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

Abstract

The role of measuring instruments of the reference level (working standards of the 1st and 2nd digits) in ensuring the uniformity of measurements of the harmonic coefficient is considered, namely, the transfer of a unit of the harmonic coefficient from the primary standard to all measuring instruments from a huge fleet of working measuring instruments. A continuous search for ways to reduce the error of these measuring instruments is required. The methods of reducing one of the components of the measurement error of the reference level – an additional error caused by distortions in the measuring cable due to a mismatch of the resistances at the output of the calibrator and the input of the meter – are investigated. Until recently, the influence of measuring cable parameters on the results of harmonic coefficient measurements was considered negligible, because harmonic coefficient measuring instruments operate at sufficiently low frequencies (less than 1 MHz). However, for modern reference-level measuring instruments, such an influence has become noticeable, and it needs to be investigated. It has been shown that for calibrators with a large (600 Ohm) output impedance, significantly exceeding the characteristic impedance of the measuring cable, this error can be significant. An analytical dependence of the indicated error on the parameters of the measuring cable and the calibrator signal, as well as the output resistance of the calibrator, is derived. Conditions have been established under which this additional error is absent or can be neglected due to its smallness. When checking harmonic coefficient measuring instruments, a measuring cable 1 m long with a characteristic impedance of 50 Ohms is most often used. For a cable with such parameters and different values of the output resistance of the calibrator, the indicated additional error was theoretically calculated and experimentally evaluated. For a signal with a fundamental frequency of 200 kHz and a uniformly decreasing distribution of harmonic levels with a calibrator output impedance of 600 Ohms, the additional absolute error in generating a harmonic distortion equal to 100 % is 3 %. The use of factory correction for the total error in the formation of the harmonic distortion coefficient of the manufactured calibrator allows us to reduce the additional error only partially. To significantly minimize the additional error of the calibrator used, it is necessary to use the measuring cable supplied with the calibrator or a cable with identical parameters. The experimental estimate of the additional error is in good agreement with the results of its calculations. It is recommended to take into account the additional absolute error in the formation of the harmonic coefficient when developing and testing instruments for measuring the harmonic coefficient.