The ADS schematic for the example amplifier for large-signal S-parameters, gain compression, and phase compression employs the P2D file called "2.45GHz_PA.p2d." The file was simulated for the nominal operating frequency of the amplifier (a CW frequency of 2.45 GHz), with the output of the amplifier terminated in 50 ohms, thereby facilitating Sparameter simulations. The large-signal S-parameter simulation block was inserted (LSSP) and the powers set to the measured input power to the device.

Figures 3 and 4 show plots of the simulated AM-to-AM compression and AM-to-PM compression, respectively, based on the use of P2D files in ADS based on measurements of the Intersil 2.45-GHz PA. Although the P2D file is effective in providing large-signal amplifier simulations, the basic large-signal measurement setup can also be used for small-signal measurements and modeling with only very minor adjustments. Figure 5 shows the setup for small-signal Sparameter analysis using the P2D file approach. Using the P2D file, the simulated small-signal S-parameter data for amplitude (gain) and phase shown in Fig. 6 resulted.

In addition to traditional one-tone or two-tone simulations, a P2D amplifier model can also be used to predict performance with the complex digitally modulated signals found in modern communications systems. Communications system performance parameters such as adjacent-channel power ratio (ACPR) and error vector magnitude (EVM), which are used to evaluate amplifier spectrum spreading outside of the desired channel(s) and the amplifier's effects on modulation quality, respectively, can be evaluated by means of a P2D amplifier model.

Figure 7 shows an example simulation schematic in which a WLAN signal is used at the amplifier input. Figure 8 shows the results of an envelope-domain simulation using a P2D model. Nonlinear spectral-regrowth behavior is apparent, predicted by the nonlinear model integrated into the P2D amplifier model. Figure 9 shows the ACPR performance of the amplifier at various input power levels. The P2D amplifier model helps identify high levels of ACPR at large input power levels, as expected.

Although this example is based on the Agilent ADS software, the P2D file modeling approach can be used with other software tools. For example, The MathWorks (www.mathworks.com) is adding support for P2D file-based behavioral models in the latest release of its popular MATLAB mathematical analysis and modeling software.

A schematic diagram for a MATLAB simulation setup is shown in Fig. 10. Figure 11 shows simulated gain-compression and phase-compression curves generated by using P2D-based behavioral models in MATLAB, with the results closely matching those of the ADS simulations in Figs. 3 and 4.

In short, the P2D-file-based behavioral model format available in simulators such as ADS from Agilent Technologies and MATLAB from The MathWorks allows users to conveniently represent a number of important nonlinear device effects in a compact format. The simulations shown in this article demonstrate how the P2D models can be used to go well beyond S-parameters in providing predictions of power-dependent device behavior such as AMto-AM conversion and AM-to-PM conversion effects. The P2D models can also be used for more advanced simulations with complex digitally modulated test signals in the envelope domain, including simulations and analysis of ACPR and EVM performance.

Editor's Note: A slightly longer version of this article, complete with an example of the P2D file format that was adapted from an example provided by Agilent Technologies in the ADS software suite, can be found on the Microwaves & RF website at: www.mwrf.com. The ADS files used for the example simulations shown in this paper will be made available for download from the following Modelithics web area: www.modelithics.com/free-models.asp. Further information is available by e-mailing info@modelithics.com.

ACKNOWLEDGMENTS
This article was adapted in part from work performed at the University of South Florida (USF) by former USF students Brad Rametta and Prabhu Patel. Collaboration with Jim Paviol and Patrick Landy, previously with Intersil, and suggestions and input provided by Chris Meuth and Cory Edelman of Agilent Technologies and Colin Warwick of The Mathworks, are also gratefully acknowledged.

REFERENCES
1. Advanced Design System software documentation, Agilent Technologies, Inc., Palo Alto, CA, Internet: www.agilent.com.
2. J.C. Pedro and S.A. Maas, "A comparative overview of microwave and wireless poweramplifier behavioral modeling approaches," IEEE Transactions on Microwave Theory & Techniques, Vol. 53, No. 4, April 2005, pp. 1150–1163.
3. Jiang Liu, Lawrence P. Dunleavy, and Huseyin Arslan, "Large Signal Behavioral Modeling of Nonlinear Amplifiers Based on Loadpull AM-AM and AM-PM Measurements," IEEE Transactions on Microwave Theory & Techniques, Vol. 54, No. 8, August 2006, pp. 31913196.
4. Chris Meuth, "Behavioral Modeling Smooths RF Design," EE Times, September 26, 2003.

FOOTNOTE
aDr. Dunleavy also holds a faculty position at the University of South Florida, 4202 East Fowler Ave., Tampa, FL 33620; Internet: www.usf.edu