Uncertainties in EMC — Calibration and Testing

In this paper, the random- and systematic-errors are presented for EMC-measurements, including the measures that can be taken in order to minimize the uncertainty due to their effect. The probability density functions (PDF) are calculated for a normal distribution, a rectangular flat PDF and a U-shaped PDF which is function of the complex reflection coefficients of source and load due to impedance mismatch that can occur. The total combined standard uncertainty was then calculated via the appropriate sensitivity coefficients, when there is no correlation between any of the input estimates and must be determined in worst-case if correlation between any contributions is suspected. Normally the level of confidence for EMC-testing is 95%, with a coverage K-factor of 2. However a coverage factor may be necessary for some exceptions depending on the ratio of the total combined uncertainty Uc(y) and system repeatability tests σ, on a reference EUT (Equipment Under Test). This can be determined with a student-T distribution with a number of degrees of freedom v, depending on the ratio Uc(y)/σ. The method was first developed for accreditation purposes for antenna-calibrations according to ANSI C63.5 at Laboratoria De Nayer and for the EMC-test radiated emissions according to CISPR22 and CISPR32. Further research was focussed on radiated immunity (IEC61000-4-3) and conductive immunity (IEC61000-4-6) and the uncertainties for pulsed-immunity standards, such as electrostatic discharge (ESD) according to IEC61000-4-2, Electrical Fast Transients (EFT) based on IEC61000-4-4 and surges (IEC61000-4-5), but these results are not presented in this paper.