Linearization QuickStart Guide

This Linearization QuickStart Guide will help you get started using the Linearization DesignGuide. Linearization DesignGuide Reference provides useful reference information.

Note
This manual is written describing and showing access through the cascading menu preference. If you are running the program through the selection dialog box method, the appearance and interface will be slightly different.

The Linearization DesignGuide has many simulation set-ups and data displays that are very useful for performing linearization of a power amplifier. The simulation set-ups are categorized by the type of technique desired:

FeedForward
RF Predistorter
Combined FeedForward and Predistortion
LINC
Analog Predistortion
Cartesian Feedback
Digital Predistortion
Memory Effects

There are also several real-time ADS Ptolemy simulation examples. The simulation set-ups are for analysis and power amplifier characterizations.

Note
This DesignGuide is not a complete solution for all Linearization techniques, but covers the most common approaches. Subsequent releases of this DesignGuide will include an expanded range of features.

Using DesignGuides

All DesignGuides can be accessed in the Schematic window through either cascading menus or dialog boxes. You can configure your preferred method in the Advanced Design System Main window. Select the DesignGuide menu.

The commands in this menu are as follows:

DesignGuide Studio Documentation > Developer Studio Documentation is only available on this menu if you have installed the DesignGuide Developer Studio. It brings up the DesignGuide Developer Studio documentation. Another way to access the Developer Studio documentation is by selecting Help > Topics and Index > DesignGuides > DesignGuide Developer Studio (from any ADS program window).

DesignGuide Developer Studio > Start DesignGuide Studio is only available on this menu if you have installed the DesignGuide Developer Studio. It launches the initial Developer Studio dialog box.

Add DesignGuide brings up a directory browser in which you can add a DesignGuide to your installation. This is primarily intended for use with DesignGuides that are custom-built through the Developer Studio.

List/Remove DesignGuide brings up a list of your installed DesignGuides. Select any that you would like to uninstall and choose the Remove button.

Preferences brings up a dialog box that allows you to:

Disable the DesignGuide menu commands (all except Preferences) in the Main window by unchecking this box. In the Schematic and Layout windows, the complete DesignGuide menu and all of its commands will be removed if this box is unchecked.

Select your preferred interface method (cascading menus vs. dialog boxes).

Close and restart the program for your preference changes to take effect.

Note
On PC systems, Windows resource issues might limit the use of cascading menus. When multiple windows are open, your system could become destabilized. Thus the dialog box menu style might be best for these situations.

Basic Procedures

The features and content of the Linearization DesignGuide are accessible from the DesignGuide menu found in the ADS Schematic window.

To access the documentation for the DesignGuide, select either of the following:

DesignGuide > Linearization > Linearization DesignGuide Documentation (from ADS Schematic window)

Help > Topics and Index > DesignGuides > Linearization (from any ADS program window)

The menu selections from FeedForward to Power Amplifier Characterization each have additional selections. The menu commands for step-by-step schematics for FeedForward and RF Predistortion are shown here.

Selecting one of these menu picks, such as Step1. Cancellation Loop Swept Coefficients, under Feedforward, copies a schematic into your current project.

Each schematic contains a sample power amplifier. The simulated results are displayed in a data display file that opens automatically, after the schematic is copied into your project. Modify the power amplifier by editing its subcircuit, or delete the device and replace it with a different one.

The schematics can demonstrate performances that can be achieved through linearization. The individual components such as the couplers, auxiliary amplifier, combiners, complex correlators, etc., can be replaced by user-defined subcircuits. The red boxes enclose parameters you should set, such as the operating frequency and power level. After making modifications, run a simulation, using ADS, and the data display will update. The linearization techniques generally consist of steps that you should follow to better understand the design procedure and ultimately realize a linearized power amplifier. The steps are also used to demonstrate the sensitivity as well as show various adaptation techniques.

The simulation results for Step 1 of the feedforward linearization are shown here.

Most of the information on this data display and others is in a format that engineers can easily understand. The visibility of equation syntaxes that should not need modification is minimized. Information about items on a data display that you would want to modify is enclosed in red boxes.

Selecting Appropriate Configurations

The Linearization DesignGuide is broken up into different linearization sub-categories, as shown in the previous sections. The specifications that you use depend on your objective and the type of technique that your system can accommodate. If, for example, you have a wideband power amplifier, you can start with the FeedForward configuration at Step 1, shown here. Then proceed through the steps until you have a better understanding of the design procedure.

Shown in the figure is the optimization procedure based on the signal cancellation loop. A complex gain adjuster's parameters Alpha_I and Alpha_Q are adjusted to minimize the fundamental components. The optimization values should be noted for future steps.

IMD Reduction Performance from Two-tone Modulation

The error cancellation loop's complex gain adjuster parameters are optimized in step 7 based on minimizing the adjacent channel power ratio. Shown here is the IMD reduction performance that can be achieved based on a two-tone modulation. In this figure, the spectral plots need to be adjusted to the users frequency and power ranges. The optimum parameters for the complex gain adjuster should be noted.

The linearization steps have provided you with the optimum complex gain adjuster parameters under given conditions. Those conditions are the operating frequencies of the tones as well as the average output power delivered by the amplifier. To proceed to a linearized power amplifier based on a user defined modulation, the same conditions must be satisfied.

Convergence of Optimum Parameters

It is important to ensure that the output power from the schematic shown here is the same as that used to obtain the optimum Beta and Alpha parameters. Optimization can be performed on the linearizer using a given modulation source. However, the simulation would take a significant amount of time. This is why it is important to achieve convergence on the optimum parameters through a systematic procedure. The source can be replaced with a user-defined modulation subcircuit. Future upgrades will contain a more in-depth list of sources.

Feedforward Linearizer Performance Based on Reverse Link Signal

The performance of the feedforward linearizer based on a reverse link IS95 CDMA signal is shown here.

The spectral improvement is easily observed, as well as the operating output power conditions and ACPR calculations. If improvement is not observed, you should check the operating conditions used during the two-tone step-by-step procedure and ensure that they are close to those used for the modulated source. Further improvement can be obtained by performing an optimization with the modulated source. Optimization with a modulation source would take a significant amount of time to converge to the optimum state but will require less iterations once we are close to the optimum values for Alpha and Beta.

In addition to the Analog/RF simulation of various linearization techniques, there are also real-time adaptive simulation using ADS Ptolemy schematics. Various forms of Feedforward linearization adaptation techniques are available, as observed in the selection menu shown here, found under ADS Ptolemy (Demos/Templates).

The demos show the stability and speed of adaptation based on Behavioral Model Power Amplifiers. A few templates are available that use co-simulation and incorporate a transistor level power amplifier.

Feedforward Linearizer Using Complex Correlator

Following is the ADS Ptolemy schematic for a FeedForward Linearizer using a complex correlator for both adaptive loops.

The operating frequencies and power levels can be altered to better reflect your configuration. The percentage bandwidth between the frequency spacing and center frequency need to be scaled proportionally. Otherwise, the bandstop filter in the second adaptive loop will need to be modified.

The outputs from these ADS Ptolemy schematics is in the form of either storage to data files or real-time X-Y Plots. As soon as the X-Y Plot windows open, you should pause the simulation and arrange the plots within the window. We can observe the initial spectral state of the FeedForward Linearizer.

Once the simulation is continued, we can observe the reduction of the intermodulation distortion as well as the stabilization of the Alpha and Beta coefficients. The other two plots labeled IMD_3rd and IMD_5th display the level of 3rd and 5th order intermodulation products as a function of time.