What is Electromagnetic Interference?

Electromagnetic Interference (EMI) usually refers to unwanted electrical noise present on a power line. This noise may “leak” from the power lines and affect equipment that isn’t even connected to the power line. Such “leakage” is called a magnetic field. Magnetic fields are formed when unwanted noise voltages give rise to noise currents. Such noise signals may adversely affect electronic equipment and cause intermittent data problems.

The EMI arises due to the presence of stray electromagnetic fields either natural or man-made. Typical sources include:


  • lightning
  • cosmic
  • atmospheric noises


  • Communication Devices
  • Heavy Machinery
  • HVAC Equipment
  • Radio Signals
  • Vehicle Traffic
  • switching of large currents

EMI is usually present in almost all electronic equipment at varying levels. Some of the sources are the resistive, inductive and capacitive switching loads such as heaters, ballast, electric motors and the power supplies. The switched mode power supplies EMI is normally kept at low acceptable levels through advanced design techniques.


These interferences have a variety of characteristics which depends on the source and that generates them. They are usually classified by the duration, bandwidth and mode of coupling.


  • Continuous interference is that whose source emits a continuous signal
  • Impulse noise, either man-made or naturally occurring such as those from ESD, switching systems, lightning and other sources of impulse noise


  • Narrowband
  • Broadband

Effects of EMI

EMI impedes the proper performance of the receiving instrument. It generates stray currents and voltages which distort the signal, data or interfere with the magnetic field of the equipment. Typical examples of affected applications are:

  • Sensitive circuits such as heart pace makers
  • Applications utilizing electromagnetic fields in its sensing circuit such as the MRI/fMRI and electron microscopy
  • Telephone systems
  • Radio and TV systems
  • Test equipment

EMI coupling mechanisms

An EMI is coupled to the receiving circuit through:

  • Radiation coupling, where the source and receiver are not in any physical contact and the EMI is through induction
  • Conduction, when there is a physical contact providing a conduction route for the interference fields travel through

Other sub- categories include capacitive, inductive, resistive and magnetic coupling.

Minimizing levels of Electromagnetic Interference

Determining the source, type and coupling mechanism of the interference, is important in determining the effective method of reducing the effects of the EMI to an acceptable level. Some of the measures to take are:

  • Turning off or moving away the source of the EMI
  • Moving away the instrument affected by interference
  • Isolating source of EMI through grounding, use of a Faraday cage, or using a room made or enclosed with a conductive material. (On the source or the affected equipment)
  • Using an active EMI cancellation mechanism to sense and cancel the magnetic fields.

There are legal requirements that address the electromagnetic compatibility of most electrical and electronic equipment. Equipment should be able to work normally when subjected to specified amounts of EMI. In addition the equipment should not emit EMI large enough to interfere with other equipment’s operations.