Spectral leakage errors when using an Agilent 3458A to measure phase at mains power frequencies

By employing standard FFT methods, the phase of a single tone of a mains energy waveform can be derived from measurements made using an Agilent 3458A. Phase errors arising from the finite sampling interval resolution can be corrected when the signal frequency is known. Knowing the form of the resulting spectral leakage errors can assist in correcting the measured phase to within 0.002°. In the SSDC mode of the 3458A, these phase errors reveal an unexpected step change in the 3458A's effective sampling interval resolution as the sampling rate is changed.