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WEMS Electronics Executive Overview EMI Filter Design Guide

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

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.

Man-made

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.

Naturally

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

Filter: Show All A B C D E F G H I L M N O P R S T U V W Z
Click on any term below to reveal the description.

A

ABSORPTION LOSS
That part of shielding effectiveness dealing with energy absorption through a metal barrier.
ACTIVE FILTER
A filter made up of both active and passive components. It may provide amplification, as well as attenuation.
ALUMINUM ELECTROLYTIC
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.
APERTURE LEAKAGE
Compromise in shielding effectiveness from holes, slits, and slots from braid, windows, cooling openings, and joints of metal boxes where EMI can get in or out.
ATTENUATION
A measure of filtering, expressed in decibels (dB). It is a measure of how effectively the filter reduces the strength of a given frequency. The higher the number, the better the filtering. In general, attenuation of 40 to 70 dB is a good target. Reduction in the quality or magnitude of an electrical signal. Suppression of EMI noise in the interconnect transmission path.

B

BAND – ELIMINATION FILTER
A filter that rejects frequencies above and below a frequency, or frequencies, of interest.
BANDPASS FILTER
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.
BANDWIDTH
The frequency interval between the upper and lower 3 dB down response of a receiver.
BASELINE
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.
BOLT STYLE FILTERS
Also referred to as Bushing Style Filters, Bolt Style Filters, and Screw Body Style Filters.
BUTTERWORTH
A filter network that exhibits the flattest possible response in its passband. The response is monotonic, rolling off smoothly at the rate of 6dB per octave, per pole.
BUTTON FILTER
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.

C

C FILTER
A filter using a capacitor.
CAPACITANCE
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.
CAPACITANCE REACTANCE
Opposition offered to the flow of alternating or pulsating current by capacitance, measured in ohms.
CHEBYSHEV
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 network that is designed to exhibit a predetermined passband ripple; in exchange, it provides more rapid attenuation above the cut-off (-3dB) frequency.
CHIP FILTER
A filter specifically designed for surface mount installation.
CISPR
The international special committee for EMI/RFI suppression.
CLAMPING VOLTAGE
The maximum voltage to which the TVS device will clip any transient voltages.
COMMON MODE INTERFERENCE
Interference which is present as a common potential between ground and all power lines, also referred to as asymmetric interference.
CONDUCTED EMI
Electromagnetic interference that exits or enters a piece of equipment via conduction on the power line.
CORNER FREQUENCY
The cutoff frequency.
CORONA
An undesirable electrical discharge resulting from lionization of gas within a capacitor.  This is a particular problem with film capacitors in AC voltage conditions.
COUPLING PATH
The conducted or radiated path by which interfering energy gets from a source to a victim.
CROSS MODULATION
Energy from one transmitter that causes the modulation to change on a received signal from another transmitter.
CROSSTALK
Electromagnetic energy bleed across dielectric materials, for example, in twisted pair cable sets or across adjacent connector contacts, disrupting the electrical signals in each respective circuit.
CUTOFF FREQUENCY
The frequency which determines the end of the passband, normally the 3 dB point.
CYLINDRICAL STYLE FILTERS
Also referred to as Coaxial Style Filters, Mini Filters, Sub-miniature Filters, Broadband Filters, Threaded Mount Style Filters, and Button Filters.

D

DECADE (FREQUENCY)
An increase or decrease in frequency by a factor of 10.  1 kHz to 10 kHz is one decade.
DECIBEL (Db)
Convenient method of expressing the ratio of signal levels i.e. input to output.
DIELECTRIC
The insulating material that separates the two plates of the capacitor.  Common dielectric include mica, ceramic, plastic films, tantalum pentoxide, and aluminum oxide.
DIFFERENTIAL MODE INTERFERENCE
Interference which is present as a potential between individual power lines, also referred to as symmetric Interference.
DOUBLE-T FILTER
A filter composed of two T filters. It has five elements, typically three inductors and two capacitors.
DPA
Destructive Physical Analysis.  A method of examining a filter or capacitor for defects, such as voids or de-laminations, by cross-sectioning.
DRAIN
Path by which charges absorbed by a conductor move to ground in a properly grounded system.

