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LANGER
DE-01728 Bannewitz
mail@langer-emv.de
EMV-Technik
www.langer-emv.com
4.1.4
Example: Measurements on a complex unit under test
In many cases, different effects of various RF sources within the unit under test will first be
superposed and lead to an amplification or reduction of the RF fields at particular frequencies. It is
therefore important for an effective reason analysis, particularly with complex units under test
comprising several modules, to dismantle the unit under test and deal with individual modules
separately.
The unit under test shown in Figure 27 has several potential disturbance emission sources. We
only consider here the interface module plugged on to the basic unit as an example:
Figure 27 RF sources on the interface module
Three RF sources are likely to cause emissions:
1) plug-and-socket connector between basic unit and interface module
2) electronic components (processor with memory chip) on the interface module
3) data streams generated by the interface module and fed into the connected cable
Deal with these three RF sources in succession. You will need measurement set-ups that largely
blank out the other RF sources of the unit under test and those of the basic unit.
1) Plug-and-socket connector between basic unit and interface module
Assumption:
The basic unit and interface module are connected to each other via data and control lines. These
lines are well protected by the basic unit and interface module in the area of the GND systems –
but in the area of the plug-and-socket connector they lie in the open air. The high-frequency shares
of the signals sent via these lines generate RF magnetic fields which can dissipate freely in the
atmosphere and can cause voltage differences between the GND of the basic unit and that of the
interface module. These voltage differences drive RF currents into the cable that is connected to
the interface module and thus cause emissions (Figure 28).
All other potential RF sources have to be largely eliminated to measure these voltage differences.
To do so, make several connections between the GND of the basic unit and the GP 23 ground
plate with copper foil adhesive tape (mainly in the area of the interface module). The basic unit's
GND in the area of the interface module and the ground plate are thus equipotential – the voltage
differences caused by other sources are short-circuited to the greatest possible extent. Also
disconnect the data cable from the interface module provided the data transfer between interface
module and basic unit is not dramatically reduced by this measure.
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ESA1
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