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Why do I need an EMI Filter?  Glossary of EMI Filter Terms

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Why do you need an EMI Filter?

Electromagnetic compatibility (EMC) engineers use the concept of “noise” to describe the unwanted signals that degrade the performance of electronic equipment. In avionics applications, both external and internal sources of EMI noise can jam sensitive navigation and tactical equipment, possibly even disrupting control of the aircraft. An aircraft carrier’s massive electronics bay might cause interference that scuttles a take-off or landing. EMI affecting satellite transmissions can cause communication failures on the battleground. For these reasons, EMI is considered a serious problem, and numerous technologies and techniques have been developed to insure electromagnetic compatibility (EMC) in data transmission systems—from shipboard to undersea, from avionics to space, from aircraft carriers to micro unmanned aerial vehicles.

Sources of EMI

Sources of EMI “noise” can be grouped into three categories: 1) intrinsic noise that arises from random fluctuations within physical systems, such as thermal and shot noise, 2) man-made noise from motors, switches, power supplies, digital electronics and radio transmitters, and 3) noise from natural disturbances such as electro-static discharge (ESD), lightning and sunspots.


Intrinsic noise sources can be very subtle and often go unrecognized. All electrical systems are potential sources of intrinsic noise, including such common devices as portable radios, MP3 players, cell phones and so on. These devices can cause interference simply by being on. This is because electrons within a conducting media or a semiconductor device create current flow when excited by external voltages. When the externally applied voltage stops, electrons continue to move, randomly interacting with other electrons and with the surrounding material. This random electron motion can create noise in conducting media even without current flow.


To protect avionics systems from man-made noise, intentional radio frequency (RF) emitters like cell phones, Bluetooth accessories, CB radios, remote-controlled toys, and walkie-talkies are banned outright on commercial airline flights. Laptops, hand held scanners and game players, while not intentional emitters, can produce signals in the 1 MHz range that can affect performance of avionic equipment. Navigation cabling and other critical wiring runs along the fuselage with passengers sitting just a few feet away. Since the thin sheet of dielectric material that forms the interior of the passenger compartment— typically fiberglass—offers no shielding whatsoever; and since commercial passenger jets contain up to 150 miles of electrical wiring that can behave like a giant antenna, it is extremely important for passengers to heed regulations on the use of potentially disruptive electronic equipment.

Obviously, these internal sources of EMI are quite dangerous to aircraft because they are so close to the systems they might affect. But external sources, such as radio and radar transmitters on the ground, or radar from a passing military plane, Cockpit avionics are susceptible to multiple sources of EMI including man-made interference from iPhones and other PEDs can be even more disruptive due to the high power and frequency of such equipment. As if the many external and internal sources of EMI were not enough of a concern, the aluminum airframe itself, in certain circumstances, can act as a resonant cavity in the 1 to 10 MHz range. Behaving much like a satellite dish, the airframe can compound the effects of both internal and external EMI by concentrating man made and naturally-occurring transient signals and broadcasting the interference into nearby equipment. A recently released report from a major aircraft manufacturer illustrates the ongoing concern with passenger-carried portable electronic devices (PED). The number of these devices on commercial airplanes has mushroomed, particularly with the advents of new classes of laptop devices such as the Apple iPad. The use of PEDs produces uncontrolled electromagnetic emissions that have the potential to interfere with avionic systems. While aircraft avionics gear is tested and qualified to rigorous electromagnetic standards PEDs are not subjected to even a fraction of the same testing and qualification regimens for electromagnetic compatibility. As system speeds have increased, the voltage levels of data signals have necessarily decreased, making them much more susceptible to performance degradation by unwanted electronic noise—particularly the combined noise of large numbers of PED’s operating within a single aircraft.

The frequency bands used by avionic systems span the electromagnetic spectrum from a few kilohertz to several gigahertz. At the low end, Omega Navigation, which is used to fix aircraft position within a network of ground based transmitters, operates in the frequency range of 10 to 14 KHz. VHF Omnidirectional Range Finders (VOR) are radio beacons used in point to point navigation. They operate from 108 to 118 MHz. Glideslope Systems used during landings operate in the 328 to 335 MHz range. Distance-Measuring Equipment (DME), which gauges the space between the aircraft and ground-based transponders operate at just over 1 GHz. Also in the spectrum above 1 GHz are global positioning, collision avoidance, and cockpit weather radar systems. Personal Electronic Devices (PEDs) operate at frequencies from 10 to 15 KHz for AM radios and up to 400 MHz for laptop computers. When the higher harmonics of these signals are taken into account, the emitted frequencies cover almost the entire range of navigation and communication frequencies used on the aircraft, and PEDs, are just a single class of EMI emitters. When the full spectrum of other radiated and conducted EMI emitters are taken into account, it becomes clear that the entire system of electronic equipment aboard commercial and military aircraft are at risk to EMI.


