Microwave Engineering Europe - December 2008 - (Page 20) 20 X-PARAMETERS can extract the intrinsic device X-parameters even in the presence of source harmonics and harmonic mismatch of the measurement system. Measurement-based X-parameter design flow The application flow for nonlinear design using X-parameters measured on the Agilent NVNA is shown in Figure 5. The extracted X-parameters are exported to a data-file in generic MDIF format, and placed in the user’s ADS PHD_Files directory. A menu pick auto-configures a PHD simulation component that corresponds to the data file with the requisite number of RF and DC ports. In the ADS schematic environment, one simply drags and drops the component onto a schematic page and one can immediately begin nonlinear simulations with the component and design nonlinear circuits. GSM Handset Amplifier The GSM handset power amplifier is designed to be integrated into a cellular phone to drive the antenna. Because the load presented to the amplifier by the antenna varies significantly depending on the phone’s surroundings, a good design requires information about how the device performance is affected by mismatch. Since GSM amplifiers are operated in saturated mode, traditional approaches such as “hot S22” are insufficient to capture this and loadpull measurements are typically relied upon. X-parameters provide an alternative approach to characterization and design which is more accurate and more complete than “hot S22”, less time consuming than load-pull, and integrates seamlessly with Agilent’s ADS EDA tool. Figure 6 shows a comparison of the output power and DC current predicted by the X-parameters to measurements taken on a load-pull system on a commercially available GSM amplifier. Accurate prediction of DC performance and RF performance — including harmonics — under mismatched conditions in simulation allows for simultaneous optimization of output power, efficiency, and distortion which was not previously possible. The ability to predict behavior under mismatch conditions also enables accurate simulation of cascaded nonlinear devices, including prediction of distortion through a chain of devices. Load-dependent X-parameters The examples presented thus far in this article, along with most of the discussion, have been limited to X-parameters measured at a LSOP consisting of DC and a single large tone at one port. For some devices, such as high power amplifiers operated under highly mismatched conditions, the incident wave at port 2 at the fundamental frequency may be large enough that it must be included in the LSOP rather Figure 4: The three independent experiments required to extract a set of three X-parameters relating the scattered wave at port p harmonic m to the LSOP (|A11| in this case) and to the small signal incident wave at port q harmonic n are shown in red, green, and blue. The terms extracted related to each measurement are underlined with the corresponding color in the equation above. Figure 5: Measurement-based nonlinear design flow with X-parameters: Diagram illustrating process flow of X-parameter measurement on NVNA, import into ADS, design and simulation. Figure 6: Comparison of X-parameter simulated (blue) and load-pull measured (red) DC current and output power contours on commercially available GSM amplifier. Microwave Engineering Europe ● December 2008 ● www.mwee.com http://www.mwee.com
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