Common mode electromagnetic noise is conducted simultaneously on all lines (supply and ground) in the same direction. But it is also coupling back to ground. Common mode chokes help to capture this unwanted noise and prevent it from being transmitted and coupling back to ground. High impedance at the unwanted frequencies helps to eliminate the unwanted noise. This is done by capturing the line and neutral signal currents and burning it off as heat through a common device such as a magnetic core.

In a common mode choke, both line and neutral windings are wound on a single core. Designing filters with common mode chokes is an economic way to combine low losses, low voltage drop, minimal signal impact with efficient EMC/EMI filtering.