Standard: MODUK - AVP 118: CHAPTER 8


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Many i tems of aircraft electrical and electronic (avionic) equipment incorporate circuits that are sensitive to interference. The degradation of performance of the equipment during the occurrence of interference can be such that the operation of the aircraft, or of particular systems, is severely affected, It is essential, therefore, that the EMC problems likely to occur are taken into account during the design end development stages of ell avionic eystems, early consideration during the outline design stage being of particular importance.

Certain types of evionic equipment incorporate components end circuits that generate interference that could affect the equipment itself, and it is possible in such cases that the control achieved by the application of specifications (such as BS 3G 100 or MIL-STD-461 and 462) will be inedequate to prevent adverse effects. It is therefore necessary to identify these internal sources of interference and to combat the likely consequences by suitable circuit design or by suppression, segregation, bonding, screening, etc.

The design of signal circuits and the choice of signal parameters will critically determine the susceptibility to interference of the equipment and suitable selection of design features will considerably aid in achieving EMC. In certain types of signal circuit it may not be feasible to eliminate completely interference to the signal. In such circuits suitable processing of the signal can be carried out to reject the interference using techniques such as sampling, logic checks, loop checks and system control. These techniques are particularly effective in dealing with interference in digital systems.

Coupling of interference via signal cables and connectors is often the most serious mechanism in a wide variety of systems. Care should therefore be taken in the selection of suitable types of cables and connectors in order to minimise interference coupling. Particular attention should be paid to'the choice of screened cables with adequate performance and special types of cable may be required in some installations. The overall EMC in a system with screened cables (especially coaxial cables) will also depend upon the types of connector used and these must therefore be selected to have a performance compatible with that of the associated cable.

The need to effect an optimal overall design of equipment may reduce the choice of techniques available to achieve EMC. A balance must be made against competing design requirements such as availability, which includes reliability and maintainability, tolerance to environmental effects and mass limitation.

It should be noted that specifications for all Service equipments which may give rise to, or may suffer from EMI, should invoke Defence Standard 59-41 'Electromagnetic Compatibility'. This explains the methods of planning and controlling EMC programmes, emphasising the need for compatibility to be studied from the initial design stages. Methods of testing and the reporting of results are covered in detail.

This chapter includes a comprehensive bibliography dealing with many EMC aspects of avionic systems. A check list of factors which are directly relevant to EMC is provided.

Organization: British Defence Standards
Document Number: avp 118: chapter 8
Publish Date: 1981-01-01
Page Count: 34
Available Languages: EN
DOD Adopted: NO
ANSI Approved: NO
Most Recent Revision: YES
Current Version: YES
Status: Inactive

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