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Electromagnetic
Compatibility (EMC) may be a new term to some, however it has been important
for many years and actually predates W.W.II. For several decades three
agencies have been the driving forces behind EMC: the U.S. Military; Europe's
Special International Committee on Radio Perturbations (Interference),
CISPR; and the U.S. Federal Communications Commission (The FCC).
History
EMC
first began to be an issue in the military environment particularly
on board ships where many types of electronic equipment, had to successfully
operate in close proximity to each other. In such an environment communication,
navigation and data processing electronics all need to function simultaneously
in the presence of strong radio frequency (RF) fields. Such RF fields
are produced by two-way communications equipment, radar transmitters
and microprocessor controlled devices. Added to this "mix" on board
a military ship is the presence of ordinance or explosives and aircraft
fuel. In such an environment it becomes transparently clear that each
device needs to be electromagnetically compatible with its environment
and not be rendered inoperative or unsafe by this environment. Also
each device added to this milieu must not unnecessarily or unintentionally
contribute spurious emissions that do not perform any particular function.
From the preceding, the origin of the two major aspects of EMC, Emissions
and Immunity, can be seen.
Due to
the global proliferation of electronic devices in non-military living
it is becoming increasingly important that EMC is maintained in civilian
settings as well. Residential and commercial environments may contain
dozens of appliances that are controlled by microprocessors, i.e.,
kitchen stoves, video cassette recorders, TVs, bread makers, personal
computers, etc. All electronic devices utilizing microprocessor technology
generate radio frequencies. For example a 100 MHz computer contains
an electronic clock that steps the microprocessor through its program.
In this case this clock frequency falls within the frequency spectrum
allocated in the U.S. for FM radio broadcasting. If precautions were
not taken by PC manufacturers, interference to nearby radio receivers
would result. Harmonics or multiples of this frequency could, if not
subdued, cause interference to other radio receivers; such as those
used by emergency medical personnel and to television receivers. It
is therefore incumbent upon manufacturers of digital electronic devices
to guarantee their products will not be incompatible with or a nuisance
to other electronic devices.
EMC
and the USA
Because of the proliferation of Information Technology Equipment (ITE)
and other microprocessor controlled electronic equipment, in the 1970's
the F.C.C. (as the authority having jurisdiction in the U.S.) implemented
limits on RF emissions from digital devices. Digital devices that
are intended to be used in residential environments are classified
as Class "B" devices. All such class "B" devices must comply with
limits set forth in part 15 of the F.C.C. rules for radiated and conducted
emissions. Before a Class "B" digital device may be sold in the U.S.,
it must conform to the requirements of the F.C.C. rules. Currently
there are no U.S. requirements for immunity testing. Products destined
for use in the U.S. Industrial, Scientific and Medical fields are classified
as Class "A" devices and may not be used in
residential environments.
EMC
and the European Union
Products sold in the European Union must carry the "CE" mark that
constitutes a declaration by the manufacturer of the products’ compliance
will all applicable harmonized Directives and Standards. Electronic
devices are subject to the EMC Directive, 89/392/EEC. Article 4 of
this document states: "The apparatus...shall be so constructed that
(a) the EMC disturbance it generates does not exceed a level allowing
radio and telecommunications equipment and other apparatus to operate
as intended; (b) the apparatus has an adequate level of intrinsic
immunity of EMC disturbance to enable it to operate as intended."
Clearly, complying with the "essential requirements" of the European
EMC Directive requires evaluation of a products emission and immunity
characteristics.
The Intrinsic
Immunity requirement dictates that an electronic apparatus be so constructed
that its’ performance will not be degraded by its normal electromagnetic
environment. For example a consumer in Europe has a right to expect
that the digital security system monitoring his home will not malfunction
if a nearby ambulance crew talks to their local dispatcher via two-way
radio communications equipment. The directive implies that manufactures
will design products to possess immunity not only to radiated RF fields,
but to other Electromagnetic phenomena as well.
Specific
immunity tests are itemized by generic and product-specific European
Norms or standards. Minimally, this means that a device’s performance
will not be adversely effected by: (1) RF fields such as radio and
TV broadcast stations and licensed two-way radio equipment; (2) Electrostatic
Discharge events (ESD); (3) and Electrical Fast Transients (EFT).
Testing of products for immunity in simulation of real-world environments
allows manufacturers to demonstrate compliance with Article 4, clause
(b) of the EMC Directive. Additional immunity testing is required
by certain product specific standards and the generic immunity
standard. These additional tests include: Conducted RF Immunity; Surge
Immunity; Power Frequency Magnetic Fields immunity; Voltage Dips and
Interrupts immunity; and Pulsed RF Fields immunity.
CE
Conformity
Conformity to the essential requirements of the EMC Directive must
be declared by the manufacturer or his authorized representative.
This is done by issuing a document called a "Declaration of Conformity"
(DOC). It is the manufacturer’s responsibility to procure and maintain
technical evidence supporting all claims of product "conformity".
This supporting evidence is assembled in a Technical Construction
File (TCF). A TCF will exist for each product sold in the European
Union. Verification of compliance (testing) may be performed by the
manufacturer or a third-party test house. In all cases though, tests
must be performed in harmony with International IEC Test Standards.
Results of EMC testing, such as the Test Report issued by a testing
laboratory, shall be included in the TCF.
A product
that meets the requirements of an appropriate "product specific standard",
or in lieu of a "product specific standard" the generic standard,
is presumed to meet the essential requirements of the EMC Directive.
In addition to the EMC Directive, other directives may be applicable
to an electronic device. Conformity with all applicable directives
must be verified and documented. Having met all requirements, the
"CE" mark may then be applied. For a period of ten years after being
placed on the European Market, the supporting technical documentation
(TCF) must be kept on file and be accessible by an authorized representative
within the European Union.
Benefits
Compliance
with the European Union’s EMC directive leads to increasingly robust
products, improvements in quality and increased customer satisfaction.
For example; ESD (electrostatic discharge) immunity testing quickly
reveals any latent vulnerability a product might have to such hazards
and promotes corrective measures that render the product immune to
such real world occurrences. The result is improved customer satisfaction
realized from reliable, solid products that provide years of trouble
free service.
C.R.S.
26-Jan-98
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