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

The ac-dc transfer with a multijunction thermal converter (MJTC) is the most precise method to determine the rms value of an ac voltage. Its ac-dc transfer difference is known with an uncertainty smaller than 1·10^-6 in the audio frequency range. At frequencies below 40 Hz down to 10 Hz, the ac-dc transfer difference of an MJTC increases, but it can be evaluated with an uncertainty smaller than 1·10^-6. However, in the planar version of the MJTC the thermal time-constant could be increased to improve its properties at low frequencies, but there is also the limit of 10 Hz for accurate measurement. The advantage of the thermal approach is the wide bandwidth of the MJTC which allows to measure all spurious effects in the rms value of the generated waveform.