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Introduction
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EMC
is about the ability of different items of electrical equipment to
work together without suffering the effects of interference. Equipment
should also operate without interfering with broadcast and communications
signals and to be immune to normal levels of such signals. EMC implies
that a system will not generate unacceptable levels of conducted or
radiated signals which could cause interference to other well designed
products. Systems should also be designed in such a way that normal
ambient levels of electrical noise will not cause degradation of performance
- they must have an adequate level of immunity. |
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Information
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Interference
Any electromagnetic activity which disturbs the normal operation of
an electrical system can be called interference. The effects may range
from increased background noise in a communication channel or corruption
of digital data, to the destruction of electronic circuits. Interference
sources may arise wherever there is an electrical current. This may
cause direct coupling to other circuits, or radiated fields which
then couple unwanted signals into other circuits. Sources are characterised
by their magnitude, frequency and bandwidth. Where the source is identified,
protection may be possible on a selective basis. Where it is not known
or is intermittent, a more general approach to protection is required.
The higher the frequency of the interference, the more easily it will
radiate. In most EMC standards, energy is assumed to be conducted
from one system to another at frequencies below 30MHz. Above 30MHz,
the transfer mechanism is assumed to be radiation. The problem of
electrical interference is becoming worse with the trend towards smaller
devices operating at higher frequencies. Higher speed switching logic
increases emissions while low operating voltages and currents, with
circuits packaged more closely together, decreases immunity.
Bandwidth and magnitude
In general, unintentional radiating sources are characterised by wide
bandwidth and have, in some cases, extremely large magnitudes. In
addition, equipment may be affected by the proliferation of narrow
band deliberate radiation sources, such as cellular radio and hand
held transmitters. Conducted sources can be of a similar nature to
radiated ones (sometimes a radiated signal induces conducted interference
on cables) and the bandwidth can be extremely wide. High power transients
can be generated, for instance when switches are closed.
Electrostatic discharge
When one insulator slides over another, an electrical charge accumulates
causing high voltages. In dry and well-insulated conditions, a centrally
heated office having a nylon carpet, for instance, a person wearing
rubber soled shoes may retain the charge for some time. Rapid discharge
occurs when the person touches a conductive surface. If that conductive
surface is part of electrical equipment, it will be exposed to a fast
electrical pulse which is capable of delivering a destructive current
to integrated circuits. This also creates a wide spectrum of radiated
interference.
Transients
Lightning is another source of transient interference. A lightning
strike induces a transient waveform on cables and can be conducted
for some distance. Electromechanical switches produce interference
through the combined interactive process of arcing, bouncing and load
circuit oscillations. In inductive circuits, interruptions can lead
to high induced voltages, transients and arcing. Extreme cases can
produce dielectric breakdown. Changes in power loading, both local
and remote, can cause variations in the supply voltage. This is known
as sag and surge, and can result in transients at the time of switching
as well as long term voltage variations. Drop-outs occur when alternative
power sources are switched in and out. If the drop-out duration is
short, machinery may not be affected, but there may be a critical
effect on data processing equipment. Transients of many times the
supply voltage can occur on both public and private supplies, creating
a wide spectrum of interference.
Narrow band sources
Single frequency signals can be described as a narrow band. A sine
wave is a pure tone and has one frequency only. A square wave, such
as that produced by a digital switching circuit, contains more than
one frequency, comprising a fundamental and harmonics. Each harmonic
represents a narrow band source. In digital switching circuitry, the
harmonic frequencies can be very high. A fast digital rise time results
in high magnitudes of frequencies many times the fundamental. If the
rise and fall times of a digital signal are increased (in other words,
the harmonics are reduced), the effects of high frequency interference
sources can be minimised. The amplitude of radiated emissions from
electronic systems increases with frequency. Thus to reduce such emissions
narrow pulses and fast switching should be avoided if possible (easier
said than done, perhaps).
Electromagnetic waves
Radiated emissions are in the form of electromagnetic waves which
can be thought of as an electric field vector (E) and magnetic field
vector (H) which are at right angles to each other and to the direction
of propagation of the wave. If an imaginary plane surface in space
is placed perpendicular to the direction of propagation, there will
be a flow of power through this surface. This is a vector quantity
and is known as the Poynting vector. The instantaneous power through
an area is given by E x H (Ohms per square metre)
Small loop radiation
For a small loop carrying a current, it is easy for a current to flow
but difficult for positive and negative electric charges to accumulate
in different positions so as to provide an electric field between
them. Thus the H field is dominant. The space close to the loop is
dominated by fields which decay at different rates, the H field decaying
faster than the E field. The E/H ratio increases with distance (r)
through a ‘transition’ when r=wavelength/2xPi, the ratio
approaches 377Ohms, the impedance of free space.
Monopole radiation
A monopole antenna acts as a high impedance source, so that the E
field predominates. Close to the source (in relation to wavelength),
this field decays inversely in proportion to distance cubed while
the magnetic field (H) decays in proportion to distance squared. In
the region when r=wavelength/2xPi, the ratio E/H approaches 377Ohms
as in the case of small loop radiation.
Differential and common mode
Differential mode interference is caused by currents flowing in loops
and its magnitude is proportional to the current, loop area and the
frequency squared. Emissions are maximum in the plane of the loop.
Differential mode emissions can therefore be controlled by reducing
frequency, loop area and current flow, and by using different orientations
within circuit layouts.
Common mode interference is caused by current flow in a monopole like
conductor (a long circuit board track or cable, for instance). Its
magnitude is governed by current level, monopole length and frequency.
Reduced emissions can be achieved by minimising currents, track and
cable lengths and frequency. Common mode currents are often caused
by poor grounding and cross coupling.
Ground impedance
The voltage developed between equipment grounds is a function of the
ground plane current and its ground plane impedance. Clearly the ground
plane impedance must be as low as possible to minimise the noise voltage
and hence the resultant noise injected into the electronic circuit.
Limits in the standards
The mechanisms for radiation from equipment are extremely complex
due to the number, nature and interaction of interference sources.
The field equations for elemental sources show that radiation is dominant
beyond a distance of wavelength/2xPi from the source. Today’s
standards therefore assume that any true measurements of emissions
must be made with the antenna at least this distance from the equipment
under test. The magnitude of any specification limits must therefore
reflect the nature and type of the equipment covered. The limits are
also devised to achieve a good balance between the cost of achieving
EMC and the technical complexity involved. |
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