In this paper, a software supported DC voltage calibrator is presented, requiring a standard programmable signal generator, a digital voltmeter, and some free software components. After detailed analysis of the programmable signal generator applying an automated process, it is concluded that intrinsic precision is insufficient for the generator to be used as a voltage calibrator. Furthermore, offline calibration techniques as polynomial interpolation and lookup table compensation are ruled out due to the generator reference instability over temperature and time. As a solution, a closed loop method based upon a voltmeter and a control software thread is proposed. The method is verified experimentally.