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CHAPTER 5: EMI MEASUREMENTS
Figure 5.4: Setup for current probe calibration
voltage output of the current transducer. Once this second sweep has been performed
and the data exported, the probe’s impedance can easily be computed for each fre-
quency simply dividing the voltage by the current references. This will generate a list
of impedance values, one for every frequency of the sweep. Calculating the logarithm
of this values will provide the Z db values, to be used with the formula 5.3.1 to translate
the voltage reading from the spectrum analyser into proper current readings.
5.4 Experimental current measurements: CM & DM
This section deals with the EMI current measurements, showing how to perform the
readings with the spectrum analyser for a two and a three wire system, with the as-
sumption that the currents flow as illustrated in Fig. 5.5.
Regarding the former case, the CM and DM currents can be acquired as depicted in
Fig. 5.6 [47, 48]. The CM configuration is clear because it has to measure the current
that flows through the earth connection, and it comes from both the line wires. The
DM configuration may seem odd but this is necessary to remove the CM component
from the line current, and it can be explained with some algebra [49]. Considering
that the line current comprises both CM and DM parts (Fig. 5.5) it is clear to see that
iA = iDM + iCM
2 and iB = −iDM + iCM
2 , hence the measurement will simply perform the
following equation:
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