The basics of the common mode choke
The impedance of an inductor is proportional to its inductance and the frequency of the signal passing through it. Inductors, therefore, can act like low-pass filters, that is they allow low frequency signals to pass through them, while they block higher frequency signals. A common mode choke is a special type of filter with two inductor windings on the same core. In order to understand common mode chokes, it is important to first understand what common mode means.