Modern communications systems rely on digital modulation techniques to transfer large amounts of information wirelessly. These modulation methods typically rely on vector signals with in-phase (I) and quadrature (Q) signal components. One of the key tests for evaluating the performance of a wireless digital communications system is an error-vector-magnitude (EVM) measurement, which can be performed with a vector signal analyzer (VSA) and a vector signal generator (VSG). But no test instruments are perfect, and noise and other imperfections from the test equipment can degrade the quality of EVM measurements. Fortunately, a five-page application note from ZTEC Instruments, “Extending the Useable Range of Error Vector Magnitude (EVM) Testing,” helps testers get the most from the VSAs and VSGs when making EVM measurements.
The application note uses the IEEE 802.11 wireless-local-area-network (WLAN) standard as an example of a digitally modulated communications system that requires EVM for evaluation. The various iterations of the IEEE 802.11 WLAN standard employ orthogonal frequency-division-multiplexing (OFDM) modulation, which encodes digital data simultaneously on multiple subcarrier frequencies for enhanced signal robustness. The standard’s number of carriers varies by channel bandwidth, with 802.11 channel bandwidths ranging from 20 to 160 MHz.
Such factors as signal instability and noise in a VSA and VSG can hinder the effectiveness of EVM measurements performed with these instruments. Signal noise and distortion can limit the minimum and maximum modulated signal levels that can reliably be measured with a VSG and VSA. For example, test signal phase noise and spurious levels can impact EVM measurement quality.
Fortunately, the application note provides several equalization techniques, as well as noise and distortion optimization methods, that can be used to improve the quality of EVM measurements with a VSA and VSG. A free PDF of the application note is available for download from the ZTEC Instruments website.
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