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Linear frequency modulated (FM), or chirp, pulse compression is a widely used technique for improving the range resolution of radar systems, although it often places quite stringent demands on FM sweep linearity. This paper examines the impact of sweep nonlinearities on the performance of frequency modulated continuous wave (FMCW) radar systems, particularly those employing simple voltage-controlled oscillator (VCO) sources, using a new and straightforward approach based on the fractional slope variation (FSV). Modeled results are presented, assuming a square-law source nonlinearity representation, showing the effect of such nonlinearities on point-target response and range resolution. These results are then related to the standard definition of linearity. Measurements from a commercial VCO are finally used to convincingly validate the work, resulting in a simple and practical method to predict the impact of source nonlinearity, as defined by the FSV parameter, on the performance of an FMCW radar system.