Microwave Engineering Europe - January/February 2009 - (Page 14)

14 TEST & MEASUREMENT An Aeroﬂex PXI system containing the RF Signal Generator (3010/302x modules), RF Digitizer (3011/303x modules) and embedded host PC is used in the test setup conﬁguration as shown in the block diagram in ﬁgure 2. A software application has been developed that utilizes the PXI functionality to iteratively step through the power and frequency levels and measure and plot EVM vs output power for various standards including WiMAX and WLAN. Measurements can be made quickly over a range of power level and frequency steps, saving time and allowing the user to produce results without difﬁculty. EVM characterization application program The EVM characterization results shown in ﬁgure 1 are achieved using a software application that controls the instruments and calculates EVM and ampliﬁer output power. The PXI RF signal generator plays an arb ﬁle containing the appropriate signal attributes needed to exercise the ampliﬁer under test with respect to the type of communication standard being characterized. The modulated RF signal is applied to the ampliﬁer input. The signal generator’s output power level is automatically stepped through a range of power levels set by the user. These power level steps are repeated over a number of carrier frequency steps. The RF digitizer automatically tracks the signal generator’s power level and carrier frequency and captures the selected number of I/Q samples from the ampliﬁer output. The captured I/Q sample arrays are transferred to the host PC where EVM and output power levels are calculated by the software application, using functionality from the appropriate measurement suites. The user determines the initial power level, step size and number of steps. The application contains a manual operation mode that allows the user to experiment with power, frequency, attenuation and other programmed settings. Experimenting with Figure 2: Ampliﬁer test conﬁguration using PXI. individual test settings allows the user to verify the test hardware conﬁguration and establish acceptable programmed settings. Once the initial test settings are established using the manual mode, the settings are transferred to the application’s automatic mode, allowing for multiple EVM and power measurements to be made. The auto mode allows the user to experiment with EVM/power measurements at any number of carrier frequency increments. The calculated EVM/output power measurements at each frequency can be written to a ﬁle. The resulting graphical plot, like the one shown in ﬁgure 1, can be written to a .png ﬁle. The arb ﬁle played by the signal generator is formatted to provide a signal for the type of communications standard being measured against. For example, to measure WiMAX 802.16e uplink EVM characteristics, an arb signal with the particular attributes of link direction, bandwidth, guard period, modulation type, and so on, would be created using the Aeroﬂex IQCreatorTM software tool. The ﬁles created with this tool can be saved in a library as well as loaded into the signal generator arb’s memory. Conclusion EVM characterization of ampliﬁers used in digital communication applications is necessary in order to verify the speciﬁed design performance of those devices. To achieve this, extensive data capture and computation must be done iteratively over ranges of many power levels and frequencies. This data capture computation process can be very time consuming, depending on the test methodology, hardware and software used to implement this process. The Aeroﬂex PXI system does perform this process much faster than traditional rack and stack instrumentation. Test times of seconds or tens of seconds have been compared against methods taking many minutes. Aeroﬂex PXI is an extremely fast, accurate solution for this type of iterative, data intensive testing. About the author Tim Culligan is a PXI product specialist and application engineer covering the Americas for Aeroﬂex Test Solutions. Mr. Culligan has a BSEE from Portland State University and has been working with Mixed Signal and RF test equipment applications for over 10 years. MICROWAVE ENGINEERING EUROPE Free subscription at: www.mwee.com/subscribe Microwave Engineering Europe ● January/February 2009 ● www.mwee.com http://www.mwee.com/subscribe http://www.mwee.com

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Microwave Engineering Europe - January 2009 News Contents Comment Using KPIs to Ensure Quality in a Converging Network Amplifier Error Vector Magnitude Characterisation Using High-Speed Modular PXI Instruments GPS: Making a Play for Femtocells Accelerating Global WiMAX Adoption: The Move to Picocell and Femtocell Base Stations Addressing PA Efficiency for Multi-Mode Wideband Handset Applications Wi-Fi: Mobile Feature or Fundamental RAN? Products Calendar

Microwave Engineering Europe - January/February 2009

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