— 1. Electromagnetic compatibility • apparatus: each finished device, or combination of finished devices, marketed as a functional, independent unit intended for the end user; • electromagnetic emission or disturbance: all electromagnetic phenomena that can alter the way an apparatus operates. Electromagnetic disturbance may be electromagnetic noise, an unwanted signal or a change in the propagation medium itself; • immunity: fitness of an apparatus to function normally without deteriorating in the presence of an electromagnetic disturbance. This means that a system formed by at least an emitter or source device, a receiver and a coupling path must be considered.
• conducted coupling occurs when the source is electrically connected to the receiver, thus by cable or conductor. It can be in the common mode if the disturbance affects both the feeder wires and return circuit, or in the differential mode if the disturbance only affects the supply cabling; • radiated coupling, which occurs through the air, includes: -- inductive coupling, when a current source generates a magnetic field which can pair with a coil of the receiver and produce disturbance by induction; -- capacitive coupling, when there is a source of voltage in the vicinity of the receiver circuit. There is also a direct radiated coupling. In this case, the field radiated by the source travels through the air and couples to the receiver, which acts like an antenna. This occurs when the distance between source and receiver is comparable to or greater than the wavelength of the electromagnetic field.
Figure 5:
EMC system
In practice, if the system is to function correctly, the electromagnetic disturbance emitted by the source must be below the emission limit established by the standards and the receiver must be immune to it, with a sufficient immunity level in relation to the disturbance itself and in any case higher that the standard immunity limit established by the regulations. As regards the coupling path, high frequency emissions can propagate in different ways (fig. 6).
1.2 Conducted coupling This coupling can be of two different types: • in the common mode: the disturbance affects both the supply cabling and return circuit by means of the parassitic capacitance to earth;
Figure 7: common mode
• in the differential mode: the disturbance only affects the supply cabling.
Figure 6: types of emission
Figure 8: differential mode
