Microwave Engineering Europe - January/February 2009 - (Page 12)

12 TEST & MEASUREMENT Ampliﬁer error vector magnitude characterisation using high-speed modular PXI instruments RF device characterization can be very time consuming. Conventional instrumentation tends to be slow even when automated. The use of high-speed PXI modular RF instruments can accelerate test speed and produce accurate results. By Tim Culligan, PXI Application Specialist, Aeroﬂex Incorporated P XI (PCI eXtensions for Instrumentation) provides a standardized modular form factor used in measurement and automation systems. RF test and measurement equipment has been developed that combines RF signal generation and analysis capabilities with the PXI standard to produce fast, accurate test equipment. Scalable PXI based systems provide small footprint, low cost test solutions that are extremely appealing to RF device manufacturers around the world. Since PXI modules are connected to the common PCI bus, the hardware connection is analogous to that of peripheral cards in a PC. Beneﬁts of this arrangement include high-speed communication between module and host processor as well as ability to easily upgrade test speed as advances in PCs become available. A prime example of a test application for PXI modules has to do with testing the modulation quality of vector-modulated signals from the output of ampliﬁer circuits used in digital communications. Ampliﬁer circuits used in the transmission of vector modulated signals are required to provide acceptable Error Vector Magnitude (EVM) performance over a speciﬁed range of power levels and frequencies. Characterization of EVM performance is necessary to ensure proper operation of ampliﬁer circuits used in digital communication applications. When performing ampliﬁer EVM characterization, it may be necessary to make EVM measurements at a number of incremental power level steps and then verify that each measured value falls within a speciﬁed range. Graphical presentation of EVM versus output power makes it easy for the designer or test engineer to see the EVM performance of a particular device over a range of power levels. As an ampliﬁer’s output power increases towards its’ gain compression point, the Figure 1: Ampliﬁer EVM versus Output power plotted iteratively at ten frequency steps. corresponding EVM performance can be degraded due to non linearity. Characterizing this behaviour is an important part of the design process. This information allows users of the device to select their operating point. Plotting EVM versus output power at a number of frequency steps graphically illustrates the device behaviour. The plots shown in ﬁgure 1 illustrate a family of EVM versus output power is curves. The bold dots on each curve represent measured output power values in dBm versus their corresponding EVM values, shown in percent. Each plot is relatively ﬂat or rises slightly from left to right as the ampliﬁer’s output power is increased within its’ speciﬁed operating range. The curve then rises sharply once the ampliﬁer’s output power nears its’ gain compression region. To produce such a family of curves requires a very large number of iterative measurements. This data collection and calculation can take a lot of time, depending on the method and the equipment used. PXI solution RF modular PXI instruments conﬁgured for this application have been proven as an accurate, high-speed solution for performing the many iterative measurements necessary for ● EVM characterization. The characterization curves shown in ﬁgure 1 are the result of calculations made at 12 power level steps, iteratively performed at each of 10 frequency steps. The resulting measurement time was only 4.3 seconds, 10 times faster than the solution it replaced. In this example, 8192 IQ sample pairs were captured at each power level step. EVM and ampliﬁer output power were calculated for each captured sample set. This capture/calculation process was repeated 120 times on a total of 120 x 8192 or 983040 captured samples. The number of IQ sample pairs necessary to make accurate measurement calculations is a limiting factor regarding test time. Another is the hardware response times of the test equipment. Greatest efﬁciency and test time optimization is achieved when capturing the least number of required samples using the fastest responding test equipment. Sample rate optimization is achieved by determining an appropriate sample rate and overall sampling interval for each type of communications signal. In the case of bursted signals, capture of only one burst is required for the Aeroﬂex measurement algorithm to make the needed calculations. Hardware response times are technology and manufacturer speciﬁc. Microwave Engineering Europe ● January/February 2009 www.mwee.com 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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