E

ED
Electrostatic discharge.
ELECTRIC FIELD
A radiated wave’s potential gradient in volts per meter (V/m).
ELLIPTIC FUNCTION
A design approach which yields maximum selectivity by introducing stopband and passband ripple.
EMC
Electromagnetic compatibility, the conditions under which all components of a system do not interfere with each other or with their environment.
EMI
Electromagnetic interference (opposite of EMC). Electromagnetic disturbance that degrades performance of electronic equipment. Same as RFI.
EMISSION
The releasing or sending forth of either conducted or radiated EMI produced by some electronic device.
EMP
Electromagnetic Pulse.  A high energy electromagnetic pulse resulting from a nuclear detonation in the stratosphere.
ESD
Electrostatic discharge. A momentary (and unwanted) discharge of built-up electrical energy, usually from an electrically insulated object to an object with a different electrical potential.
ESL
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.
ESR
Equivalent series resistance.  The extent to which the capacitor acts as a resistor when charging and discharging in an electronic circuit expressed as a resistance in series with the capacitor

F

FEED-THROUGH CAPACITOR
A three terminal capacitor where the third terminal is grounded.  The capacity is between the line passing through the capacitor and ground.
FEED-THROUGH FILTER
A filter designed for bulkhead mounting.  All signals must “feed-through” the bulkhead.
FERRITE
Powdered magnetic material in form of beads, rods, and rings used to absorb EMI on wires and cables.
FIELD STRENGTH
Radiated voltage or current per meter corresponding to electric or magnetic fields.
FILM CAPACITORS
Capacitors made out of any plastic, polyester, polypropylene, and polystyrene, being the most common.
FILTER
An electronic circuit that offers minimum opposition to the passage of certain signals, but offers maximum opposition to others.
FILTER CONNECTOR
Connector that houses contacts that provide EMI suppression in addition to its normal function or transmitting electrical energy. Filtered connectors are typically specified for high speed signal paths. Filtering is accomplished through the integration of capacitors into the contact to separate high frequency noise from low frequency signals.

G

GAIN
Ratio of output to input at a certain frequency.  The filter both amplifies and attenuates if the gain is greater than one.
GROUNDING
A conductive path to earth designed to eliminate electrical shock by shunting away dangerous currents.

H

HERF
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.
HI-POT
High-potential test. A test for determining the breakdown point of insulating materials and spacing also referred to as dielectric withstanding voltage.
HIGHPASS FILTER
A filter that passes frequencies above a specific frequency.

I

IMPEDANCE
The sum of the resistive and reactive components in an AC circuit, which represents the total resistance of that circuit (symbol is Z).
IMPULSE NOISE
A transient electrical disturbance, usually repetitive.
INDUCTANCE
For filter connectors, inductance presents high reactance to high frequencies and low reactance to low frequencies: it passes low frequencies and blocks high frequencies
INSERTION LOSS
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.
INSULATION RESISTANCE (IR)
The electrical resistance of an insulating material, typically measured between two conductors or a conductor and ground. The value is typically several million ohms (megohms).

L

L FILTER
A filter using an inductor (or several inductors) only.
LC OR CL FILTER
A filter using both a capacitor and an inductor.
LEAKAGE CURRENT
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.
LEMP
Lightning electromagnetic pulse. EMP caused by a lightning strike.
LINEAR POWER SUPPLY
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.
LOWPASS FILTER
A filter designed to pass frequencies below a specific frequency.

M

MAGNETIC FIELD
A radiated wave’s current gradient, expressed in amperes per meter (A/m).
METAL-OXIDE VARISTOR (MOV)
A device whose resistance varies with voltage, used to suppress transient voltages. Varistor is a shortened form of variable resistor.
MIL-PRF-15733
A general military specification for all types of radio frequency interference filters and capacitors.
MIL-PRF-28861
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.
MIL-STD-202
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.
MIL-STD-220
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.
MIL-STD-790
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.
MULTICIRCUIT FILTER
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.
MULTILAYER CERAMIC (MLC)
A ceramic capacitors manufactured by stacking many alternating layers of ceramic dielectric and conductive plates.

N

NARROWBAND
EMI Interference whose emission bandwidth is less than the bandwidth of the EMI measuring receiver or spectrum analyzer.
NEMP
Nuclear electromagnetic pulses. EMP caused by a nuclear explosion.
NOISE
A random or persistent electrical disturbance that can obscure or reduce the clarity of quality of an electrical signal or function.
NOTCH FILTER
A filter designed to reject a small band of frequencies, passing the low frequencies and the high frequencies.

