On making accurate measurements with a harmonic distortion meter
James Ross Macdonald · IRE Transactions on Audio · 1957
It is pointed out that an avoidable error occurs in making total harmonic distortion measurements by the usually recommended method with art instrument incorporating art average-responding output meter. The error arises from the fact that the average value of a full-wave rectified distortion signal can be reduced by adding a small phase-shifted fundamental component to the distortion. The error is always in the direction to make equipment seem better than it actually is and can amount to ten per cent or more. It can be readily avoided by changing the instrument operating procedure.