What is EMI
Electromagnetic Interference (EMI) is a disturbance in an electrical circuit. This disturbance occurs from another source through electromagnetic induction or electromagnetic radiation. In plain English, EMI is when an electrical device is interfered with by a source of energy. A prime example is the static on an analog TV when a vacuum cleaner or microwave is operated. EMI is most often created by man-made sources. However, weather events such as the sun, northern lights and lightning can create EMI as well.
EMI is regulated by most countries through product testing to specifications and standards. Electrical devices are not permitted to originate EMI levels outside of the standard levels. Likewise, devices must be able to operate correctly even when exposed to certain levels of EMI. These requirements apply to nearly every industry. Consumer electronics, automobile, medical, industrial, aerospace, nuclear, transportation, military, and energy devices must all be tested.
When conducting EMI testing, there are four primary focuses. These are conducted susceptibility, conducted emissions, radiated susceptibility and radiated emissions. The following paragraphs will address each one.
Conducted susceptibility is best understood by defining the two words. Conducted refers to wired. Conducted energy is energy transferred through a cable or wire conducted to a device. Most commonly, this is the power cable. Susceptibility refers to effect. Is a device effected by signals or energy received on the cable? Putting it together, conducted susceptibility is the ability of a device to operate without being impacted by energy being received from a cable or cord. Essentially, wiring can be an antenna of radiated RF signals. These induce currents on conductors and may produce voltages in circuitry. This energy can impact operations. The ability of a device to not be impacted by this RF interference is its conducted susceptibility. A device not impacted by RF is referred to being immune. That is where the terms EMC immunity and EMI immunity comes from.
As previously stated, EMI can be man-made or occur naturally. Conducted signals that are natural can include lightning and surges.
During conducted susceptibility testing, the EMC test lab will wire the device and then introduce signals on the cables. A range of frequencies will be broadcast to determine if the device is susceptible or immune. Certification standards determine the frequencies to be tested.
Similar to conducted susceptibility testing, conducted emissions testing is focused on cables. However, instead of looking at what is being transmitted to the device, we now look at what the device is transmitting. Conducted emissions are unwanted RF being generated from a device and being transferred to other devices through cables and cords. This unwanted RF is called RF interference. RF interference can interrupt the operations of other devices.
During conducted emissions testing, the device will be wired. The EMC test lab will then read the signals being transmitted from the device to the cables. The signals being transmitted will be compared to the signals listed in the standard.
Radiated susceptibility is similar to conducted susceptibility in that it focuses on the ability of a device to operate without interruption. However, instead of looking at the cables, it focuses on radiated signals. The radiated signals from one device can impact another device. This RF interference or RF noise most commonly comes from unintentional radiators. This noise is unwanted frequencies outside of their intended transmission frequency band. The testing involves introducing RF. This introduction usually involves a signal generator and current clamp.
Radiated susceptibility testing involves transmitting signals at the device. The signals to be transmitted are determine by the EMC test standard. The objective is to determine if the device’s operations will be impacted. If they are, the manufacturer and the standard will determine if the impact is acceptable. Different device types have different definitions of failure. For instance, a flicker on a child’s game might be acceptable. However, a flicker on a plane’s navigation system is not.
Radiated emissions testing determines the RF noise is being generated by a device. Every electronic device must operate within a predetermined spectrum. If a device is generating RF outside of that spectrum, it could interfere with other devices.
To test radiated emissions, the EMI test lab will operate the device and receive the RF it is transmitting. A determination will then be made as to whether it is acceptable or not. A device that is transmitting too much RF may need hardened. Often times, hardening means increased shielding, gasketing or filtering. Devices with unacceptable amounts of emissions can create dangerous situations.
Types of interference
There are two types of interference. Continuous interference is just what it sounds like. It is when a source continuously emits RF at a given range of frequencies. One example is the hum from a power supply. Wireless and RF transmitters are often continuous. Pules interference is the other type. This is referred to as transient disturbances. These occur when the source emits a short-duration pule of energy. One example is a power line switch. Another example is an electrostatic discharge.
There are various ways of reducing EMI. Grounding or earthing can reduce EMI. Using shielded cables can improve immunity. Shielded cables have an outer layer of conductive material. These cables are also grounded. Having a shielded enclosure also improves immunity. These enclosures also reduce emissions. The best material for shielding is metal. However, there are other materials that can have shielding properties. There are some paints that have reflective characteristics. Other materials using specialty fibers and resins can be RF shielded as well.
Keystone Compliance is a leading EMC test lab. We have seven EMC test chambers. Our lab also has several ground planes. We are experts in shielding effectiveness and indirect lightning testing. We offer quick turnaround on quotes and reports. Our pricing is affordable. Hundreds of manufacturers partner with us each year. Contact us to work with an EMC lab that will meet all of your needs.