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Path to Electromagnetic compatibility (EMC)--part 1



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  • 1 Introduction
    • 1.1 Sources
    • 1.2 Coupling mechanisms
    • 1.3 Equipment sensitivity
  • 2 Simple source models
  • 3 Signal waveforms and spectra
  • 4 EMC limits and test levels
    • 4.1 Emissions
    • 4.1.1 Conducted emissions
    • 4.1.2 Radiated emissions
    • 4.1.3 Power frequency harmonics
    • 4.2 Immunity
    • 4.2.1 Electrostatic Discharge (ESD)
    • 4.2.2 Electrochemical fast transients
    • 4.2.3 Radio Frequency (RF) fields
    • 4.2.4 Dips, surges and voltage interruptions
    • 4.2.5 Magnetic fields
  • 5 Design for EMC
    • 5.1 Basic concepts
    • 5.2 Shielding
    • 5.3 Cable screens termination
    • 5.4 PCB design and layout
    • 5.5 Grounding
    • 5.6 Systems and installations
  • 6 EMC Measurements
    • 6.1 Emissions
    • 6.2 Immunity
  • 7 EMC Standards
    • 7.1 Generic standards
    • 7.2 Important product standards
    • 7.3 Basic standards

Introduction

Electromagnetic Compatibility (EMC) is achieved when co-located equipment and systems operate satisfactorily, without malfunction, in the presence of electromagnetic disturbances. For example, the electrical noise generated by motor-driven household appliances, if not properly controlled, is capable of causing interference to domestic radio and TV broadcast reception. Equally, microprocessor-based electronic control systems need to be designed to be immune to the electromagnetic fields from hand- held radio communication transmitters, if the system is to be reliable in service. The issues covered by EMC are quality of life, spectrum utilization, and operational reliability, through to safety of life, where safety-related systems are involved.



The electromagnetic environment in which a system is intended to operate may comprise a large number of different disturbance types, emanating from a wide range of sources including: mains transients due to switching e radio frequency fields due to fixed, portable and mobile radio transmitters electrostatic discharges from human body charging powerline surges, dips and interruption power frequency magnetic fields from power lines and transformers In addition to having adequate immunity to all these disturbances, equipment and systems should not adversely add electromagnetic energy to the environment above the level that would permit interference-free radio communication and reception.

Sources:

The essence of all EMC situations is contained in the simple source-path-receptor model.

___ Source, path, receptor model.

Sources comprise electromechanical switches, commutator motors, power semiconductor devices, digital logic circuits and intentional radio frequency generators.

A The electromagnetic disturbances they create can be propagated via the path to the receptor such as a radio receiver, which contains a semiconductor device capable of responding to the disturbance, and causing an unwanted response, i.e. interference.

For many equipments and systems, EMC requirements now form part of the overall technical performance specification. The EU's EMC Directive, 89/336/EEC, was published in 1992 and came into full implementation on 1 January 1996. All apparatus placed on the market or taken into service must, by law, comply with the Directive's essential requirements, that is it must be immune to electromagnetic disturbance representative of the intended environment and must generate its own disturbance at no greater than a set level that will permit interference-free radio communication. The Directive refers to relevant standards which themselves define the appropriate immunity levels and emission limits. More information on the EMC Directive and its ramifications is available in the reference.

Coupling mechanisms:

The path by which electromagnetic disturbance propagates from source to receptor comprises one or more of the following:

conduction; capacitive or inductive coupling; radiation

These paths are outlined.

Capacitive and inductive coupling path, I I

___ Coupling mechanisms for electromagnetic disturbance -- Radiation path

Coupling by conduction can occur where there is a galvanic link between the two circuits, and dominates at low frequencies where the conductor impedances are low.

Capacitance and inductive coupling takes place usually between reasonably long co- located parallel cable runs. Radiation dominates where conductor dimensions are comparable with a wavelength at the frequency of interest, and- efficient radiation occurs.

For example, with a personal computer, the radiation path is more important for both emission and immunity at frequencies above 30 MHz, where total cable lengths are of the order of several meters. Designers and installers of electrical and electronic equipment need to be aware that all three coupling methods exist so that the equipment can be properly configured for compatibility.

Equipment sensitivity:

Analogue circuits may respond adversely to unwanted signals in the order of millivolts.

Digital circuits may require only a few 100 millivolts of disturbance to change state.

Given the high levels of transient disturbance present in the environment, which may be in the order of several kilovolts, good design is essential for compatibility to occur.

Next: EMC Simple source models; Signal waveforms + spectra

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Updated: Saturday, 2013-03-30 15:45 PST