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CHAPTER 1: INTRODUCTION
Regarding the differential mode, the differential voltage across wires a and b, for exam-
ple, is simply:
V dm = Va−Vb (1.2.3)
Obtaining the DM current for a phase wire, a for example, is not so straightforward
because the measured line current Ia includes both the differential and the common
mode, therefore the latter must be subtracted from that measured to obtain the desired
value:
Ia dm = Ia− Icm
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(1.2.4)
As mentioned earlier, the high dV/dt transitions, typical of power converters, gener-
ate EMI. They are due to the transient charge currents mainly in inter-wire capacitance,
semiconductor p-n junctions and parasitic capacitances toward ground. These currents
appear as spikes superimposed to the fundamental signal. The currents that close to-
ward ground constitute the CM emissions, the others generate the DM ripple which
has a pattern strictly related to the switching frequency and the used modulation.
1.3 Instruments for EMI measurement
There are specific rules that define the instruments to be used for EMI measurements,
they are stated on the standard that imposes the limitations on the EMI, according to
the specific environment the equipment will be used in. The standards for Aircraft
systems which are of interest for the application of this research work, are identified
on the DO-160E document, and they define the mode of acquisition for the equipment
under test. The measuring instrument is a spectrum analyser, used with the peak de-
tector and a resolution bandwidth of 1kHz. To understand the imposed limits, a brief
description on how the instrument works is needed; to this aim the description found
on the Agilent’s AN 150, is very appropriate:
A spectrum analyser is a frequency-selective, peak-responding voltmeter calibrated to display
the rms value of a sine wave.
Basically it decomposes the signal in all of its harmonics and then it records the ampli-
tude for every frequency constituting the measured signal. It has to be pointed out that,
because of its implementation, it loses the phase information, therefore from the read-
ings it is not possible to reconstruct the original signal as it would be with the Fourier
Transform. Even so this is not an issue because we are interested only in the power of
the interference, therefore only the magnitude is required.
A spectrum analyser can be represented with the block scheme in Fig. 1.5. The main
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