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EMI/EMCa primer

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Intersystem EMI

Intersystem compatibility concerns the compatible operation of the equipment with all

other systems with which it must operate or interact with, including its environment.

For example, in a military context, it may imply that restrictions are placed upon how

close an aircraft may fly to a ground based transmitter.

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Intrasystem EMI

Intrasystem compatibility refers to the compatibility among each of the subsystems that

are required to operate as a whole.

Each subsystem must not interfere with any other, and each must be free of interference.

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EMC test activities

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Emission and susceptibility

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Shielding

Filtering

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Any electromagnetic disturbance that interrupts, obstructs, or otherwise degrades the

equipment performances.

It can be induced unintentionally, as a result of spurious emissions, vulnerabilities, etc.

It can also be induced intentionally, as in some forms of electronic warfare

Electromagnetic Interference

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Integration, and increasing complexity make electronic systems and components more

vulnerable to electrical influences.

New silicon chips tend to be more susceptible to EM disturbance than the

devices they replace.

Miniaturisation and proliferation of electronic/ electrical equipment (wired and

wireless) bring the sources and victims of disturbance closer together, which increases

the interference potential

Trends

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NOISE AND INTERFERENCE

Noise is any electrical signal present in a circuit other than the desired signal.

This definition excludes the distortion products produced in a circuit due to

nonlinearities.

Although these distortion products may be undesirable, they are not considered noise

unless they are coupled into another part of the circuit.

Interference is the undesirable effect of noise.

If a noise voltage causes improper operation of a circuit, it is interference.

Noise cannot be eliminated, but interference can.

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We may split the transfer of electromagnetic energy (with regard to the prevention of

interference) into four subgroups:

radiated emissions

radiated susceptibility

conducted emissions

conducted susceptibility

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Overview of the electromagnetic interference problem

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Three conditions are simultaneously required: the presence of a source of perturbations

(perturbing or aggressor system), the presence of a perturbed system (victim), and at least

one coupling path to transmit the noise from source to victim.

It is important to know how the aggressor is coupled to the victim, since in many practical

situations, reducing the coupling coefficient is the only way to fight interference.

Differential Mode Coupling - an EM field is said

to be coupled in differential mode if the induced

currents flow in opposite directions.

The perturbing currents Ipare induced in the

wires connecting two circuits belonging to the

system. Z1and Z2have no influence on Ip.

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Common-Mode Coupling - a perturbing field is

coupled in common mode if the resulting currents

(apart from the return) all flow in the same

direction.

The vulnerability area includes the ground plane;

consequently, Z1, Z2affect both the amplitude and

the spectrum of the induced currents.

Antenna-Mode Coupling - In this case,

circuits 1 and 2, as well as the connecting

wires (including the ground plane), all act

as receiving antennas with respect to theperturbing field.

The currents carried by the

interconnecting wires and the ground

plane all flow in the same direction.

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Two main situations are encountered:

1) Both aggressor and victim belong to the same system or facility. In this case, the

interference is inside the system of interest.

2) The source of perturbation belongs to one system and the victim to another (for

example, the aggressor is a mainframe computer and the victim is a radio receiver). The

interference then appears between two distinct systems.

In some situations, the same equipment is both aggressor and its own victim.

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To reduce interference, there are three possible approaches:

reduce the emission of perturbations at the source,

reduce the electromagnetic susceptibility of the victim,

reduce the coupling between aggressor and victim.

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Electromagnetic Compatibility (EMC) - The capability of electronic systems, equipment

or devices to operate in their intended electromagnetic environment without suffering or

causing unacceptable degradation of performance as a result of electromagnetic

interference.

Note that EMC has two aspects: the emission of perturbations and the susceptibility to

perturbations.

Electromagnetic Susceptibility - A relative measure of a device or systems inability to

perform without degradation in the presence of an electromagnetic perturbation

- Lack of electromagnetic immunity.

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Electromagnetic Immunity - A relative measure of a device or systems ability to

withstand EMI exposure while maintaining a predefined performance level.

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A system is electromagnetically compatible with its environment if it satisfies 3 criteria:

1. It does not cause interference with other systems.

2. It is not susceptible to emissions from other systems.

3. It does not cause interference with itself.

There are three ways to prevent interference:

1. Suppress the emission at its source.

2. Make the coupling path as inefficient as possible.

3. Make the receptor less susceptible to the emission.

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ELECTRICAL DIMENSIONS AND WAVES

Physical dimensions of a radiating structure such as an antenna are not important, per se,

in determining the ability of that structure to radiate electromagnetic energy.

Electrical dimensions of the structure in wavelengths are more significant in determining

this.

Electrical dimensions are measured in wavelengths.

When a circuit is electrically small ( < /10), network theory and lumped circuit models

suffice.

Otherwise, EM theory & distributed circuit models should be used

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