Electromagnetic Interference
Simply put, electromagnetic interference (EMI) is the effect of electromagnetic fields (e.g. radio frequency) on sensitive electronic equipment and systems. It spreads over a surprisingly wide area and can cause serious damage to your system. Fortunately, there are two possible solutions to this problem: filtering and shielding.
EMI filtering and shielding are two different techniques used to address the problem of EMI in electronic devices or systems. With electronic systems, there are concerns about radiated and conducted sensitivities as well as radiated and conducted emissions, which are EMI noise from the system.
But what's the difference?
Filtering
A brief history of EMI, EMC and protection needs
In the 1930s, when radios quickly became a household necessity around the world, users began to notice the strange effects of frequencies on other electronic and electrical equipment. This phenomenon, known as "electromagnetic interference", clearly demonstrated the need for a way to protect equipment from erroneous radio signals.
The solution to these problems came in 1933, when the International Special Committee on Radio Interference (CISPR) of the International Electrotechnical Commission (IEC) in Paris issued the earliest recommendations for minimizing EMI. Emission limitations occur when systems are interconnected to ensure that noise generated in one system does not interfere with the performance of another.
The need for more comprehensive EMI filtering and shielding methods and equipment for defense applications was recognized as early as 1967, when the aircraft carrier USS Forrestal, deployed in the Gulf of Tonkin in support of U.S. forces in Vietnam, suffered a catastrophic fire when an electronic anomaly triggered a Zuni rocket. This resulted in 134 deaths and nearly 200 injuries. It was later determined that the root cause of the accident was an incorrectly installed shielded cable connector and multiple electromagnetic fields on the flight deck, which caused the rocket to catch fire and ignite the fuel tank of a nearby airplane.
Later, as cell phones and other portable electronic devices became more commonplace in the 1990s, several airlines realized that signals from these and other devices could interfere with avionics, thus impeding the safe flight or navigational capabilities of aircraft. Military leaders have found that electromagnetic interference, whether natural or manmade, can seriously interfere with Global Positioning System (GNSS) technology, rendering unmanned aerial vehicles unable to fly reliably. GNSS interference and EMI caused by military activities is a notable issue in civil aviation flight safety that continues to this day.
After radio interference, many other forms of EMI were recognized, including microwave, infrared, and gamma and X-rays, each of which can be eliminated by a range of methods and equipment.The CISPR recommendations form the basis of the modern international standard for EMI prevention and define how electronic equipment can coexist without interference affecting performance. This is called electromagnetic compatibility.
Electromagnetic compatibility (EMC) defines the acceptable operational capability of electrical and electronic equipment when exposed to external electromagnetic sources and when limiting unwanted electromagnetic energy generated internally.
EMC consists of three aspects: limiting the electromagnetic interference radiation generated by the equipment, the susceptibility of the equipment to external sources of electromagnetic interference, and the immunity of the equipment when operating in a given environment.
Different types of electromagnetic interference
EMI can generally be categorized into four types based on the source or bandwidth of the original signal. Narrowband EMI is generated by cell phone, radio or TV transmissions, while broadband EMI has a wider radio spectrum and is generated by unintentional radiation from signal sources such as power transmission lines. Depending on the source of radiation, EMI can be categorized as intentional, unintentional, inter-system or intra-system:
Unintentional or "non-functional" EMI is generated by unintentional equipment such as welding equipment, DC motors, computers and power lines.
-Intentional EMI (IEMI) is EMI emitted by specially designed equipment, often as part of electronic warfare systems such as electronic countermeasures and electromagnetic pulse (EMP) weapons.
-Intersystem EMI is interference generated between two components of a system or device, while intersystem EMI occurs between two or more independent systems.
Different types of EMI
What is EMI Filtering?
EMI filtering is an important consideration in the design and manufacture of electronic equipment to ensure reliable operation and compliance with electromagnetic compatibility (EMC) regulations.
EMI filtering is the process of reducing or eliminating interference caused by electromagnetic signals in an electronic device or system. It is a technique used to filter out unwanted electromagnetic signals (noise) that can interfere with the operation of electronic equipment or cause electromagnetic interference.
EMI filters are usually placed at the input or output of a device and can be panel mounted or PCB mounted. They consist of a combination of passive components such as capacitors, inductors, and resistors designed to attenuate or suppress EMI in a specific frequency range. They are commonly used in electronic equipment such as power supplies, audio equipment, computer systems, and other electronic equipment with high reliability and EMC requirements.
EMI filters are usually low-pass filters that allow low-frequency signals to pass through while blocking high-frequency signals or "noise." Capacitors block certain frequencies while allowing others to pass. Capacitors discharge into the ground plane, thereby reducing high frequency signals. Inductors work differently; they absorb high-frequency energy and convert it to heat, thus suppressing high-frequency noise. Custom filters can be "tuned" to meet specific frequency requirements by "tweaking" the combination of capacitors and inductors.
EMI filters can be divided into two types: differential mode filters and common mode filters. Differential-mode filters attenuate interference that occurs between two signal lines, while common-mode filters attenuate interference that occurs between a signal line and ground.
What is EMI shielding?
EMI shielding is the process of reducing electromagnetic radiation emitted by electronic devices or systems and preventing external electromagnetic signals from interfering with the operation of these devices.EMI shielding is necessary because electromagnetic radiation can interfere with the operation of other electronic devices and cause malfunctions or errors.EMI shielding involves the use of conductive materials, such as copper or aluminum, to block or attenuate electromagnetic signals from entering or leaving the device.
EMI shielding can be accomplished by placing conductive shielding (called a Faraday cage) around the device or by coating the device with conductive material. The conductive shielding or coating creates a barrier that prevents electromagnetic signals from entering or leaving the equipment, thus reducing the risk of electromagnetic interference.EMI shielding can be applied to electronic components, circuit boards, cables, or even entire electronic devices or systems.
Filtering vs. Shielding
The choice between filtering and shielding depends on a variety of factors, such as the sensitivity of the device and the amount of EMI that will be generated. Conductivity, size and cost also play a role in choosing the best EMI solution.
Shielding takes a comprehensive approach to EMI management, while filters target specific EMI areas. Shielding reflects incoming energy while still generating some absorption. This energy is converted to heat and therefore requires some type of thermal management. In addition, the quality of the material used for shielding affects not only the output, but also the weight of the system, i.e. thicker shielding is more effective, but also heavier. While shielding reflects and suppresses EMI, filters eliminate EMI by addressing the vulnerable points in the system that generate the most interference. As a result, filters can be customized to meet specific needs.
