EMC Immunity

Every electrical and electronic produces electromagnetic emissions, both radiated and conducted (if another conductive structure is in close enough proximity). Radiated emissions can occur from any non-DC signal stimulating a significantly long conductor, which then is acting as an antenna transducing conducted electrical signals into EM signals, or radiated emissions. These emissions can be from the switching of a switch-power supply, digital circuitry, AC power being carried through a device, any signal generators, such as an oscillator/resonator, etc. Conducted emissions are different from radiated emissions, and instead of being transduced to EM energy and radiated, they are conducted into nearby cabling, typically the power supply cord or communication cords.

Radiated Immunity


Radiated immunity is the use of radiated EM fields to stimulate a test product to replicate potential EMI that are commonly encountered in normal product use. Examples of radiated immunity are products being exposed to electrical motors initiating or winding down, nearby wireless communication transmitters, etc. Radiated Immunity testing is designed to ensure that a product will survive a minimum of radiated EM at a specified amplitude and frequency. This test is essentially the opposite of the radiated emissions testing. An external signal generator is used to feed a RF tone to an amplifier that is connected to an antenna, which directs the transduced signal into an radiated EM field toward the test product. Some type of modulation of the signal is typically used to somewhat replicate communication signals. The signal level and frequency range depends on the application, with industrial and military/automotive applications typically requiring the highest signal levels.

This type of testing can cause a wide range of issues with product operation, which typically depend on the field strength and frequency of the stimulus and how a product is designed. Where the radiated energy is picked up within a product depends on the frequency of the signal and the relative lengths and geometric structure of the product. For products with RF ports and antennas, these areas are particularly susceptible, as they will most optimally couple energy from the EM field stimulus.

Conducted Immunity


Conducted immunity testing is based on the concept that nearby power cabling can result in interference being coupled into the primary power and signal cables. In this testing an amplified signal generator stimulus is injected as a common mode disturbance into a device’s cabling. This can be done using CDNs, BCI probes, EM clamps, or direct injection of a common mode signal using a resistor. Common mode filtering can be instrumental in enhancing a device's conducted immunity. Additional filtering at sensitive ports, such as analog input ports, can also aid in conducted immunity performance.

Electro-static Discharge (ESD)


ESD testing is extremely common in EMC standards, as it is a natural phenomenon that virtually any electrical or electronic product that comes into contact with users can be exposed to. ESD is the electrical discharge that occurs when an object or person builds up a substantial amount of static charge (typically kilovolts). When a user or object has such charge build up and comes into close proximity to another object or user that does not have the same charge potential, then the discharge occurs equalizing the electrostatic potential. This can result in a significant air discharge or even direct contact discharge of thousands of volts of electrical potential.

ESD Testing is typically done to imitate this natural phenomenon, where a product is subjected to kilovolts of discharge either as an air discharge or direct contact discharge to the parts of a product that are typically exposed to users (human touch). Usually the testing is done at a standard low discharge level and the level is ramped up to whatever is deemed the appropriate maximum level. At each level a product is tested to ensure it still retains all of the product's functions at each level.

ESD Testing is particularly challenging for many products, and results in frequency failures. Common failure modes included damage to I/O ports and communication interfaces, display damage, memory and digital storage corruption, logic errors, and product resetting or mode switching.

Electrical Fast Transient (EFT)


EFT tests are designed to replicate the fast switching of inductive loads in real applications. This could be motors/relays, large runs of cables/bundles of cables, electrical utility switches, or lamp ballasts. EFT pulses are typically in the nanoseconds of rise time and are tens of nanoseconds long, but they are usually delivered in a series. Test devices are typically injected with EFT pulses through an AC power cable, but signal/control ports may also be tested using some type of capacitive coupling. Without proper powerline filtering and protection on sensitive ports, EFT pulses can lead to damaged ICs, other electronic hardware, damage to communication circuits, analog circuit errors, and clicking in audio products.



Surge testing is designed to replicate low frequency power surges, such as from lightning strikes or nearby lightning events. This also includes power switching events, insulation faults on the power grid, reactive load switching, blown fuses, and other relatively rapid power faults. A surge immunity test generally involves a surge simulator/generator injecting a signal into the AC power lines of a device, but DC power lines or signal/control ports may also be tested. Surge protection is critical in prevention of damaged internal hardware from surge exposure.

Magnetic Field


The most common type of magnetic field testing is exposing a product to the magnetic field driven by a mains power line type signal. This type of testing is to ensure that a product behaves properly when exposed to nearby AC power lines. There are other types of magnetic field testing that are more similar to radiated immunity testing, but only using a magnetic field stimulus. Devices that don’t contain devices that are typically sensitive to magnetic field stimulus are often exempted from magnetic field testing, but devices with relays, hall elements, electrodynamic microphones, magnetic field sensors, etc. are generally tested.

Voltage Drops, Dips, & Interruptions


This type of testing is designed to ensure that a product operates properly and safely when the power supply voltage or AC mains voltage dips rapidly or is reduced to a lower level for an extended period of time (brownout condition). These power supply fluctuations can lead to repeated rebooting or restarting of a product and possible failure modes related to repeated start and stops from low power. Equipment that is directly and continuously powered by AC mains or DC power supplies may struggle with this test, and ensuring that a device safely shuts down and resets from a power supply spike or brownout is helpful.

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