As we surround ourselves with more and more electronic devices, we need to stay aware of electromagnetic interference (EMI). EMI can have consequences that range from a static-filled song on the radio to interruptions to advanced military and medical equipment. EMI plays a role in everything from the smartphone in your pocket and appliances in your home to medical, aerospace, railroad and industrial equipment and components.
Almost every electronic device is capable of releasing EMI, so its role in engineering can’t be understated. The primary methods used to control EMI are shielding and filtering. These two options offer different types of protections and functionality.
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What Is EMI?
Electromagnetic interference is emitted by almost every electronic device due to the way the electricity moves through the components. When emitted EMI reaches another piece of technology, it can interfere with its operations and cause it to malfunction. Every time a radio crackles, you’re hearing EMI at work, as its waves interfere with the signal reaching your radio. At some levels, EMI is called radio frequency interference (RFI).
Of course, the effects can be much harsher than a radio signal. When EMI affects devices like pacemakers, aircraft and other technology critical to safety and health, it becomes a serious threat — especially if it’s strong enough to damage the devices. Plus, we add devices to our routines all the time. You can probably look around the room and identify 10 different electronics. If EMI was allowed to run rampant, these devices would interfere with each other and nothing would work. The more devices, the more EMI gets released.
To ensure EMI stays in check and doesn’t cause problems, the Federal Communications Commission (FCC) regulates devices, with other governing bodies taking over outside the United States. The FCC makes sure products can operate within a certain EMI threshold and won’t produce so much EMI that they would impede the functionality of other devices.
Designing components with EMI in mind can help engineers avoid detrimental effects from EMI interference. Designers typically turn to shields and filters, which both attempt to reduce the device’s susceptibility to interference. Shields and filters may reflect the waves away from the device’s inner components and keep EMI contained within a device to prevent it from affecting others. The overall goal is to suppress and deflect noise from EMI without adding too much weight or cost to the item.
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Types of EMI
There are a few different ways to categorize EMI based on its source, length, movement mechanism and frequency range. We can split up EMI into natural or human-made sources:
- Human-made: As you may have guessed, human-made EMI comes from the electronic sources we’ve discussed, like devices and circuits. It comes from smaller, residential sources like laptops, cellphones and microwaves, but also from more powerful industrial sources such as electric motors, telecommunications networks, power grids, medical equipment and railroad systems.
- Naturally occurring: EMI can come from several natural sources, like electrical storms, solar flares, snowstorms and even static electricity. Many sensitive electronics need to factor in these threats to prevent mishaps during unpredictable weather events.
The duration of the EMI will also play a role in its effects:
- Continuous: Continuous emissions come from ongoing sources, such as an electrical circuit that’s always running or background noise coming from an electrical storm. Continuous emission strength can vary widely.
- Impulse noise: This EMI appears in short bursts. It might occur with events like electrostatic discharge, lightning or a cellphone signal interrupting a radio.
Finally, we look at the bandwidth:
- Narrowband: These emissions are typically human-made and, as the name implies, cover a narrow band of the electromagnetic spectrum or even a single frequency.
- Broadband: Broadband EMI is much stronger and spans a larger portion of the spectrum. These signals can divide up into impulse and random sources. Broadband emissions often come from unintended transmissions, like electric power transmission lines.
EMI Shielding
EMI shielding basics are all about forming a protective barrier around the sensitive components of a device. Shields are usually made of metal or a polyester material and can be machined into many different shapes and formats. These shielding layers might be in the form of a solid plate or case, which can be placed around the sensitive components of a device. They can also be combined with cables and printed circuit boards (PCBs) through technology like metallic foil, braided wires and PCB shields.
Whatever form it takes, the shield prevents radiated emissions from getting out and getting in via good ground connections. This conductive surface reflects most of the incoming energy, redirecting it away from the device. It also absorbs some of the energy and converts it to heat. While this is great for reducing EMI, it can create a design challenge because high temperatures call for thermal management strategies that add to the complexity of a device.
To address the high temperatures, some shields function as heat sinks with holes that release the heat and keep things cool. However, a hole also creates a gap in the EMI shielding. Depending on the sensitivity of the item, these gaps might need to be fixed with other methods, such as filters, which we’ll discuss next.
Shields can vary widely in terms of thickness, weight, conductivity, operating temperature range, compression force and costs, so engineers have to be creative to find the right balance of qualities required for the product at hand. For example, a thicker shield is more effective, but it adds weight to the item. The weight, conductivity and characteristics of different metals make the material an important choice for EMI control.
Other materials, like polyester mesh or fabrics, can add flexibility and come in different formats. This allows for more design adaptability. Instead of adding a heavy, rigid casing, you could use a braided cable that’s lighter and more flexible. Of course, those may cost more than a simple metal plate. The right solution really depends on the product.
Common EMI shielding materials include:
- Pre-tin plated steel: Stainless steel is a relatively cost-effective choice that is good for lower frequencies. Carbon steel performs well at these frequencies and offers protection from corrosion and rust.
- Copper alloy: Copper is a particularly reliable material with a very high rate of conductivity. It costs a bit more than some of the other materials and is heavier than aluminum.
- Aluminum: Aluminum is strong, very lightweight and has high conductivity. It’s also one of the most affordable metals, making it a cost-effective option for many applications.
