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As wireless communications technologies such as Wi-Fi (IEEE 802.11) and cellular (LTE, 5G NR) continue to demand higher data throughput, the modulation schemes used to encode information have grown increasingly complex. Modern systems employ quadrature amplitude modulation (QAM) with orders up to 4096QAM, where each symbol carries twelve bits but the constellation points are extremely close together. This makes modulation accuracy critical: even small deviations in amplitude or phase can cause bit errors. Error vector magnitude (EVM) has become the primary metric for quantifying this accuracy. This white paper covers the fundamentals of digital modulation, defines EVM and its calculation methods, explores the common sources of EVM degradation, and explains how constellation diagrams can be used to visually diagnose the root causes of modulation impairments in practical wireless systems.