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EMC101
Why EMC?
In order for virtually any electronics product to be sold in most countries,
the products need to satisfy local government regulations limiting the
amount of electromagnetic emissions (radiated and conducted) it produces
(electromagnetic compliance or EMC). Radiated emission is caused by
the product acting as a radio station, emitting by-products of its internally
generated electrical signals. Conducted emission consists of high-frequency
signals superimposed on cables connected to the product. Even if agency
approval is not an issue, excessive electromagnetic emissions may interfere
with the normal operation of other products. Locating the sources of such
emissions can help to restore normal operation. The 3M
™
ScanEM-C Probes
deal mostly with radiated emission. They can also help with many cases of
conducted emission where the cable “infected” with the conducted radiation
becomes an antenna, and, in turn, produces radiated emission.
Near-Field Primer
The measurements done for agency approval are performed at a distance
of three to ten meters. At these distances, the nature of the electromagnetic
field is determined by the source and the distance from the source. As the
test antenna gets closer to the source, the nature of the electromagnetic
field changes. Near the source of radiation, the field produced is mostly a
function of the properties of the source. If the field is generated by an object
with high current and low voltage, the field produced will be magnetic in
nature, or H-field. If the source has high voltage but little or no current, the
resulting field is electric, or E-field. It is important to remember that there is
no such thing as a purely electric or purely magnetic field. A conductor with
a substantial current causing an H-field still carries some small voltage that
