common mode filter
Common-mode filters are filters that suppress unwanted electromagnetic common-mode signals. They are used in high- frequency technology in transmission and antenna technology for the suppression of superimposed RF interference signals.
Such interference signals, which can propagate as sheath waves on the shielding ofcoaxial cables, occur when the matching between balanced and unbalanced signals is not correct, in terms of impedance and cable matching. Common-mode filters are also used in high-speed interfaces such as the USB interface or the HDMI interface to ensure electromagnetic compatibility between the interface and the peripheral device.
Common-mode filters act like current-compensated chokes. They are based on the transformer principle and consist of a ferrite core with the windings surrounding it. The useful signal is conducted as a push-pull signal and the common-mode interference is removed from the useful signal.
The attenuation range of common-mode filters is designed for specific frequencies, and their characteristics correspond to those of high-pass filters. This means that lower frequencies are attenuated more strongly, while higher frequencies are attenuated less strongly.