CISPR specification and measurement uncertainty of the time-domain EMI measurement system

Traditionally emission measurements are performed in frequency domain by EMI-Receivers. The drawback of EMI receivers is the long measurement time, up to several hours for a single frequency scan. By emission measurements in time-domain and simulation of several thousand EMI receivers by a digital signal processing unit the measurement time can be reduced by several orders of magnitude. In this paper the measurement uncertainty of FFT-based measurement systems are discussed. FFT-bases measurement have due to the digital implementation low calibration effort, however the measurement uncertainty of isolated pulses is dependent on the overlapping of the calculation during the Short time fast Fourier transform. The dynamic range is investigated, and it is shown that even the isolated pulse can be measured correctly in the quasipeak detector mode. The error in dependence of the overlapping for a gaussian window function is investigated and compared with the requirements in CISPR 16-1-1. Procedures are presented that allow to determine the measurement uncertainty caused by the the limited overlapping for FFT-based measurement systems. The procedures are applied to the time-domain EMI measurement system in the frequency range up to 1 GHz.