Analyzing The Results - 3M ScanEM-C User Manual

Analyzing the Results

If the 3M ScanEM-C Electromagnetic Near-Field Probe indicates the
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presence of an electromagnetic field, it may not necessarily mean that your
product has emission problems. It is normal for an electronic device to
produce some electromagnetic noise. As you pass the ScanEM-C Probe over
the tested product, you may notice that the level of the electromagnetic field
changes. You may find that several areas in the tested product have higher
than average emission levels. These areas require special attention. Should
the product fail an agency emissions test, those areas are suspect. It is a good
idea to take whatever steps necessary to reduce the excessive EM field level
at these areas to as close to the background level as possible prior to the
agency test.
Often the ScanEM-C Probe may indicate excessive EM field strength
in places where high frequency signals are not normally expected—i.e.
AC power lines. This happens because high-frequency signals produced
elsewhere "ride" on these lines and generate an electromagnetic field.
Many low-frequency signals contain a sufficient amount of high-frequency
harmonics for the ScanEM-C Probe to detect. If any of these non-sinewave
signals are generated by your equipment, it may be necessary to reduce the
emission level in order to satisfy EMI regulations. For a far-field agency
test, one or two spots on the product with high emission levels detected in
the near-field may be less problematic than a broad area with a somewhat-
higher-than-average EM level.
High-level radiation registered with a near-field probe is not necessarily
an indication of a problem in the far-field. Conversely, a near-field probe
may not show the field strength of the area that is causing the compliance
failure. In order to understand why, one needs only to realize that in the
near field, the probe picks up the signal only at a particular spot, while in
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