Occurring Naturally occurring noise sources such as ESD, lightning or other energy surges also present significant life safety and equipment damage potential. A poorly grounded device can transmit dangerous energy from a transient surge to a technician, user or any other passerby. Sensitive semiconductors and other components can be damaged or destroyed. Solutions to naturally occurring noise include:

  • Eliminate static buildup at the source
  • Insulate the device properly
  • Provide an alternative path for the discharge or surge to bypass the circuit
  • Use of EMI Filters with Transient Suppression

In conclusion, it is becoming more and more apparent that EMI/EMC is a growing concern for both the military and commercial industries for all forms of electronic equipment. In response to the increasing demand for low-cost and effective EMI/EMC solutions, WEMS Electronics is here to solve your most challenging requirements and would welcome any opportunity to demonstrate our expertise on your new & upgrade program requirements.

EMI Filter Glossary

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A filter made up of both active and passive components. It may provide amplification, as well as attenuation.
A capacitor that uses a thin aluminum oxide layer on pure aluminum foil as a dielectric. An electrically active solution called electrolyte is part of the cathode of the capacitor.
The ability of a filter to reduce interference on incoming or outgoing power lines. Expressed in dB (see Decibel).


A filter that rejects frequencies above and below a frequency, or frequencies, of interest
A filter that rejects frequencies above and below a frequency, or frequencies, of interest.
A document that establishes a procedure for controlling all changes to design sheets, outline drawing, cross sectional drawing, part procurement drawing, high reliability documentation and all procedures and processes, both assembly and test, used in conjunction with the manufacturing of parts to a customer’s drawings
Also referred to as Bushing Style Filters, Bolt Style Filters, and Screw Body Style Filters.
A filter specifically designed to have a maximally flat passband.
A cylindrical, or coaxial, style filter with a straight or nail head lead for a terminal. These filters are normally “C” or “L” circuits and are epoxy sealed on both ends.


The technically correct term for capacity. It is the measure of the amount of electrical charge stored in a capacitor expressed in farads. One farad is one coulomb of charge at one volt.
Opposition offered to the flow of alternating or pulsating current by capacitance, measured in ohms.
A filter design approach providing greater selectivity (for a given number of elements) than a Butterworth design by introducing some ripple in the passband.
A filter specifically designed for surface mount installation.
The international special committee for EMI/RFI suppression.
Interference which is present as a common potential between ground and all power lines, also referred to as asymmetric interference.
Electromagnetic interference that exits or enters a piece of equipment via conduction on the power line.
The cutoff frequency
An undesirable electrical discharge resulting from lionization of gas within a capacitor. This is a particular problem with film capacitors in AC voltage conditions.
The frequency which determines the end of the passband, normally the 3 dB point.
Also referred to as Coaxial Style Filters, Mini Filters, Sub-miniature Filters, Broadband Filters, Threaded Mount Style Filters, and Button Filters.


An increase or decrease in frequency by a factor of 10. 1 kHz to 10 kHz is one decade.
Convenient method of expressing the ratio of signal levels i.e. input to output.
The insulating material that separates the two plates of the capacitor. Common dielectric include mica, ceramic, plastic films, tantalum pentoxide, and aluminum oxide.
Interference which is present as a potential between individual power lines, also referred to as symmetric Interference.
Destructive Physical Analysis. A method of examining a filter or capacitor for defects, such as voids or de-laminations, by cross-sectioning.


Electrostatic discharge.
A design approach which yields maximum selectivity by introducing stopband and passband ripple.
Electromagnetic Compatibility
Electromagnetic Interference. Same as RFI.
the releasing or sending forth of either conducted or radiated EMI produced by some electronic device.
Electromagnetic Pulse. A high energy electromagnetic pulse resulting from a nuclear detonation in the stratosphere.
Equivalent series inductance. The extent to which the capacitor acts as though there were an inductor in series with the capacitor. ESL is generally undesirable especially at high frequencies.


A three terminal capacitor where the third terminal is grounded. The capacity is between the line passing through the capacitor and ground.
A filter designed for bulkhead mounting. All signals must “feed-through” the bulkhead.
Capacitors made out of any plastic, polyester, polypropylene, and polystyrene, being the most common.
An electronic circuit that offers minimum opposition to the passage of certain signals, but offers maximum opposition to others.


Ratio of output to input at a certain frequency. The filter both amplifies and attenuates if the gain is greater than one.


