The common mode choke is a device often used when attempting to reduce the EMI (Electromagnetic Interference) produced in an electronic circuit. Many modern systems carry signals in differential mode; that is, the signals are transmitted along two wires where one carries the signal from its source and the other carries the signal's return (which is generally at ground potential). The distinguishing characteristic of a differential signal is that its two signal components travel in opposite directions along parallel wires, which may be comprised of (for example) a twisted pair or coaxial cable.

Often, EMI problems are caused by common mode current. Common mode current is defined as those signal components which flow in the same direction along the two wires in a differential system. All cables and wires have the capactity to act as antennas. Noise from external sources (radio stations, power supplies, nearby unrelated electronic equipment) can induce a current in, say, the outer conductor of a piece of coax. If the outer conductor is tied to ground, as is often the case, an induced current might begin to flow away from ground potential along the line designated for the signal return. This can result in "mixing" with the signal itself, which distorts the signal.

The most basic common mode choke consists of two windings, or coils, with a common core. The schematic symbol for the common-mode choke looks like two parallel inductors:

```
o------UUUU------o
* ----

* ----
o------UUUU------o

```

Such an arrangement will allow differential signals to pass unimpeded because of the principles of inductance and magnetic flux. (Quit yawning!) In our common mode choke, the two coils of wire share the same core. When current flows in either of the coils, magnetic flux is induced in the core. Since the two components of a differential signal are equal in magnitude but opposite in direction, there will be two flux components induced in the core which will cancel one another out. Any common-mode signals put through the coils will introduce flux to the core that adds, rather than cancels, resulting in impedance of the signal.

References:

http://www.bravoelectro.com
http://www.k6bj.org