Common Mode Chokes

When we said a common mode choke, it means it was wound both line and neutural windings on a single core.

Since the noise would be common mode noise usually, the way to lower (if not eliminate) the unwant noise frequencies is to use a common mode choke

As below picture show us details of Common-Mode Signals Defined:

Such signals can arise from one or more of the following sources: Radiated signals coupled equally to both lines, An offset from signal common created in the driver circuit, A ground differential between the transmitting and receiving locations.