus of gain in dB as a function of Pin, the power of the CW input signal.
   • Tick phase[CW Gain] [par=Freq] to display, for different frequencies, the phase of gain in dB as a function of Pin, the power of the CW input signal.
   The graphs show the curves of data (from measurement or simulation) in red lines and the extracted model in blue lines. The legend recalls the error NMSE between model and data. If the number of curves makes the graphs unreadable, click on Configure button to reduce the density of curves and/or limit the input power range and frequency band. Select Volterra Model HPA-U-HF [DC consumption] in Figures section to display power consumption of the device depending on input power and frequency.
5. Check measurement aberration and noise measurement
   The first extraction is an opportunity to verify the data, especially if there is measurement noise or measurement error. This type of phenomena can make it difficult to extract a model and can lead to aberrant results when the model is subjected to more complex signals. VISION provides some tools to limit or eliminate these phenomena in order to avoid doing measurements again. In Applications window, click on your device, here HPA_example1, to show up Settings tab in the Workspace window. Click on Extraction Options section to reveal some options:

   • Measurement aberration and noise polish filters: for the U-HF model, the option CW power gain aberrations is available to filter the noise that can be encountered on the measurement data. The user must choose the filter order appropriately.
   • Extraction power and frequency range tune: depending on the needs or observations on the data, one can modify the range of input power and/or the frequency band of the data with which the extraction of the model can be performed.

   
   

   

   
   The option Frequency grid oversampling increases the frequency step in order to improve the approximation of the phase, especially if this one presents a variation greater than $2\mathrm{\pi }$ in radian according to the frequency.
6. Tune power and frequency range
   If the first extraction is not satisfactory, it is necessary to increase the order of approximation power and/or frequency.

   1. Start by increasing the order of approximation power as long as the error NMSE decreases significantly. Check graphically the comparison between the data and the model.
   2. Then, increase the order of approximation frequency as long as the error NMSE decreases significantly. Check graphically the comparison between the data and the model.
   3. If the error is not small enough, restart in step a from the current settings
   The user can find in the following table an example of the extraction process. Here, the parameters of extraction 007 allow to have the smallest error between the data and the model.

   Table 1. Extraction settings HPA_example1

   Extraction	Power approximation order	Frequency approximation order	NMSE (dB)
   001	1	1	-43.32
   002	2	1	-45.70
   003	1	2	-43.32
   004	1	3	-43.32
   005	1	4	-43.29
   006	2	4	-46.77
   007	2	5	-46.74

   
   

   

   
   

   You can label an extraction as a reference to differentiate it from others for use in System Architect. Select the appropriate extraction of your device in the Applications , right-click on it, and subsequently select the add to favourites option.

   
   

   

   
   

7. Apply a test plan
   It is recommended to perform basic simulations after an extraction to check the behavior of the model in the face of signals different from those used for its identification. VISION provides tools to simply configure signals and perform simulations directly after model extraction. In Applications window, click on your device, here HPA_example1, to show up Settings tab in the Workspace window. Click on Test plan section to reveal two options:

   • Automatic tests: this option allows you to perform simulations with 2-tone and pulse signals whose settings are set automatically, except for the pulse width period.
   • Normal test: this option allows simulations with CW, 2-tone, pulse and white noise signals. You can also provide your own IQ file by specifying the path of the file.

     
     

     

     
     

   To set up a test, select Normal test and click on New test button. A new test is added to the list with the default name "test1". Then, the test highlighted in blue can be configured. Here, a configuration example of 2-tone test:

   • Name: 2tone-test1
   • Signal type: 2TONE
   • Number of samples: 1024
   • Carrier frequency: 3 GHz
   • Power sweep: -30 to 10 dBm with 2 dBm step
   • Output figure: IMD3
   • Frequency sweep: 1 to 60 MHz with 1 MHz step

   Tick your newly configured test in the list and click on Extract button to run the test after the extraction process. The simulation results are saved and can visualized at any time by designating in the tree the associated extraction. Click on the Test graphs tab to see IMD3 results according to frequency and power.