ETS-Lindgren 5411 GTEM! Test Cell - Manual (Page 39)

GTEM! Use
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Monitoring of EUT performance is via a cable to any externally located monitor
unit. Typical precautions must be taken, such as are used in shielded enclosure
immunity testing with EUT performance monitors. An example would be
grounding the shield of the cable to the performance monitor to the bottom of
the GTEM!. Once the setup is complete, the signal generator is turned over the
test frequency range while monitoring the performance of the EUT for response
to the applied test signal. The levels of the test signal are adjusted by controlling
the signal generator output while monitoring for the minimum field level at the
location of the isotropic probe.
Note that the electric field strength and the sweep speeds are often set by the
test requirements document(s). Care should be taken not to exceed specified
sweep speeds. An additional factor is that, with the availability of automated
testing, it is possible to sweep at the specification-required speed without
consideration of the performance of the EUT. If the EUT must be stepped
through a number of modes at each frequency, then even slower sweep speeds
may be needed.
Radiated Emissions Testing – General
In addition to immunity testing, the GTEM! may be used for radiated emissions
testing. An item placed in the test volume under the septum can be evaluated for
radiated emissions as easily and as simply as an immunity test is accomplished.
By the reciprocity principle in electromagnetic theory, if the application of an
RF voltage generates a field, then the introduction of a device that radiates a field
in the volume under the septum will produce an RF voltage at the GTEM! input
connector. The voltage produced will be proportional to the intensity of the
radiated field.
The main development that brought the GTEM! forward as a practical radiated
emissions device was the three-position correlation algorithm (derived by
Wilson et al. at ABB, based on results from NIST), which allowed the direct
comparison of data taken in a GTEM! to data acquired on an Open Area Test
Site (OATS). The GTEM! feed connector voltages produced by radiated
emissions from the EUT at each of three orthogonal positions are measured.
Then at each frequency, an equivalent set of dipole antennas that would produce
the same voltages at the GTEM! connector are defined through computer
computation. Once the equivalent antennas are defined, the field intensities for
comparison to the given specification limit are computed from the set of
equivalent dipoles at each frequency, given the separation and geometry of the
test setup on an OATS.
The simplest GTEM! to OATS correlation algorithm uses three EUT positions.
Various other rotation schemes have been described developed. Please contact
ETS-Lindgren for additional details.