Comparison of three accurate methods to measure AC voltage at low frequencies

Three methods for the determination of the RMS value of AC voltage at low frequencies are compared: the step calibration of a digitally synthesized source, the optimized sampling method, and the AC-DC voltage transfer with a multifunction thermal converter. The three methods agree within 1/spl times/10/sup -6/ with an uncertainty of 2.5/spl times/10/sup -6/ in the frequency range from 10 Hz to 100 Hz.