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Agilent 8 Hints for Making and Interpreting EVM Measurements Application Note Introduction Defining EVM Error vector magnitude (EVM) The error vector is the vector dif- measurements can provide a great ference at a given time between deal of insight into the perfor- the ideal reference signal and mance of digital communications the measured signal. Expressed transmitters and receivers. With another way, it is the residual proper use, EVM and related mea- noise and distortion remaining surements can pinpoint exactly after an ideal version of the signal the type of degradations present has been stripped away. EVM is in a signal and can even help the root-mean-square (RMS) value identify their sources. of the error vector over time at the instants of the symbol (or Primarily a measure of signal chip) clock transitions. quality, EVM provides both a sim- ple, quantitative figure-of-merit for a digitally modulated signal, and a far-reaching methodology for uncovering and attacking the underlying causes of signal impairments and distortion. EVM measurements are growing rapid- ly in acceptance, being already the required modulation quality measurement in such important technology standards as 3GPP W-CDMA and IEEE 802.11a/b/g WLAN, and they are poised to appear in several upcoming standards. This application note provides useful tips that will assist in accu- rately making and understanding EVM measurements. Depending on the technology, EVM The magnitude of the error vec- The spectrum of the error vec- is reported as a percentage of the tor versus time measurement tor (or error vector spectrum) square root of the mean power of shows the error vector magni- is the frequency spectrum of the ideal signal, as a percentage of tude variations as a signal the error vector time. the square root of the average sym- changes over time--that is, at bol power, or as a percentage of and between symbol decision The EVM troubleshooting tree the peak signal level, usually timing points. shown in Figure 2 is a useful defined by the constellation's cor- tool for analyzing vector modu- ner states. The EVM value can lated signals with EVM mea- also be reported in