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a tutorial or summary about the basics of the ways to design for EMC, i.e.
design techniques for EMC so that electromagnetic compatibility, EMC emissions
and susceptibility are minimised.
Any electronic circuit that has signals that change in levels
will tend to radiate some power as any interconnections, and wires will act as
radiating antennas, however short they may be. Similarly circuits will tend to
pick up radiated signals from other transmitters whether these sources are
transmitting intentionally or not. These Electromagnetic Interference, EMI,
problems can prevent adjacent piece of equipment working alongside one another.
With the vast growth in the usage of electronic equipment, this problem of
Electromagnetic Compatibility, EMC has become a particularly important topic. As
a result it is necessary to design for EMC from the outset of a new electronics
development project and implement the various design techniques for EMC into the
whole concept of the product. Only by taking account of the design for EMC
aspects at the concept stages of a development, can any precautions be
implemented correctly.
In years gone by transmitters might prevent local domestic
televisions from displaying their picture. In the worst case the whole picture
could disappear, or there may be some patterning of the picture. With these and
many other examples of the results of poor EMC regulation becoming more
widespread, it became necessary to improve matters. Now with modern electronic
equipment it is possible to operate mobile phones and other wireless devices
near almost any electronics equipment with little or no effect. This has come
about by ensuring that equipment does not radiate unwanted emissions, and also
making equipment less vulnerable to radio frequency radiation. In this way,
these aspects of design for EMC have paid major dividends in today's world where
thee is a huge amount of electronic equipment being used.
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Note on EMC testing:
With the importance of Electromagnetic Compatibility,
EMC, it is now a requirement in most countries that all electronic
products to undergo EMC testing and pass an EMC test to show its
compliance with the relevant EMC standards for that country. EMC testing
addresses elements including radiated and conducted emissions, radiated
and conducted susceptibility, transient immunity, surge immunity, and
ESD immunity. It is usually necessary to have a certificate of EMC
compliance, demonstrating that the relevant EMC tests have been passed,
before a product can be sold.
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Design for EMC compliance
When designing an electronic circuit card it is necessary to
take a number of precautions to ensure that its EMC performance requirements can
be met. Trying to fix the EMC performance once the circuit ahs been designed and
built will be far more difficult and costly. Accordingly there are a number of
areas that can be address during the design to ensure that the EMC performance
is optimised:
- Circuit design for minimum radiation
- EMC filters
- Circuit partitioning
- Grounding
- Screened enclosure
- Screened lines and cables
By adopting these precautions, the EMC performance of the
circuit can be greatly enhanced. However it will still need to undergo EMC
testing to ensure that it meets the required performance.
Circuit design for minimum radiation
One of the chief areas that needs to be borne in mind for EMC
/ EMI compliance is the RF radiated emissions arising from connecting cables and
the susceptibility to receiving interference. It is found that they form the
major coupling path for interference in any product. Often these cables need to
carry high frequency signals, possible data, and this can present some
challenges in terms of improving their EMC / EMI performance.
Any cable will receive and radiate signals, especially when
it approaches a quarter wavelength, or odd multiple thereof because it forms a
resonant circuit. However even when the cable does approach these lengths,
electromagnetic compatibility, EMC can be a problem.
One solution is to filter the cables entering and leaving the
unit. While this does reduce the level of EMI, it may also degrade the
performance of the circuit. If high speed data needs to be carried, then any
sharp edges will be removed by the filters, and in the worst case, the signal
may be attenuated to such a degree that the system does not work. Thus a careful
balance may need to be made for the filter between the equipment performance and
the electromagnetic compatibility, EMC requirements.
In these circumstances the signals can be carried in a
differential format. The signal cables can then be constructed as a twisted
pair, and could even be screened. In this way the high frequency signal cane be
carried, but its susceptibility to radiation and reception is reduced, because
anything received will appear on both lines and cancelled out. Additionally
radiation does not occur for the same reason.
EMC Filters
The possibility of introducing EMC filters has already been
mentioned. It can form a useful tool for the EMC engineer to use in many
instances. A EMC filters are particularly useful for lines that only carry low
frequency signals. Power input cables, or other lines that carry status voltages
are particularly good candidates for filtering. Here EMC filters can remove any
high frequency components, leaving the low frequency elements on the line that
will not radiate much.
EMC filters should be placed at the entry point to the unit,
and should be tightly bonded to the chassis. In this way no signals can enter
the unit and radiate into it prior to being removed by the filter.