- Fabric: Different “fabric” materials are also available and can be handled like everyday fabrics except they’re embedded with metals that function as shields.
Aside from metal plates, you can also find EMI shields made into gaskets to fill any gaps. Gaskets are designed to accommodate changes in metal due to heat and cold, ensuring appropriate coverage over time. Other types of shields include tape, foils and films. These flexible and lightweight materials are versatile and help protect devices in many configurations. Foils can surround wires or be built into them as a braided shield, while PCBs can have shielding built in. Foil tapes can even hug odd shapes and corners while resisting corrosion.
Other shielding options include foams and silicone. Shielding foam is flexible and lightweight and can provide fire resistance and durability. By embedding metal in silicone, it can also be an effective shielding material. It’s flexible and malleable and resists water, sunlight and low and high temperatures. Places where you can find shielding include surrounding cables, in seams and within doors and panels.
Another type of barrier against electromagnetic radiation is the Faraday cage or Faraday shield. Named after 19th-century scientist Michael Faraday, these structures look like cages and redistribute incoming EMI around the exterior of the cage, canceling it out in the interior. Faraday cages are in everything from microwaves to the rooms of magnetic resonance imaging (MRI) scanners.
EMI Filtering
Filtering has a similar end goal, but instead of blocking EMI in general, filters work like they would for water or air purifiers. EMI filtering means removing unwanted components — frequencies — while letting the desirable ones pass through. To do this, the noise is either diverted to the ground, absorbed into the filter or reflected back to its origin. The filter is made up of capacitors and inductors:
- Capacitors: A capacitor inhibits direct current (DC) to prevent EMI from getting in, but it continues to allow alternating current (AC) through. These are also called shunting capacitors.
- Inductors: The inductors hold energy within a magnetic field as the current passes through, allowing it to reduce the total voltage.
These two components work together, as the capacitor feeds EMI to the inductors. These are arranged in a series and reduce the current’s voltage as it moves through them. The goal is to bring interference down to nothing, which is called shorting to ground. Filters are placed at specific points on a circuit to control the flow across various frequencies.
The basic foundation of an EMI filter is impedance matching. For the best attenuation, the impedance matching between the input, output, power supply and load side of the filter should be as large as possible.
Generally, EMI filters will be either:
- Single-phase filters: These filters are best for smaller equipment such as consumer electronics, appliances and industrial applications like power supplies and telecommunications.
- Three-phase filters: A three-phase filter is best for more intense or sensitive devices like medical equipment and industrial machinery. These filters block higher levels of noise and are good for stricter EMI suppression.
Both types use conduction to reduce the electromagnetic noise. Often, these are low-pass filters, sifting out the high frequencies and allowing the low ones through. Different filters suppress certain frequencies, diverting the noise or absorbing it after the filtering process.
Keep in mind that filters only work on conducted EMI, which moves through a physical connection such as a cable. Radiated EMI travels through the air and doesn’t require a physical point of entry. The filter can be integrated in a device or work separately.
How EMI Filtering and Shielding Work Together
Although filters and shields use different mechanisms to shield from EMI, they make a great pair and are often used together because they fill the gaps the other creates. In shielding applications, you’ll often need holes in the shield to function as heat sinks, allowing thermal energy to be released and pulled away from the sensitive components. While these heat sinks will reduce the overall effectiveness of the shield, they’re necessary for controlling heat and preventing the device from being damaged or creating safety risks.
By placing filters at these heat sink points, engineers can help minimize the EMI that would otherwise move through the holes. The filters reduce the high-frequency noise that enters or exits and can help bring down conducted EMI where the shield cannot.
In many cases, both filters and shields are necessary to accommodate the different EMI environments throughout a device. The two work together to create more comprehensive EMI protection than either one could accomplish alone. The nature of the device plays a role in whether both are needed or if one could do the job.
Choosing Between EMI Filtering, Shielding, or Both for Electronics
The decision between which EMI solution to use involves many different factors, most notably how sensitive the device is and how much EMI it is likely to generate. These will determine your device’s electromagnetic compliance (EMC), or its ability to function as intended without problems from EMI and without causing problems for other devices.
Whatever your application, you’ll want to consider factors like the weight it can support, what level of conductivity you’ll need, how much you want to spend on the system and how thick it can be.
If your application calls for medical or military use, you’ll need to ensure the shield and filter are graded for those environments. These sensitive and often high-powered devices must meet particularly stringent standards. These standards may vary by region, but they provide good baselines to work from.
In the U.S., we have FCC 47 CFR Part 15, with Class A and Class B. Class A is for use outside of the home, such as offices and industrial settings, while Class B applies to home use. Other regions have differing requirements, so you’ll want to look into them. Even if you’re not sure exactly which standards will apply to a device, they provide useful guidelines.
Contact AerosUSA for EMI Solutions
EMI is a tricky concept to design around. The knowledgeable professionals at AerosUSA are ready to help you find the right solution for your application. We have years of experience in cabling and electric engineering and can assist with applications in everything from consumer devices to industrial equipment and sensitive medical and military technology.
Our wide inventory of cables includes various shielding and filtering solutions such as sleeves, braided wire and more. Explore the selection today or reach out to a team member to find the right product for your design.
