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Introduction to EMI

Electromagnetic Interference (EMI) describes a wide range of environmental electrical disturbances, which can adversely affect the performance of electrical equipment. There are several types of EMI, including conducted line interference, radiated interference, transient interference, ambient interference and electrostatic discharge (ESD).

Electrical noise is subdivided into four categories:

  • Gaussian noise: similar to the sound of static from a TV set tuned off of a commercial channel. It is a flat level of noise that can be measured with a spectrum analyzer.
  • Impulsive noise: Similar to Gaussian noise, except its level is not flat but varies rapidly with time.
  • Broadband noise: a measurement of noise in a particular bandwidth of the spectrum analyzer, usually 1 Mhz.
  • Narrowband noise: refers to signals that are “continuous” such that they only exist at particular frequencies.

Magnetic fields are directly related to electrical fields and are analyzed separately using the same techniques.


EMI testing arose as a result of World War II and the recognition of the fact that the detonation of nuclear weapons creates very strong electromagnetic fields that can instantly destroy electronic equipment hundreds of miles away. Radar was also found to cause localized disturbances. It was necessary to develop theories on how electromagnetic fields propagated from these devices and how they caused equipment malfunctions.

In the early 1970’s it was recognized that the same theories developed in World War II could be applied to the problems of localized disturbances affecting computers and televisions. The broadcasting industry petitioned the FCC to regulate electronic equipment to reduce the levels of interference and provide for clear reception of programming. This was the beginning of large-scale regulation of all electronic emitters with frequencies greater than 10 Khz.

Testing for EMI

There are two ways in which a specialized EMI testing laboratory can help. First, testing your device for EMI emissions before it goes into production allows you to make the design modifications necessary to meet all applicable standards. Agencies typically require regulation to acceptable levels of noise in the broadband and narrowband categories. Midwest EMI Associates handles everything for you, including interpretation of what is acceptable, how to correct deficiencies, and how to submit data properly to the regulating agency.

Second, testing your device for susceptibility to EMI radiated from other sources helps you increase your device’s reliability by adding protective measures if necessary. The science of fortifying electrical equipment against the effects of EMI interference is called susceptibility engineering. The staff at Midwest EMI have many years experience in this field.

Military Testing

The Department of Defense regulates EMI testing of military equipment. Testing in accordance with Military Standard 461C usually requires security clearances and is very intensive. Submission of data is very meticulous and detailed and the Government may require special certification. Our chief testing engineer at Midwest EMI Associates is certified by the National Association of Radio and Telecommunications Engineers, Inc. (NARTE).

Medical Testing

Presently, testing of medical devices is mandatory and is covered by either the FDA Reviewer's Guide or by the European Medical Device Directive. To market in the USA, companies must pass several required tests and have their 510K submittals approved. Our specialty in these areas provides a turnkey solution including 510K preparation, FMEA analysis, EMC testing, and submittal to the European Notified Body. We provide guidance to assure you achieve your requirements.

Preventing EMI and its Effects

A variety of techniques and standard have evolved to analyze EMI disturbances. Specialized antennas, spectrum analyzers and a host of other apparatus can measure simulated or actual EMI effects. Agencies such as the FCC and the European VDE organization routinely regulate commercial equipment. IEEE and ANSI have written numerous protocols. There are many sources of interference that can cause EMI problems, among them unshielded cables and PC board traces carrying high-frequency clocks. Midwest EMI Associates detects and helps you eliminate any EMI problems from your equipment design. We can also help you write meaningful protocols for your future requirements.

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