EMI is usually classified into two types depending on how it is propagated:

Conducted EMI is noise fed back from a system onto the AC or DC power line or signal lines. This noise is in the frequency range of 150KHz to 30 MHz. It usually has a common mode component and a differential mode component. The common mode component appears as a voltage on both line and neutral leads with respect to ground or earth while the differential mode appears between the line and neutral leads. To suppress conducted EMI, LC networks are usually used.

Radiated EMI comes in the form of electromagnetic waves radiating directly from the circuitry and leads of a system. A common example is the AC power cord of the system which can act as a transmitting antenna for radiated EMI. Ranging from 30 MHz to 1GHz, this type of noise can be effectively suppressed by metal shielding around the source.

Note: The scope of this application note will be limited to the subject of conducted EMI.