The Science of Electromagnetic Compatibility (EMC)
EMC is the situation in which electrical and electronic
devices and systems work as intended, both within themselves
and in their electromagnetic environment.
Electromagnetic interference (EMI) is said to exist when
unwanted voltages or currents are present so that they
adversely affect the performance of a device or system. Such
voltages or currents may reach the victim circuit or device
by conduction or non-ionizing radiation. In all cases, EMI
arises because of a combination of three factors: a source,
a transmission path, and a response, at least one of which
is unplanned. EMI control refers to the process of making
design changes or adjustments of signal or noise levels in
order to achieve electromagnetic compatibility (EMC).
The purpose of shielding is to confine radiated energy to a
specific region, or to prevent radiated energy from entering
a specific region. The most effective shield is a solid
metallic enclosure, made of a permeable metal (for example,
steel or iron) if frequencies below 100 kHz are to be
shielded, or any metal of higher frequencies are to be
shielded. However, the solid shield does not permit light,
air, water, or other substances to be passed through it, so
shields with holes, including screen, braids, and honeycomb
arrangements, as well as conductive glass may be needed. The
widespread use of plastic enclosures has made thin film
shields vital in achieving the needed shielding
effectiveness in the use of such enclosures.
Any electrical filter offers relatively little opposition to
the passage of certain frequencies or direct current (dc)
while blocking the passage of other frequencies.
Accordingly, filters play a significant role in reducing
conducted interference to the extent that such interference
has a spectral content different from that of desired goals.
A filter may be either reflective or lossy. Reflective
filters present an impedance mismatch to unwanted
frequencies, thereby returning them to the input, where as
lossy filters absorb unwanted frequencies. A filter may be
designed on a time-domain basis as well as on a
frequency-domain basis.
Digital systems, such as computers, tend to interfere with
analog systems, such as voice and video communications, more
readily than analog systems interfere with digital systems.
Therefore, data streams to be transmitted over analog voice
circuits are converted to a quasi-analog tone form first.
Computer clocks also may have to be shielded and their
output circuits may have to be filtered to prevent
interference to communication equipment. In addition,
personal computers must be connected to television receivers
so that the video output of the PC does not reach the
television antenna, which would radiate such signals.