Electromagnetic interference

Electromagnetic interference (EMI) or radio frequency interference (RFI) is a term to describe the effect of ambient electromagnetic signals withing a circuit. This can be created through an electromagnetic flux or electromagnetic radiation from an unintended source, either external or from a portion of the circuit that is not directly connected. This tends to cause a fluctuation in voltage and current, causing communication interference, inducing improper part function, and potentially damaging sensitive components.

Sometimes EMI can be intentional such as with radio jamming and other forms of electronic warfare (the interruption of data transmissions).

Technical Information
EMI can be a significant issue in RF circuits. As a result, components in high-frequency devices usually demodulate the input signal to a lower frequency for internal use to help prevent EMI effects on internal components.