The Emission Spectrum

Conducted noise example

The parasitic emission is the amount of energy plotted in dBµV within a certain frequency, usually from range 1 to 1000MHz. The spectrum shown in this slide has been obtained by placing a 1 ohm resistance in the supply path, to pick the conducted noise flowing to the ground supply of this 16 bit micro-controller. The spectrum analyzer gives a frequency domain representation of the ISS current versus time. A Fourier transform of the time domain signal would give a similar result. The noise floor of the measurement apparatus is around 20dBµV in this case. This means that a signal lower than 20dBµV cannot be detected with this equipment. The customer usually specifies a maximum level of energy that the IC should not overcome, in order to warrantee an electromagnetic compatibility of the electronic equipment.

Processing the spectrum

The measured spectrum, shown on the left part of the figure is a very precise evaluation of the energy versus frequency. It usually consist of several thousands of points, which lead to huge experimental data which no not ease comparisons. In many case, a small post-process program only considers local maximal energy. Furthermore, within a given decade, only a reduced set of maximum is kept (10 for example). Also, we call f_Amax the frequency with the maxi energy Amax. This allows for an immediate comparison between spectrum.

Trend: investigate ever higher frequencies

The global trend is towards an increase of emission parasitics, over a wider frequency range. The 32-bits micro-controllers have a spectrum mainly between 100 and 500 MHz, with some harmonical contents in mobile phone frequency bands (0.9, 1.8, 1.9 GHz). It is expected that 64 bit processors will emit even higher harmonics, at even higher frequencies.