 Electromagnetic
Compatibility and Interference [Top]
Electronic devices operating normally in their intended environment,
without conducting or radiating excessive amounts of electromagnetic
energy, or being susceptible to such energy from internal or external
sources, are in the state of electromagnetic compatibility, or EMC.
Electromagnetic interference, EMI, is radiated or conducted energy
that adversely affects circuit performance, and thus disrupts a
device’s EMC. Many types of electronic circuits radiate or are susceptible
to EMI and must be shielded to ensure proper performance. Establishing
basic electromagnetic compatibility in any electronic device generally
requires two distinct approaches. The first approach is to reduce
EMI generated from internal sources. As shown in Figure 1, this
is best accomplished by designing an electronic circuit or device
so that it inherently generates less EMI. Residual EMI may then
be suppressed or contained within the enclosure by appropriate filtering
and shielding methods. Filtering cables at the point where they
enter or leave the enclosure will reduce conducted emissions.
Radiated EMI
may be eliminated or reduced by the use of shielded enclosures
and shielding materials. The second method for establishing EMC
in a device is to improve its immunity (or reduce its susceptibility)
to interference from external EMI sources. Figure 1a illustrates
an EMI-susceptible device. Susceptibility to external EMI may
be reduced or even eliminated by designing circuits and choosing
components which are inherently less sensitive to interference.
As in the case of internal sources, conducted EMI may be reduced
with filtering devices on incoming and outgoing leads as shown
in Figure 1b, and susceptibility to externally radiated EMI may
be reduced with use of effective shielding, as illustrated in
Figure 1c.
Electromagnetic
Fields [Top]
Radiating electromagnetic waves consist of both an E-field (electric)
and an H-field (magnetic) oscillating at right angles to each
other, as illustrated in Figure 2. E-fields are created from voltage
sources, such as logic chips or clocks switching between their
zero and 5-volt states. H-fields result from current sources,
such as motors and transformers.
Figure
1
Reducing EMI Radiation and EMI Susceptibility
Figure
2
E-Field/H-Field Schematic
EMI
Regulations [Top]
Government regulations in the US and many other countries prohibit
electronic products from emitting EMI that could interfere with
radio and television receivers. European regulations also include
EMI immunity levels. Manufacturers of commercial electronic products
generally contend with three types of EMI problems:
- Suppression
of internally generated signals to prevent excessive levels
of radiated and/or conducted emissions
The FCC in the United States, CSA in Canada, VCCI in Japan,
AUSTEL in Australia, and legislation by EU (European Union)
member countries all set certain standards for EMI emission
levels that commercial electronic devices must meet before being
sold in those countries. Many electronic products sold in the
US must be tested and verified or certified for compliance with
the FCC’s Part 15 regulations.
- External
ambient interference with equipment operation
Many companies establish their own specifications for immunity
to EMI over a range of phenomena. These may include electrostatic
discharge (ESD), radiated immunity, and electricfast transients
(EFT). This is not yet a requirement in the US; however, EU
regulations currently do include immunity requirements.
- Internally
generated emissions interfering with equipment operation
EMI from one circuit can interfere with the function of another
within the same system or subsystem. Typically called cross-talk,
this problem is the most common source of system susceptibility.
Cross-talk frequently occurs in densely packaged mobile or portable
equipment.
Design
assistance that can make
a real difference in time and cost …
As the world’s largest manufacturer of EMI shielding materials,
Chomerics offers its customers unparalleled knowledge of system
design and regulatory compliance. We encourage you to contact
our Applications Engineering Departments in Woburn or Marlow for
practical advice on product selection. Featured in this catalog
is Chomerics’ comprehensive product line for commercial applications.
Since 1961, vigorous product development in pace with the rapid
advances in electronics has made Chomerics a primary force in
EMI shielding technology.
If you don’t find the ideal solution, call
us.
We’re probably working on it!
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