This paper describes a new calibration system developed at INMETRO to guarantee traceability of low-frequency alternating current (ac) voltage calibrations (<1 kHz) to a programmable Josephson ac waveform synthesizer. The automated full-synchronous system allows the synchronization of a commercial calibrator or signal synthesizer with a programmable Josephson voltage synthesizer system to be made by employing digital sampling and signal processing techniques, aided by adaptive digital control. It allows accurate determination of the spectral content of ac signals with minimum human intervention. System operation, measurement techniques, and data analysis are detailed.