EMI/EMC - Frequently Asked Questions (FAQ)
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Q1.  What is EMI?
A.  Electromagnetic Interference (EMI) is defined as electromagnetic energy from sources external or internal to electrical or electronic equipment that adversely affects equipment by creating undesirable responses (degraded performance or malfunctions). EMI can be divided into two classes: continuous wave (CW) and transient.

Q2.  What is EMC?
A.  Electromagnetic Compatibility (EMC) is defined as an electrical system's ability to perform its specified functions in the presence of electrical noise generated either internally or externally by other systems. The goal of EMC is to minimize the influence of electrical noise.

Q3.  Why is EMI/EMC testing required for commercial and military equipment?
A.  EMI/EMC testing is designed to ensure that electrical/electronic equipment will perform properly in its expected electromagnetic environment, thus maintaining an acceptable degree of Electromagnetic Compatibility (EMC).

Q4.  What are some basic test categories and their associated requirements?
A.  The following EMI Tests are performed as determined by the equipment's intended application: Conducted Emissions (CE), Radiated Emissions (RE), Conducted Susceptibility (CS), and Radiated Susceptibility (RS). The Emissions Tests (CE & RE) record any undesirable emissions from the test article. This data is plotted against the applicable specification limits. The Susceptibility Tests (CS & RS) determine the test article's ability to operate in the typical operating environment. The test article is exposed to electromagnetic signals at the levels and frequency ranges required by the applicable commercial or military specification.

Q5.  Where are EMI/EMC tests performed?
A.  There are many EMI/EMC testing labs throughout the U.S. and many other countries. Shielded room test chambers and open area test sites (OATS) may be used depending on the test requirements document.

Q6.  What is an EMI Receiver?
A.  An EMI Receiver is a tunable, sensitive voltmeter used to measure electric and magnetic field strengths. Most are similar to specialized spectrum analyzers, but are characterized by having preselectors, several detector functions, a housing shielding effectiveness of at least 90 dB and other unique additions. The frequency coverage of high-end receivers is typically 30 Hz to 22 GHz. Measurement bandwidths are variable to cover broadband and narrowband measurements.

Q7.  What is RFI?
A.  Radio Frequency Interference (RFI) is considered as part of the EMI spectrum, with interference signals being within the radio frequency (RF) range. This term was once used interchangeably with EMI.

Q8.  What equipment should be furnished as part of an EMI/EMC test?
A.  A typical EMI/EMC test should include the test article and all associated cables, interfaces, power supplies, software, and other support equipment as needed to simulate the actual operational configuration.

Q9.  What is an emitter?
A.  In EMI applications, this term applies to unintentional radiators, particularly those that are the source of interference. Otherwise, the term refers to the intentional radiators such as transmitter antennas.

Q10.  What is a receptor?
A.  A device that receives conducted or radiated electromagnetic emissions. In EMI applications, a receptor has the potential for being susceptible to undesired interference. It is considered to be a victim if it is susceptible to EMI from the emissions received.

Q11.  What is crosstalk?
A.  Crosstalk results from the coupling of conducted emissions between two pairs of wires, one pair carrying emissions from a source and the other pair connected to a susceptible device. In a printed circuit board (PCB), crosstalk involves interaction between signals on two different electrical nets. The one creating crosstalk is called an aggressor, and the one receiving it is called a victim. Often, a net is both an aggressor and a victim.

Q12.  What is susceptibility?
A.  The inability of equipment/systems to perform without degradation in the presence of an electromagnetic disturbance. Susceptibility is often characterized as a lack of immunity. The threshold of susceptibility is the level of interference at which the test article begins to show a degradation in performance. This is often frequency-dependent.

Q13.  Why should EMI/EMC testing be done at a certified Test Lab?
A.  In industry, it has been common for EMI/EMC tests to be conducted in Open Area Test Sites (OATS) and other non-shielded manufacturer facilities. However, due to the increased use of electronic office equipment and wide-area communication systems such as cellular phones, pagers, and high powered radio signal sources, it has become more difficult to conduct accurate emissions tests using OATS. The increased ambient emissions from the external sources preclude accurate results in many of these test environments. Indoor shielded chambers, such as a certified Test Facility or Lab, provide a good environment for EMI/EMC testing because they attenuate ambient emissions from the surrounding area.

Q14.  What is ambient level?
A.  The nominal level of radiated and conducted electromagnetic signal and noise existing at a specified location. This is usually considered to be a function of the entire electromagnetic environment including atmospheric noise and interference generated from within the measuring set-up.

Q15.  Where can I find more information on the subjects of EMI and EMC?
A.  This web site includes sections for related documents and organizations as well as an EMI/EMC Links section with many other related resources.

* Refer to sections listed in the web site Index for additional information.


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