Assuring EMC Compliance in an Avionics Equipment Context

Author : Andrew Lawson, TÜV SÜD

09 August 2023

In the IEC 50 (161) documentation electromagnetic compatibility (EMC) is defined as; ‘The ability of a device, equipment or system to function satisfactorily in its electromagnetic environment without introducing intolerable electromagnetic disturbances to anything in that environment.’

Example sources of electromagnetic interference (EMI) may be of natural origin, such as static and lightning. Alternatively, they can be humanmade. Either way, aerospace systems mandate that they are properly addressed. 

Humanmade EMI can be categorised as being either an intentional transmission or unintentional interference. The sources of unintentional EMI include:
•Various forms of electronics hardware in the vicinity - like switched mode power supplies (SMPS), digital circuitry, etc.  
•Electric power - in relation to either the generation, distribution or conversion of power, with the resulting transients and harmonics causing interference issues.
•Communication signals - such as fixed and mobile broadcast, radar and other navigation aids.
•Electromechanical hardware - like motors, pumps, etc.
•Aircraft ignition systems.

An EMC problem will consist of 3 elements. These are a source, a coupling mechanism and a victim. The source produces intentional or unintentional EMI - electromagnetic fields, currents and voltages. 

There are several potential coupling mechanisms, namely; 
•Conducted - through a direct connection from source to victim.
•Conducted - due to a common impedance (e.g. ground system).
•Near field induced - because of co-located cables which have significant mutual capacitance or inductance.
•Far field radiated - getting into equipment via apertures.
•Far field radiated - being applied onto cables where the coupling can be differential or common mode.

The operation of the victim equipment may be affected by the coupled interference leading to EMI susceptibility.

Avionics platforms and installations will be subject to high power intentional transmitting sources along with other sources of interference. They must therefore be designed to ensure EMC with the humanmade electric, magnetic and electromagnetic environments in which they are to be deployed. Likewise, they must also offer compatibility with natural environment sources.

EMC standards
Manufacturers need to ensure their equipment meets with safety legislation requirements, and EMC standards contribute to ensuring the functional safety of such equipment. However, EMC testing alone does not provide safety assurance - as it would not be practical to test for all conditions. This means it does not consider foreseeable faults, misuse, ageing, production tolerances, environmental factors, testing uncertainty, etc. Therefore, the focus should be on design and risk analysis to identify how EMC susceptibility would cause the product to fail, the consequences if it does fail, plus any design mitigations.

Unfortunately, all avionics electromagnetic phenomena are not covered in a single standard. RTCA DO-160 is an avionics environmental standard and therefore includes a wide range of non-EMC phenomena, as well as EMC test methods and limits for equipment. RTCA DO-357 is the user guide to DO-160. DO-357 provides additional background information for the associated test procedures and requirements of DO-160. It also includes the rationale for requirements, guidance in applying the requirements, commentary, possible troubleshooting techniques and lessons learnt from laboratory experience.

DO-160 - purpose & applicability
RTCA DO-160 Section 1 requires the equipment performance standards to be specified, which define the minimum functional performance of different categories of equipment. The manufacturer can either adopt an RTCA or EuroCAEMOPS or specify their own equipment specification. These equipment performance standards are required during susceptibility testing to ensure that the equipment continues to function correctly.

DO-160 - definition of terms 
RTCA DO-160 Section 2 calls for the severity of limits, known as ‘categories’, to be decided upon. DO-160 lists categories for each test. Note that the category reference is not the same in all tests. Due to the large number of possible categories, they are not listed here but reference should be made to the DO-160 standard and the accompanying DO-357 user guide. The category information can be entered onto the environmental qualification form in the DO-160 appendices.

DO-160 - conditions of tests
RTCA DO-160 Section 3 provides the general test requirements relating to the following:
•Connection and orientation of equipment.
•Order of tests, plus multiple test articles.
•Combining tests.
•Environmental conditions. 
•Test instrumentation.
•Multiple unit equipment.
•Equipment under test (EUT) configuration for susceptibility tests.