An acronym for High Energy Radiated Fields. This type of radiated susceptibility testing generates electric fields in excess of 1000 volts per meter to simulate close lightning strikes, normally applicable to aircraft testing.
High-potential test. A test for determining the breakdown point of insulating materials and spacing also referred to as dielectric withstanding voltage.
A filter that passes frequencies above a specific frequency.


A measure of the total opposition to current flow in an AC circuit.
The measured signal at a given point of the circuit at a specified frequency without a filter in the circuit. This is compared to the signal level obtained at the same point with the filter inserted into the circuit.
The measured signal at a given point of the circuit at a specified frequency without a filter in the circuit. This is compared to the signal level obtained at the same point with the filter inserted into the circuit.


The current that flows from the power line to ground potential when power is applied to the device in question. If the device becomes ungrounded, it could become a shock hazard.
A power supply having an output voltage bearing a direct or linear relationship to the input voltage. Transformer power supplies with capacitive input filters are categorized as linear power supplies.
A filter designed to pass frequencies below a specific frequency.


A general military specification for all types of radio frequency interference filters and capacitors.
A general military specification specifically intended for EMI filters and capacitors that utilize ceramic dielectric capacitors. MIL-PRF-28861 is a more stringent specification than MIL-PRF-15733 and will eventually replace some of the filter slash sheets in MIL-PRF-15733.
A military specification delineating the electrical and environmental test methods for electronic parts. MIL-STD-202 is referenced in filter specifications MIL-PRF-15733 AND MIL-PRF-28861.
A military specification delineating the test methods for measuring the insertion loss of a filter in a 50 ohm system. This document does not specify minimum limits and cannot be correlated to the requirements of MIL-STD-461.
MIL-STD-461 (462, 463)
A joint service military specification that limits the level of conducted and radiated emissions (EMI) emanating from military subsystems and systems. This specification also establishes minimum levels of conducted and radiated susceptibility including EMP. EMI filters do not meet MIL-STD-461. EMI filters cause systems to comply with the requirements by reducing the conducted emissions.
A military standard that defines the criteria for electronic parts reliability assurance programs which must be adhered to by manufacturers qualifying parts to specifications where established reliability is required.
A custom engineered filter assembly whose main function is to provide filtering for primary input power lines (AC and DC) and, in some cases, the output or signal lines.
A ceramic capacitors manufactured by stacking many alternating layers of ceramic dielectric and conductive plates.


A random or persistent electrical disturbance that can obscure or reduce the clarity of quality of an electrical signal or function.
A filter designed to reject a small band of frequencies, passing the low frequencies and the high frequencies.


An increase or decrease in frequency by a factor of two. 6 MHz to 12 MHz is one octave. The insertion loss of a filter increases at a rate of 6 dB per filter pole per octave in the reject band. The insertion loss of a Pi circuit filter will increase by 18 dB from 6 MHz to 12 MHz.


The band of signal frequencies to which a filter offers minimum opposition.
A filter comprised of only passive, reactive and resistive elements (resistors, capacitors, and inductors). The maximum gain of this type filter is less than one.
An electrical circuit comprised of passive components (resistors, capacitors, and inductors).
Percent Defective Allowed. The number of defective parts (failures) expressed as a percent of the lot, that are permitted for a given test or group of tests.
A type of filter intended to prevent power line noise from entering electrical or electronic equipment. These filters also prevent digital signals from leaking out onto the power supply line.


See stop bend.


A term used to describe a filter’s ability to distinguish between frequencies.
Also referred to as Eyelet Style Filters and Sleeve Mount Filters.
The band of frequencies to which a filter offers maximum opposition.
The ease with which an item of electronic equipment is affected by the influence of an outside RF signal source, which is being conducted or radiated in the equipment.
A power supply which uses transistors to switch a rectified and filtered DC voltage at 20 kHz or higher rate. This high frequency voltage is then applied to the primary winding of a transformer. An advantage of using this technique includes high efficiency in the power conversion process. Also, smaller transformers can be used, which results in a lightweight power supply.


A general term for the protection and prevention of classified sensitive information being extracted electromagnetically. Electronic eavesdropping.
Ratio of a filter’s output to its input voltage as a function of frequency.
A mathematical expression used in the synthesis of a filter design. Mathematically expressing the transfer characteristic.
A feed-through capacitor formed on large presses which extrude a mix of ceramic powders, organic binders and solvents. After firing, the tubes are cut to the desired length, .5 to 1.0 cm, using high speed a diamond saw. Electrodes are then applied to the OD and the ID of the tube.
A filter having a corner, or center, frequency which can be adjusted or tuned.