  We describe a detailed radial velocity error budget for the NASA-NSF Extreme
Precision Doppler Spectrometer instrument concept NEID (NN-explore Exoplanet
Investigations with Doppler spectroscopy). Such an instrument performance
budget is a necessity for both identifying the variety of noise sources
currently limiting Doppler measurements, and estimating the achievable
performance of next generation exoplanet hunting Doppler spectrometers. For
these instruments, no single source of instrumental error is expected to set
the overall measurement floor. Rather, the overall instrumental measurement
precision is set by the contribution of many individual error sources. We use a
combination of numerical simulations, educated estimates based on published
materials, extrapolations of physical models, results from laboratory
measurements of spectroscopic subsystems, and informed upper limits for a
variety of error sources to identify likely sources of systematic error and
construct our global instrument performance error budget. While natively
focused on the performance of the NEID instrument, this modular performance
budget is immediately adaptable to a number of current and future instruments.
Such an approach is an important step in charting a path towards improving
Doppler measurement precisions to the levels necessary for discovering
Earth-like planets.
