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near-field, the probe picks up the signal only at a particular spot, while in a
far-field test, the field strength greatly depends on the antenna properties of
the radiating object. As an example, a short trace on a PCB with a strong
emission level may be much less dangerous than a longer one with a lower
emission level. A strong but small emission leak from an otherwise shielded
enclosure may be much less of a problem than “lukewarm” radiation from
a large area or around a long seam in the shield. If the offending frequency
is well-known and if the field carrying it is not the strongest one in the
product, a spectrum analyzer may be a more appropriate tool. If the 3M
™
ScanEM-QC Electromagnetic Near-Field Probe is used in the early stages
of development where it is not yet clear what frequency will be of a concern,
the self-contained mode will prove to be quick and effective. In some cases
the ScanEM-QC Probe’s own indication may actually do a better job than
a spectrum analyzer in searching for the highest emission levels, since
the spectrum analyzer won’t provide a total signal level. It is difficult to
subjectively judge it by looking at the screen.