Circuit partitioning
This element of the circuit design is important to ensure
that the circuit can pass its EMC test. It must be accomplished at the very
earliest stages of the design in view of the fact that it governs the whole
topology of the circuit and the mechanical construction.
The first stage of the partitioning process is to segregate
the circuit into EMC critical and non-critical areas. The electromagnetic
compatibility, EMC critical areas are those areas which contain sources of
radiation, or may be susceptible to radiation. These areas may include circuits
containing high frequency circuitry, low level analogue circuits and high speed
logic including microprocessor circuits.
The Non-critical EMC areas are those which contain areas that
are unlikely to radiate signals or be susceptible to radiation. Circuits
including linear power supplies (not switch mode power supplies), slow speed
circuits and the like.
Once this action has been completed, the layout for the
design can be undertaken. The critical or sensitive regions can be screened or
and filters added as necessary at the interfaces to prevent EMI being radiated,
or to protect these circuits from the effects of EMI.
By isolating the EMC critical areas, it is possible to add
the relevant measures both at the initial stages of the design, or possibly
later. Having an interface provides the possibility for optimising the overall
performance to meet its EMC test. This may result in the addition of further
filtering, screening, etc, or it may even enable cost reductions to be made if
some of the measures are not required.
Grounding
The grounding scheme within a unit is of particular
importance for its EMC performance. Poor grounding can lead to earth loops that
can in turn lead to signals being radiated, or picked up within the unit and
hence poor electromagnetic compatibility, EMC performance results.
To help ensure that the earth or grounding system works
satisfactorily, it is worth bearing in mind its function. It can be said to be a
path that enables a current to return to its source. It should obviously have a
low impedance, and it should also be direct. Any loops, or deviations may give
rise to spurious effects that can give rise to EMC problems.
Planning earth or grounding systems is not trivial. It is
more challenging than it appears, but essential for a good EMC performance.
Lengths must be kept to a minimum because above frequencies more than only a few
kilohertz the impedance is dominated by inductance, and lengths of a few
centimetres make a significant difference, even at low frequencies.
To overcome these effects, thick wires should be used if
possible, and on printed circuit boards ground planes must be used. Critical
tracks must be run above the ground plane, and they should be routed so that
they do not encounter any breaks in the ground plane. Some times it is necessary
to have a slot or break in a ground plane, and if this occurs a critical track
must be routed over the plane, even if it makes it slightly longer.
These and other approaches can be adopted to ensure that the
grounding system is able to reduce the EMC problems to a minimum. Considerable
thought should be given to the grounding, as it may not be easy to change at a
later time.
Screened enclosure
Although screened enclosures may not be an option that is
preferred from a cost viewpoint, placing the unit in a conductive enclosure that
is grounded will significantly improve the performance. All filtering can then
be undertaken at this interface and the conductive wall will provide a barrier
to radiation, thereby improving both the emissions and susceptibility elements
of the EMC performance.
Where cost and possibly aesthetics are important it is
possible to spray the inside of cabinets with conductive paint, although the
level of screening provided will not be nearly as good as if a fully conductive
metal case is used. Where high levels of EMC performance are required care
should be taken to choose a case where the continuity of the screen is not
breached. The case should ideally be made of as few elements as possible. At
each joint there will be the possibility of radiation passing through. Where
joints to occur they should be as tight as possible and they should have good
continuity between them.
Some metal cases using a prefabricated style of construction
with anodised aluminium panels do not offer good EMC performance, although they
are aesthetically more pleasing than some RF tight cases. A balance has to be
made dependent upon the performance required and the EMC tests that need to be
undertaken.
Screened lines and cables
When lines and cables need to pass into or out of a unit, the
cables can be screened to prevent any radiation of the signals being carried or
pick up of external signals. However when screened cables are needed for
electromagnetic compatibility EMC applications, the screen must be bonded tot he
equipment signal ground as soon as it enters the unit, otherwise unwanted
signals may be radiated or picked up and this would compromise the EMC
compliance.
Summary
The electromagnetic compatibility, EMC performance is of
electronic equipment today is a great importance and as a result it is necessary
to design for EMC. In order to enable the unit to pass its EMC testing and be
placed on the market, it is necessary for it to conform to the directives and
regulations in force. For a unit to be successful, it is necessary for it to be
designed to provide a high level of electromagnetic compatibility, EMC
performance and reduction of EMI.
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