The short time fourier transform and the spectrograms to characterize EMI emissions

Spectrograms are useful diagnotic tools because they allow to improve our understanding of unwanted transient disturbances and in general of the so called sophisticated signals which include frequency-modulated pulses, coded groups, phase-modulated signals, etc. Unlike the limits specified by EMC standards which usually make reference to graphs of amplitude versus time or amplitude versus frequency, spectrograms allow users to analyze the behavior of disturbance signals in the time and frequency domains in the same diagram with the advantage of improving our knowledge of interference sources. The Short Time Fourier Transform, which is the key issue of spectrogram technology, is briefly described discussing the implications of the Uncertaity Principle, which has a major impact in the analysis of wideband and narrowband signals.