Near Field Tools for Debugging
Far field tests can accurately tell whether a product
passes or fails as a whole, but cannot pinpoint the source
of a problem. Using only the far field test, you can’t isolate
problems down to specific components or locations — like
too much RF energy “leaking” from an opening in a metal
enclosure or a cable radiating too much RF energy. The
near field test is a good way to locate such emission
sources and is typically performed using a spectrum
analyzer and near field probe.
Near field probes are electromagnetic pickups used to
capture either the electric (E) or magnetic (H) field at the
area of interest.
Manufacturers provide kits of probes that offer the
best compromise between size, sensitivity, and frequency
range, and you may need all the sizes in your toolkit to
solve your problem.
Selection between an H-field or E-field probe may be
driven by the location of a signal in your design, or by the
nature of its source (voltage or current).
For example, the presence of a metal shield may
suppress the E-field, making it necessary to use an H-field
probe for the application. Near field probes must be used
to pick up the signal near the device under test.
Most hobbyists don’t need to worry much about FCC
rules and compliance testing. That is, until the hobby
produces an interesting design with market potential.
Even for small-scale manufacturing, you risk running
afoul of FCC rules around electromagnetic emissions
without the stamp of approval from an FCC approved test
Pre-compliance testing using low cost USB based
spectrum analyzers can give you the confidence that your
design will meet FCC requirements and you’ll sail through
compliance testing the first time out. NV
FIGURE 4. Conducted emissions testing discovered an over-limit condition at the lower end of the spectrum in a
low cost power supply.
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