In EMI applications, the main focus of a scanner system is on locating real electromagnetic interference (EMI) sources distributed in a device under test, the DUT. Accordingly, the scanning surface is located in the highly reactive region of the DUT to enable a precise spatial localization of the electric charges and current surface densities directly from the mapped pattern of probe signals. Typically the separation between the scanning surface and the DUT is much smaller than the largest physical dimension of the DUT. Typical distances are 1 mm for scanning of PCBs and 30 μm for scanning of integrated circuits on a die level. In order to quickly localize field emission in the frequency domain, time domain detection techniques together with signal processing based on fast Fourier transform could be employed, e.g. utilizing a digital storage oscilloscope as a signal receiver.
