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LANGER
DE-01728 Bannewitz
mail@langer-emv.de
EMV-Technik
www.langer-emv.com
4 Measurement set-up variants
4.1 Measurement of the common-mode component
4.1.1 Unit under test with one cable terminal
If only one cable, e.g. the power supply cable, is connected to the unit under test, its leads are
filtered first through the GP 23 ground plate and then through the HFW 21 (Input: Power and
output COM). The RF current flows through the parasitic capacitance between the unit under test
and ground plate to the ground plate and back to the unit under test through the HFW 21 via the
power supply cables (Figure 22 and Figure 23).
Figure 22
Figure 23
The capacitance between the unit under test and ground plate is determined by the size and
distance of the unit under test. The RF current flows through this parasitic capacitance. This may
be caused by electric fields on the surface of the unit under test which are generated by large
surface clocked bus systems.
In some cases it will not be possible for you to pass the single cable through the current
transformer, e.g. if
- more than two wires are needed,
- the current consumption of the unit under test is too high,
- the supply voltage is too high.
In such cases you should connect the unit under test via the usual cable and connect its GND
directly to the ground plate via the HFW 21 by as short a cable connection as possible. A large part
of the RF current which is normally fed to the cable will be diverted via the HFW 21 and measured.
You can make this procedure more effective by inserting a choke into the cable.
The distance between the unit under test and
the ground plate influences the magnitude of
the RF current:
- If the unit under test is operated in an open
space (computer mouse on a wooden table),
choose a large distance between the unit
under test and ground plate (> 5 cm).
- If the unit under test is installed near a
metal surface (control system in a washing
machine), simulate this distance.
The scope of delivery includes a foam block
(25 mm high) which you can use as a
distance piece.
- 16 -
ESA1
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