What are EMI and EMC and how do you develop EMI/EMC compliance?



EMI is Electromagnetic Interference


Electromagnetic interference occurs when an electronic device, or devices, adversely affects or interferes with the operation of another electronic device. Any electrical or electronic device has the potential to electronically interfere with the operation of another electromagnetic device because of the physics governing radio waves.


As electrons move, they create electromagnetic waves that spread through free space and potentially interact with each other. Devices such as switching power supplies, motor controllers, computers, cell phones, and many other types of electronic equipment can all cause electromagnetic interference. We will discuss how to properly conduct EMI testing.


A great example of electromagnetic interference that many people can relate to was when a vacuum cleaner was used in proximity to a TV with an antenna. The vacuum cleaner’s motor generated broadband electromagnetic noise (across a wide frequency spectrum) and the picture on the TV was distorted or other obvious interference was noticeable.



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EMC is Electromagnetic Compatibility


Electromagnetic compatibility (EMC) is essentially the opposite of EMI. EMC means that the device is compatible with its electromagnetic environment and it does not emit levels of electromagnetic energy that interfere with other devices in the vicinity.


In the military world EMC is typically used for intra and inter-system compatibility. A typical example is when all of the systems of an aircraft are operational, it is very important that each system can perform its intended function and not interfere with or be interfered with other systems in the aircraft or in the general area of the aircraft (typically flight line equipment). We will also discuss how to properly conduct EMC testing.


Typically in the laboratory we verify the ability for equipment to withstand electromagnetic interference by bombarding it with electromagnetic energy this is EMC testing. Testing can be either conducted, or radiated EMC testing. The military world typically refers to this as susceptibility testing were the commercial world typically refers to it as immunity testing. It amounts to essentially the same thing and is a difference of perspective – similar to the glass is half full vs. half empty, if the equipment is not susceptible is like saying it is immune.



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EMI/EMC Compliance Development


We conduct EMI testing and EMC testing to be in compliance with a variety of test methods:


Conducted Emissions



  • the electromagnetic emissions emanating from the Equipment under Test (EUT) via power and/or interconnecting cables is measured. This may be frequency vs amplitude measurements or time domain measurements (such as switching transients). Measurements are made with a variety of methods in current or voltage depending on the specification. Each specification describes the required measurement parameters and limits for the measured emissions.

Typical Conducted Emission Setup for EMI testing/EMC testing

Current Measurement

Current Measurement1

Voltage Measurement

Voltage Measurement2



Radiated Emissions



  • the electromagnetic emissions emanating from the Equipment under Test (EUT) are received via a sensor or antenna placed in proximity to the equipment. This is typically frequency vs amplitude measurements, but other types of measurements are possible. Measurements are made with a variety of sensors depending on the specification and type of measurement (Loop Sensor, Vertical Monopole Antenna, Linearly Polarized Antennas, etc.). Each specification describes the required measurement parameters and limits for the measured emissions.


Typical Radiated Emission Test Setup EMI testing/EMC testing

Typical Radiated Emission Test Setup3 EMI testing/EMC testing


1 MIL-STD-461F, CE101 Test Setup


2 MIL-STD-461F, CE102 Test Setup


3 RTCA/DO-160E, Section 21



Radiated Susceptibility (Immunity) Test Setup for EMI testing/EMC testing

Radiated Susceptibility Test Setup Photograph for EMI testing/EMC testing



Conducted Susceptibility (or Immunity)



  • the ability of the Equipment under Test (EUT) to operate properly in the presence of electromagnetic interference applied to its cabling. This can be its power lines, interconnecting lines and in some case grounding lines or case directly. The interference can be many different types such as swept current and/or voltage or various transient types and can be applied in a multitude of ways. Each specification describes the required test parameters and levels for testing.


Typical Radiated Emission Test Setup EMI testing/EMC testing

Conducted Susceptibility Test Setup (Bulk Current Injection) for EMC testing



Radiated Susceptibility (or Immunity)



  • the ability of the Equipment under Test (EUT) to operate properly in the presence of radiated electromagnetic interference. This can be applied to its case and/or cables. The interference can be many different types such as swept field or various transient types and can be applied in different ways such as Loop Antennas, TEM cells, Striplines, Linearly Polarized Antennas, reverberation chambers, etc. Each specification describes the required test parameters and levels for testing.


Radiated Susceptibility Test

Radiated Susceptibility Test


Using testing we can determine if the EUT meets the design and application requirements for EMI testing and EMC testing. There are specific specifications for military, commercial, aircraft, automotive, rail and other applications.



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