We present a novel method of generating an ac voltage using a programmable Josephson voltage standard. The ac voltage is generated by Δ-Σ (Delta-Sigma) modulation of multiple Josephson levels. Compared with an ac voltage generated by stepwise approximation, the Δ-Σ method reduces the low frequency harmonics of the generated signal. We demonstrate this method by generating a 90 Hz sine wave and measuring the spectrum with a digitizer.