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LANGER
DE-01728 Bannewitz
mail@langer-emv.de
EMV-Technik
www.langer-emv.com
2) Measuring the effect on the unit under test
Simultaneous measuring of the RF current on many lines, such as those of a ribbon cable, is not
possible. In such cases you have to carry out the measurement according to Figure 33:
Do not measure the RF current directly on the signal lines but on another cable (e.g. on the
opposite power supply connection) or, if there is no other cable present, directly on the GND as
described under 4.1.1. Replace the data lines with capacitances relative to the GP 23 ground plate.
Figure 33
4.2 Measurement of the differential-mode component
Beside the common-mode currents measured so far (the RF currents in all wires of a cable travel
in the same direction), differential-mode currents, which are mainly generated in the power supply
unit of DC/DC converters or by clocked loads (e.g. via PWM) are of interest in the lower frequency
range. The RF current transformer enables these measurements via the DIFF port (Figure 34).
According to the disturbance emission mechanism described under chapter 1, this differential-
mode current does not result in any disturbance emissions. But since it also flows in the ground of
the chassis or GND connections between different devices, voltage differences develop between
these devices. These in turn cause compensating currents (common-mode currents) which cause
disturbance emissions.
The differential-mode currents in units under test containing a DC/DC converter or a similar device
have to be measured and, if necessary, reduced by capacitors or chokes because these currents
are partially transformed into critical common-mode currents according to the described
mechanism.
The output signal is damped (saturation effect) depending on the power consumption of the unit
under test (see point 7) in the frequency range below 10 MHz if the RF differential-mode current is
measured on the supply lines of the unit under test. This effect does not occur above this
frequency or if the measurement is carried out via the COM common-mode output.
Figure 34
Measuring the differential-mode current via the
Measuring the common-mode current via the COM
- 22 -
ESA1
DIFF port
or
port
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