Some test and measurement applications require higher dynamic range for multi-tone signals than a signal generator can generate. Moreover, for other applications it can be interesting to improve the performance of a “low cost” signal generator. The spectral purity of generated signals can be improved by using pre-distorted base-band signals. In this paper, a dynamic grey-box model is described. A pre-distorter is used in order to improve the dynamic range. The results, which are based on measurements, show an improved dynamic range for a three-tone signal of approximately 9 dB and an improved ACPR of 5 dB for a WCDMA signal.