Time synchronized near-field and far-field for EMI source identification

The evaluation of a product in terms of radiated emissions involves identifying the noise sources. Spectrum analyzer (SA) measurements alone are unable to identify noise sources when multiple sources are responsible for emissions at a particular frequency. In this paper, an approach using combined near-field and far-field measurements is proposed. This method consists of recording signals from a near field probe and from an antenna in the far-field using a high speed oscilloscope and analyzing the relationship between them via different post processing methods. The noise source can be identified by varying the location of near-field probe and searching for the probe signal that best correlates to the far field signal. A variety of post processing methods have been employed in this work. The Short Term Fast Fourier Transform (STFFT) is used to visualize the time dependence of the frequency content. Envelope correlation, coherence factor, and cross-correlation methods are further explained and tested for their ability to identify possible sources of emission problems.