Electromagnetic interference is disturbance
that affects an electrical circuit due to
either electromagnetic induction or electromagnetic
radiation emitted from an external source.
The disturbance may interrupt, obstruct, or
otherwise degrade or limit the effective performance
of the circuit. These effects can range from
a simple degradation of data to a total loss
of data. The source may be any object, artificial
or natural, that carries rapidly changing
electrical currents, such as an electrical
circuit, the Sun or the Northern Lights.
EMI can be intentionally used for radio jamming,
as in some forms of electronic warfare, or
can occur unintentionally, as a result of
spurious emissions for example through intermodulation
products, and the like. It frequently affects
the reception of AM radio in urban areas.
It can also affect cell phone, FM radio and
television reception, although to a lesser
extent.
Types
Narrowband EMI or RFI interference typically
emanates from intended transmissions, such
as radio and TV stations or cell phones.
Broadband EMI or RFI interference is unintentional
radiation from sources such as electric power
transmission lines.
Conducted electromagnetic interference is
caused by the physical contact of the conductors
as opposed to radiated EMI, which is caused
by induction. Electromagnetic disturbances
in the EM field of a conductor will no longer
be confined to the surface of the conductor
and will radiate away from it. This persists
in all conductors and mutual inductance between
two radiated electromagnetic fields will result
in EMI.
Susceptibilities of different radio technologies
Interference tends to be more troublesome
with older radio technologies such as analogue
amplitude modulation, which have no way of
distinguishing unwanted in-band signals from
the intended signal, and the omnidirectional
dipole antennas used with broadcast systems.
Newer radio systems incorporate several improvements
that enhance the selectivity. In digital radio
systems, such as Wi-Fi, error-correction techniques
can be used. Spread-spectrum and frequency-hopping
techniques can be used with both analogue
and digital signalling to improve resistance
to interference. A highly directional receiver,
such as a parabolic antenna or a diversity
receiver, can be used to select one signal
in space to the exclusion of others.
The most extreme example of digital spread-spectrum
signalling to date is ultra-wideband, which
proposes the use of large sections of the
radio spectrum at low amplitudes to transmit
high-bandwidth digital data. UWB, if used
exclusively, would enable very efficient use
of the spectrum, but users of non-UWB technology
are not yet prepared to share the spectrum
with the new system because of the interference
it would cause to their receivers. The regulatory
implications of UWB are discussed in the ultra-wideband
article.
Interference to consumer devices
In the United States, the 1982 Public Law
97-259 allowed the Federal Communications
Commission to regulate the susceptibility
of consumer electronic equipment.
Potential sources of RFI and EMI include:
various types of transmitters, doorbell transformers,
toaster ovens, electric blankets, ultrasonic
pest control devices, electric bug zappers,
heating pads, and touch controlled lamps.
Multiple CRT computer monitors or televisions
sitting too close to one another can sometimes
cause a "shimmy" effect in each other, due
to the electromagnetic nature of their picture
tubes, especially when one of their de-gaussing
coils is activated.
Electromagnetic interference at 2.4 GHz can
be caused by 802.11b and 802.11g wireless
devices, Bluetooth devices, baby monitors
and cordless telephones, video senders, and
microwave ovens.
Switching loads, such as electric motors,
transformers, heaters, lamps, ballast, power
supplies, etc., all cause electromagnetic
interference especially at currents above
2Amp. The usual method used for suppressing
EMI is by connecting a snubber network, a
resistor in series with a capacitor, across
a pair of contacts. While this may offer modest
EMI reduction at very low currents, snubbers
do not work at currents over 2Amp with electromechanical
contacts.
Switched-mode power supplies can be a source
of EMI, but have become less of a problem
as design techniques have improved, such as
integrated power factor correction.
Most countries have legal requirements that
mandate electromagnetic compatibility: electronic
and electrical hardware must still work correctly
when subjected to certain amounts of EMI,
and should not emit EMI, which could interfere
with other equipment.
Radio frequency signal quality has declined
throughout the 21st Century by roughly one
decibel per year as the spectrum becomes increasingly
crowded. This has inflicted a Red Queen's
race on the mobile phone industry as companies
have been forced to put up more cellular towers
that then cause more interference thereby
requiring more investment by the providers
and frequent upgrades of mobile phones to
match.
History
Since the earliest days of radio communications,
the negative effects of interference from
both intentional and unintentional transmissions
have been felt and the need to manage the
radio frequency spectrum became apparent.
In 1933, a meeting of the International Electrotechnical
Commission in Paris recommended the International
Special Committee on Radio Interference be
set up to deal with the emerging problem of
EMI. CISPR subsequently produced technical
publications covering measurement and test
techniques and recommended emission and immunity
limits. These have evolved over the decades
and form the basis of much of the world's
EMC regulations today.
In 1979, legal limits were imposed on electromagnetic
emissions from all digital equipment by the
FCC in the USA in response to the increased
number of digital systems that were interfering
with wired and radio communications. Test
methods and limits were based on CISPR publications,
although similar limits were already enforced
in parts of Europe.
In the mid 1980s, the European Union member
states adopted a number of "new approach"
directives with the intention of standardizing
technical requirements for products so that
they do not become a barrier to trade within
the EC. One of these was the EMC Directive
and it applies to all equipment placed on
the market or taken into service. Its scope
covers all apparatus "liable to cause electromagnetic
disturbance or the performance of which is
liable to be affected by such disturbance".
