Emissions and immunity
Any EM emission, natural or 'man-made', is potentially a disturbance to any other susceptible device in the environment. It may either put it out of action or, in many cases, cause it to malfunction. So there are two sides to the EMC equation:
An aircraft system may be disturbed by a
laptop or cell phone used on board.
- source equipment whose controllable emissions must be limited; and
- equipment that needs to have adequate immunity to those disturbances in its environment to which it is exposed.
Typical sources include, for example, power lines, electronic circuits, electric motors, radio and radar transmitters. Equipment that is disturbed, is often called 'susceptible' equipment by EMC specialists. It can include virtually anything that uses or can detect EM energy, such as radio receivers, domestic appliances or electronic circuits of any kind.
The last-mentioned are clearly of increasing concern as they come in a vast and growing array of applications from the smallest hand-held device or modem to the control circuits at an electricity generating plant serving an entire region.
EM disturbances present, to the extent of generating an interference, may work in more than one direction, disrupting more than one device, or multiple sources may have a cumulative effect on a single piece of equipment. Thus an air traffic control radar may affect the display of a laptop computer being used in an aircraft as well as other vital devices in use on the ground. At the same time, emissions from the same laptop computer may combine with those from a mobile phone to disturb systems in the aircraft.
On the emissions side of the equation, therefore, the aim of EMC is to ensure that equipment does not disturb other equipment, radio services, power or other networks. On the immunity side, the aim is to ensure that equipment is not affected by, for example, radio transmissions, mains-borne disturbances, electrostatic fields and other phenomena.