Uncertainty and design budgets applied to error vector magnitude (EVM) for digital modulation systems

EVM is a key metric for RF components and subsystems in most digital modulation systems. Understanding the way in which different distortion mechanisms contribute to the overall EVM is the key to determining how to construct a system budget for EVM. Not only must each block in a transceiver chain have a sufficiently small EVM, test equipment uncertainties are also a significant proportion of such a budget. This leads to the question of how EVM contributions and uncertainties add, and how to ensure that an indicated result is within specification. Some contributions add coherently - to raise or (possibly) lower the composite value; and some add randomly, in a noise-like fashion, such that the EVM increases according to an RSS addition. This paper investigates both random and coherent distortion effects, and demonstrates how to treat each type correctly. EVM examples for GSM/EDGE, W-CDMA, and 802.11a are presented.