O

OCTAVE (FREQUENCY)
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

P

PASSBAND
The band of signal frequencies to which a filter offers minimum opposition.
PASSIVE FILTER
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.
PASSIVE NETWORK
An electrical circuit comprised of passive components (resistors, capacitors, and inductors).
PDA
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.
PEAK IMPULSE CURRENT
The maximum current that the TVS device can handle as the result of a transient Peak Pulse Power Dissipation. A TVS device’s capability for dissipating energy. Peak pulse power dissipation is calculated by multiplying the clamping voltage by the peak impulse current (PPP = VCLAMP x IMAX) .
PI FILTER
A filter using two capacitors and one inductor for a low-pass configuration; named after the shape of the schematic symbols.
PLANAR CAPACITOR ARRAY
A monolithic construction technique for capacitive filters, arrays offer a compact, low-profile form factor well suited for use in connectors. A single array can accommodate all contacts in a connector; filter values can be the same for all contacts or have different values for each contact.
POWER CONDITIONING
Reduction of EMI pollution on power mains by inserting filters, isolators, regulators, or an uninterruptible power supply (UPS).
POWER LINE FILTER
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.

R

RADIATED INTERFERENCE
EMI or noise transmitted from any electrical system, from power-lines to mobile telephones.
RADIO FREQUENCY INTERFERENCE (RFI)
A special class of electromagnetic interference in which radio frequency transmissions cause unintentional problems in equipment operation.
REACTANCE
Resistance to changes in voltage (capacitive reactance) or current (inductive reactance). Reactance is frequency dependent.
REFLECTION LOSS
Shielding effectiveness due to energy reflection from impedance mismatch between incident field and metal barrier.
REJECT BAND
See STOP BAND.
RFI
Radio frequency interference. A type of EMI that occurs in the radio frequency bands.
ROLL-OFF
The frequency in an attenuation curve at which a filter begins to reduce the quality or magnitude of an electrical signal.

S

SELECTIVITY
A term used to describe a filter’s ability to distinguish between frequencies.
SHIELDING EFFECTIVENESS (SE)
The ratio of field strengths (absorption and reflection losses) before and after installing a shield.
SHOT NOISE
The noise caused by random fluctuations in the motion of charge carriers such as electrons in a conductor.
SKIN DEPTH
The calculated metal layer thickness through which some 63 percent of the surface current flows.
SLOPE
Filtering goes from no filtering to maximum filtering over a range of frequencies. The slope shows attenuation versus frequency. A steep slope is desirable, but requires more complex filtering.
SOLDER-IN STYLE FILTERS
Also referred to as Eyelet Style Filters and Sleeve Mount Filters.
STOP BAND
The band of frequencies to which a filter offers maximum opposition.
SURGE
A sudden voltage increase on the power mains.
SUSCEPTIBILITY
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.
SWITCHING POWER SUPPLY
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.

T

T FILTER
A filter having two filter elements in the pass path and one element in the ground (filter) path. For a low-pass EMI filter, the circuit has two inductors and one capacitor. Used when both the source and load have low impedances.
TEMPEST
A general term for the protection and prevention of classified sensitive information being extracted electromagnetically. Electronic eavesdropping.
TRANSFER CHARACTERISTICS
Ratio of a filter’s output to its input voltage as a function of frequency.
TRANSFER FUNCTION
A mathematical expression used in the synthesis of a filter design. Mathematically expressing the transfer characteristic.
TRANSFER IMPEDANCE (Zt)
The quality of cable shield performance calculated by the ratio of the coupled voltage to the surface current, in ohms per meter (Ω/m).
TRANSIENT
A short-duration voltage surge due to a lightning strike or other dynamic event.
TUBULAR CERAMIC CAPACITORS
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.
TUNABLE FILTER
A filter having a corner, or center, frequency which can be adjusted or tuned.
TVS
Transient voltage suppression. Any of several techniques to suppress electromagnetic pulses by clamping voltage transients to acceptable levels. The most common TVS device used for filter connectors is the Zener diode.

U

UPS
Uninterruptible power supply.

V

VOLTAGE STANDING WAVE RATIO (VSWR)
VSWR is an efficiency measurement of how radio frequency is transmitted from a power source though a transmission line, into a termination (or load).

W

WAVEFORM
For lightning events, measure of electrical transient exposure level and surge severity.

Z

ZENER DIODE
A semiconductor device that allows current to flow in the forward direction like a standard diode, but also permits current in the reverse direction when the voltage exceeds a certain level. Used for transient suppression.