We provide techniques to generate and characterize precision wideband millimeter-wave modulated signals. We use predistortion to obtain a significant improvement in signal quality and an associated reduction in the error vector magnitude (EVM) of the signals generated by an arbitrary-waveform-generator-based source. We adapt a recently developed microwave uncertainty framework to the problem and use it to estimate the uncertainties in the modulated-signal measurements. Models of uncertainties related to all calibration and measurement procedures within the traceability path are included in a sensitivity analysis and Monte Carlo simulations that maintain correlations between time- and frequency-domain errors. We demonstrate EVM values of approximately 1.6% $\pm$ 0.5% for a 1-GSymbol/s 64-state quadrature-amplitude-modulated signal at 44 GHz.