Electromagnetic Interference can be classified into two broad categories
Narrow band and Broadband
Narrow band interference originates from devices and frequencies used for communications such as radio, television and cell phones. The term narrowband implies that the interference is mostly in one particular frequency or a narrow set of frequencies. This type of interference can be easily filtered out.
Broadband Interference originates from sources such as electric power transmission lines. This radiation is also called unintentional radiation as the radiation is not deliberate by incidental to the power transmission. Broadband Electromagnetic interference occupies a large part of the electromagnetic spectrum. It does not have a limited frequency range like narrowband interference. This kind of EMI is mostly caused by electric phenomena such as corona, switching transients, power line radiation .
Fluorescent lights, brush arcing in motors and contact chattering in relays can also cause broadband interference in electronic devices located close by.
Electromagnetic interference can also be classified into Conducted interference and Radiated interference.
In conducted interference, the interference is transmitted through physical contact (conduction) between the devices. An example may be noise in a television when an electric appliance in the house is switched on.
Radiated interference occurs when the interference radiates out of the conductors into space and couples with other devices. For instance, the radiation from a power line can get coupled to a telephone line running alongside.
Another method of classification of Electromagnetic interference is based on the duration.
Continuous interference and Impulse Noise
Continuous interference comes from sources such as power devices or from natural sources. This interference is of long duration.
Impulse Noise is of short duration. This is caused usually by switching actions.