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LANGER
DE-01728 Bannewitz
mail@langer-emv.de
EMV-Technik
www.langer-emv.com
2.3 HFW 21 RF current transformer
The RF current transformer (Fehler! Verweisquelle konnte nicht gefunden werden. and
) measures high-frequency currents of up to 1 GHz on lines and diverts these currents to the base
plate.
Figure 9 HFW 21 RF Current Transformer
The RF current transformer enables separate measurements of RF common-mode and differential-
mode currents. In practice, you can do this by connecting the RF current transformer into the
power supply circuit of the unit under test, for example. The supply voltage is connected to the
"POWER" socket connector of the current transformer via laboratory cable with a 4 mm plug and
the unit under test is connected to the "COM" or "DIFF" output via one of the supplied plugs. The
RF currents emitted by the unit under test are thus passed through the transformer, measured
(output above the 50 Ohm SMB socket) and diverted to the metal transformer ground plate through
capacitive coupling.
If the unit under test is connected via the "COM" output, the RF current transformer measures the
common-mode currents flowing on both lines whereas the differential-mode currents are measured
via the "DIFF" output.
The RF output voltage is independent of the direct current flowing through the transformer if
common-mode measurements are carried out. In case of differential-mode measurements, the RF
output voltage decreases according to the diagrams shown in section 7.
Please note:
A current surge occurs on switching on depending on how the unit under test is connected. If
the electrolytic capacitors, for example, are charged without limiting the current and the unit
under test is connected via the HFW 21 differential-mode output, there is a risk of damage to
the preamplifier or spectrum analyzer input!
Always connect the HFW 21 to the PA 203 or spectrum analyzer after the unit under test has
been switched on in such cases!
The same thing occurs during a short circuit in the unit under test if the electrolytic capacitors
of the external power supply unit are abruptly discharged. There is no risk if the HFW 21 is
operated via the common-mode output because these current pulses are compensated within
the transformer.
Current peaks also occur if the supply of the unit under test is connected to earth and thus to
the ground plate with an external power supply unit and the voltage in the unit under test is
short-circuited to the GP 23 ground plate during the measurement:
Figure 10 HFW 21 RF Current Transformer
(schematic)
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ESA1
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