This was the first time there was a legal
requirement on immunity as well as emissions
on apparatus intended for the general population.
And although there may be additional costs
involved for some products to give them a
known level of immunity, it increases their
perceived quality as they are able to co-exist
with apparatus in the active EM environment
of modern times and with fewer problems.
Many countries now have similar requirements
for products to meet some level of EMC regulation.
Standards
The International Special Committee for Radio
Interference or CISPR, which is a committee
of the International Electrotechnical Commission
sets international standards for radiated
and conducted electromagnetic interference.
These are civilian standards for domestic,
commercial, Industrial and Automotive sectors.
These standards form the basis of other regional
and national standards most notably the European
Norms written by CENELEC.
EMI in integrated circuits
Integrated circuits are often a source of
EMI, but they must usually couple their energy
to larger objects such as heatsinks, circuit
board planes and cables to radiate significantly.
On integrated circuits, important means of
reducing EMI are: the use of bypass or decoupling
capacitors on each active device, rise time
control of high-speed signals using series
resistors, and VCC filtering. Shielding is
usually a last resort after other techniques
have failed, because of the added expense
of shielding components such as conductive
gaskets.
The efficiency of the radiation depends on
the height above the ground plane or power
plane and the length of the conductor in relation
to the wavelength of the signal component).
At lower frequencies, such as 133 MHz, radiation
is almost exclusively via I/O cables; RF noise
gets onto the power planes and is coupled
to the line drivers via the VCC and ground
pins. The RF is then coupled to the cable
through the line driver as common-mode noise.
Since the noise is common-mode, shielding
has very little effect, even with differential
pairs. The RF energy is capacitively coupled
from the signal pair to the shield and the
shield itself does the radiating. One cure
for this is to use a braid-breaker or choke
to reduce the common-mode signal.
At higher frequencies, usually above 500 MHz,
traces get electrically longer and higher
above the plane. Two techniques are used at
these frequencies: wave shaping with series
resistors and embedding the traces between
the two planes. If all these measures still
leave too much EMI, shielding such as RF gaskets
and copper tape can be used. Most digital
equipment is designed with metal, or conductive-coated
plastic, cases.
RF immunity and testing
Any unshielded Semiconductor will tend to
act as a Detector for those radio signals
commonly found in the domestic environment.
Such a Detector can demodulate the high frequency
cell phone carrier and produce low-frequency
demodulated signals. This demodulation manifests
itself as unwanted audible buzz in audio appliances
such as microphone amplifier, speaker amplifier,
car radio, telephones etc. Adding on-board
EMI filters or special layout techniques can
help in bypassing EMI or improving RF immunity.
Some ICs are designed to have integrated RF
filters or a special design that helps reduce
any demodulation of high-frequency carrier.
Designers often need to carry out special
tests for RF immunity of parts to be used
in a system. These tests are often done in
an anechoic chamber with a controlled RF environment
where the test vectors produce a RF field
similar to that produced in an actual environment.
RFI in radio astronomy
Interference in radio astronomy, where it
is commonly referred to as radio-frequency
interference, is any source of transmission
that is within the observed frequency band
other than the celestial sources themselves.
Because transmitters on and around the Earth
can be many times stronger than the astronomical
signal of interest, RFI is a major concern
for performing radio astronomy. Natural sources
of interference, such as lightning and the
sun, are also often referred to as RFI.
Some of the frequency bands that are very
important for radio astronomy, such as the
21-cm HI line at 1420 MHz, are protected
by regulation due to spectrum management.
However, modern radio-astronomical observatories
such as VLA, LOFAR and ALMA have a very large
bandwidth over which they can observe. Because
of the limited spectral space at radio frequencies,
these frequency bands can not be completely
allocated to radio astronomy. Therefore, observatories
need to deal with RFI in their observations.
Techniques to deal with RFI range from filters
in hardware to advanced algorithms in software.
One way to deal with strong transmitters is
to filter out the frequency of the source
completely. This is for example the case for
the LOFAR observatory, which filters out the
FM radio stations between 90-110 MHz. It
is important to remove such strong sources
of interference as soon as possible, because
they might "saturate" the highly sensitive
receivers, which means that the received signal
is stronger than the receiver can handle.
However, filtering out a frequency band implies
that these frequencies can never be observed
with the instrument.
A common technique to deal with RFI within
the observed frequency bandwidth, is to employ
RFI detection in software. Such software can
find samples in time, frequency or time-frequence
space that are contaminated by an interfering
source. These samples are subsequently ignored
in further analysis of the observed data.
This process is often referred to as 'data
flagging'. Because most transmitters have
a small bandwidth and are not continuously
present radio devices), most of the data remains
available for the astronomical analysis. However,
data flagging can not solve issues with continuous
broad-band transmitters, such as windmills,
digital video or digital audio transmitters.
See also
Electromagnetic radiation
Faraday cage
Radio receiver
Signal noise
Twisted pair
Interference
References
External links
ARRL, RFI
Interference Handbook
EMC Design Fundamentals
Clemson's EMC Page
EMC Tutorials
