Information
EMI Pre-Compliance Guide RIGOL Technologies
RIGOL Technologies CO., LTD. 09.09.2021
A characteristic for radiation of antennas is the far field. According to the description above,
the characteristic wave impedance Z
F
= E / H which is in far field, is 377 Ω. The idea of a TEM
13
cell is the medium of a coax cable which is suitable for a TEM wave propagation. The
characteristic wave impedance in a coax cable is independent of the mechanical structure
and always 377 Ω. A TEM cell is displayed in figure 13
14
. A common use for pre-compliance
analysis is, to use an open TEM cell with disadvantage to see influence from environment.
But this could be evaluated as a reference measurement with deactivated DUT. The two
bends on the roof are necessary for the matching, but results into a frequency limit (max
frequency range) is the cut off frequency based on the generation of higher wave guide modes
on these bends which have an influence on test results. This cell can be used for susceptibility
stress of a DUT but can also be used for repeatable pre-compliance radiated emission
approximations which are sufficient for pre-compliance analysis. The connection to a
spectrum analyzer needs a DC blocker because the lowest cut off frequency of a coaxial cable
is 0 Hz (= DC) which could destroy a spectrum analyzer. For impedance matching of septum
15
it is necessary to use a 50 Ω Termination on second port to avoid reflections. The DUT is
placed between septum and lower shielding plate. A test setup is visible in figure 14. The
advantage of open TEM cells, failures can also be analyzed in parallel with a near field probe
to detect the root cause area of DUT.
13
TEM wave is a transversal electromagnetic wave which describes a homogeneous plane wave.
14
Source of picture: „Elektromagnetische Verträglichkeit, Adolf J. Schwab / Wolfgang Kürner, ISBN: 978-3-642-
16609-9, edition: 6, §5.7.3 (page: 231)“
15
Septum of TEM cell is the center core of coaxial structure. The mechanical design is to reach the impedance of
50 Ω over specified frequency range.
Figure 14
:
Repeatable radiated emission analysis with an open TEM cell