EMC Design Techniques
- 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 pieces of electronics 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.
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 has 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
- 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 can 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.
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. 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.
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.
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. Sometimes 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.
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.
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.
By Ian Poole
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