Harmonic distortion calibrator: additional uncertainty due to distortion in the measurement cable

Reference-level measuring instruments (working standards of the first and second digits) play a role in ensuring the uniformity of measurements of harmonic distortion, namely, the transfer of a unit of harmonic distortion from the primary standard to all measuring instruments from a huge fleet of working measuring instruments. There is a need for a continuous search for ways to reduce the error of these measuring instruments. In this work, methods for reducing one of the components of the measurement tool error of the reference level, namely, an additional error caused by distortions in the measuring cable due to a mismatch of the resistances at the calibrator output and the input of the meter, are examined. Until recently, the influence of measuring cable parameters on the results of harmonic distortion measurements has been considered negligible because harmonic distortion measuring instruments operate at sufficiently low frequencies (less than 1 MHz). However, for modern reference-level measuring tools, such an effect has become noticeable and warrants an investigation. For calibrators with large output impedance (600 Ω), which significantly exceeds the characteristic impedance of the measuring cable, the additional error due to distortion in the measuring cable can be significant. An analytical dependence of the indicated error on the parameters of the measuring cable, the calibrator signal, and the output resistance of the calibrator is derived. Conditions are established under which this additional error is absent or can be ignored due to its smallness. When testing harmonic distortion measuring instruments, a measuring cable 1 m long with a characteristic impedance of 50 Ω is frequently employed. For a cable with these parameters and different output resistances of the calibrator, we calculate and experimentally evaluated the indicated additional error. For a signal with a harmonic fundamental frequency of 200 kHz and a uniformly decreasing distribution of harmonic levels with a calibrator output impedance of 600 Ω, the additional absolute error is 3% in generating a harmonic distortion equal to 100%. The use of factory corrections for the total error in the formation of harmonic distortion of the manufactured calibrator allows only a partial reduction of the additional error. To significantly minimize the additional error of the calibrator used, using the measuring cable supplied with the calibrator or a cable with identical parameters is needed. The experimental estimate of the additional error agrees well with the calculated value. This work recommends considering the additional absolute error in the formation of harmonic distortion when developing and testing instruments for harmonic distortion measurements.