This describes how to set up the equipment and the test facility and test instrumentation requirements. Since these apply to all the environmental tests, some will not be applicable to EMC (e.g. combining test conditions).

DO-160 tests
Section 15 to 25 specify the EMC test requirements. These are as follows:
15. Magnetic effect.
16. Power input.
17. Voltage spike.
18. Audio frequency conducted susceptibility - power inputs.
19. Induced signal susceptibility.
20. RF susceptibility - Radiated and conducted.
21. Emission of RF energy.
22. Lightning induced transient susceptibility.
23. Lightning direct effects.
25. Electrostatic discharge (ESD).

Some tests are subdivided into multiple sub-tests which correspond to separate tests in other standards. When writing test plans, it is often easier to consider these sub-tests as tests in their own right - as they have their own test method and limit. Section 19 is a good example as it is comprised of 5 different tests.

EMC general test requirements
A screened room must be used for both RF emission and susceptibility measurements to provide a low level of ambient noise (emissions) and to prevent radiated interference infringing the UK’s 2006 Wireless Telegraphy Act (susceptibility). Some low frequency and conducted tests do not require a screened room. 

A basic screened room has the disadvantage that it is a metal box and will resonate at certain frequencies, thereby increasing the measurement uncertainty (40dB has been quoted). To improve the performance of the screened room, an RF absorber is attached to the walls so as to provide absorption of the RF energy to prevent reflections and resonances. 

Key issues in selecting a facility for test are:
•Adequate size for the EUT - with the antenna being 0.3m minimum from absorber.
•Door size for EUT access.
•Semi-anechoic lined (to meets Def Stan 59-411 Part 3 requirements).
•Penetration panels for test antenna/sensor and drive equipment.
•Ambient levels 6dB below limit.
•Power filtered (levels 6dB below limit).
•Remote monitoring for susceptibility.
•Cleared of all unnecessary items and personnel.

The choice of test facility depends on a whole range of factors - such as accreditation, price, capability, location and good reputation. Choosing an accredited facility is not usually mandatory, but reduces the risk to the client who will otherwise have to satisfy themselves that a non-accredited facility has performed the testing correctly. 

Contents of a test plan
A control plan or procedure may be produced at the start of a project and is normally used for large or complex systems. It demonstrates to a purchaser that a defined EMC strategy is in place with regard to contractual compliance. It provides EMC guidance throughout the project lifecycle. Also, it defines the management organisation, responsibilities, EMC requirements, design approach, test and qualification programme.

The test plan provides all the information needed to enable a test facility to perform the necessary tests. Generally, there is one test plan per equipment/system. The test report is produced by the test facility as a record of the testing activities carried out. This demonstrates how the equipment complies with the test standard against the requirements of the control plan/test plan, and hence contractual requirements. Guidance on the content of these documents is provided in DO-160, which requires the test categories and minimum performance specification to be documented on an environmental qualification form, examples of which are given in DO-160 Annex A. The cost of testing and the timescales involved can become excessive unless good engineering judgment is applied in the form of a technical rationale for the selection of tests.

The generic content of a test plan should include:
•A full description of EUT - power supply, interfaces, cable lengths, size and weight.
•Modes of operation - exercising all functions for emission and susceptibility.
•Test configuration and layout - with a block diagram and layout on the bench/floor, grounding arrangement.
•Drive and support equipment. 
•Test requirements and rationale (covering any tailored requirements).
•Susceptibility performance criteria and the method of monitoring - parameters to be monitored, failure criteria, etc.
•Sweep speed, dwell time and susceptibility modulations.
•Test procedures and limits.

To achieve consistency throughout the phases of EMC testing, it is essential to formalise the details of a test plan for the project. A test plan/procedure should therefore be developed and agreed with the client project manager prior to the commencement of EMC testing. It should be sufficiently detailed to enable any test to be repeated by another approved test house. Without a formalised test plan, the results of the EMC test may vary considerably - due to possible variations in testing arrangements, thus obscuring the effects of any modifications during development of the equipment to the production stage.

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