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EMI Theory
Electromagnetic Compatibility and Interference
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
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. Hfields result from current sources, such as motors and transformers.
EMI Regulations
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
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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.
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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.
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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 m ake a real difference in tim e 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 Department for practical advice on product selection. Since 1961, vigorous product development in
pace with the rapid advances in electronics has made Chomerics a primary force in EMI shielding technology.