Technical Information

EMC- The Basics

According to the EC Directive 89/336/EEC, electromagnetic compatibility is the ability of two or more electronic or electrical systems or units to co-exist without either disturbing the correct operation of the other. If this situation does not exist then one unit is said to be interfering with the other. Measures need to be taken to limit this disturbance and these measures depend upon whether the unit is subject to conducted or radiated electromagnetic disturbance.

Where displays are concerned, interference occurs by the transmission of an interfering signal in the form of an electromagnetic wave and, for the frequencies with which we are usually concerned, is termed radio frequency interference (rfi) or electromagnetic interference (emi). In some cases the level of disturbance can be controlled by adjusting the electronic circuitry.

The methods by which it is possible to protect against the effects of such a disturbance is by placing a shield with low impedance in its path. Where Optical Filters USA is concerned, the shield is an emi shielded window ground to the metal frame. Upon meeting such a shield, the signal will be partly reflected at the surface due to the resistive part of the impedance mis-match (conductivity of the shield) and that which continues through will be reduced by the reactive part of the shield impedance (the magnetic permeability of the material). This magnetic permeability is dominant at lower frequencies (KHz and MHz).

The shielding effectiveness is made up of a reflected component (R) and an absorbed component (A) and can be defined as follows:

S.E. = R + A dB

The attenuation is therefore measured as a ratio of the incident signal and the signal passing through the barrier.

The EC Directive

The EC Directive 89/336/EEC related to Electromagnetic Compatibility and states that "Member States are responsible for providing adequate protection for radio communications and the devices, apparatus or systems whose performance may be degraded by electromagnetic disturbance produced by electrical or electronic apparatus against the degradation caused by such a disturbance."
This directive deals with both emissions and immunity of the apparatus, requiring that it should not degrade the performance of the other unit nor should its operation be degraded by the other unit.

The EC Directive sets out the legal requirements for the apparatus and is mandatory for compliance. Member States are free to establish their own Standards to specify the technical aspects of this requirement.

EN55022:1998
This standard asserts the technical specifications for compliance to the Directive. It is concerned with enforcing the EC Directive in order that no apparatus should emit or be subjected to emissions that will degrade performance.
This standard deals with two classes of Information Technology Equipment:

Class B ITE covers the following categories:

  • Equipment with no fixed place of use, powered by a battery unit.
  • Telecommunication terminal equipment powered by a telecommunication network.
  • Personal computers and auxiliary connected equipment.

This primarily specifies the acceptable disturbance levels for apparatus used in a domestic environment where the use of TV and radio broadcast receivers may be expected within a distance of 10 meters from the display. The tests are carried out in an open field site to simulate the real environment with the receiving antenna at 3 meters from the equipment under test.

Frequency range 
MHz
Quasi-Peak Limits 
dB
30-230 30
230-1000 37

The lower limit shall apply at the transition frequency.

Class A ITE concerns apparatus being used in a more remote environment. The acceptable levels are not so restrictive, although the equipment is required to carry a warning explaining that the apparatus may cause radio interference. For class A the receiver is placed at 30m from the equipment under test and the radiated disturbance levels are as follows:

Frequency range 
MHz
Quasi-Peak Limits
dB
30-230 40
230-1000 47