Ferrite rings

Ferrite rings for filtering interference frequencies

Why ferrite rings on coaxial cables?
Many electrical and electronic devices generate high-frequency interference. This interference is often conducted out of the device via connected coaxial cables and can lead to problems in complying with EMC regulations. The entire cable or its shielding acts as an antenna.

Sheath wave filters are used to attenuate this so-called common mode interference.

These consist of ferrite rings that are placed around the cable. Split snap-on ferrites are available for retrofitting; these are also known as hinged ferrites.

Ferrite rings are components made of magnetic ferrite. When such a ferrite is placed around a coaxial cable, an increased inductance is created at this point.
Together with the capacitance already present in cables, a ferrite acts as a PI low-pass filter, filtering out high-frequency interference from the transmitted signal.

The effect of the low-pass filter depends on the number of windings around the ferrite and the capacitance of the cable. For example, thinner coaxial cables can be wound around the ferrite several times to achieve a higher attenuation of the interference signal. However, it should be noted that the cable must not be attenuated too much in order not to jeopardise the transmission of the useful signal! A compromise must be found in order to reduce the interference emission and transmit the useful signal without loss.

Removing ferrites from coaxial cables does not impair the transmission of the signal, but can lead to problems in complying with EMC standards and to interference with neighbouring devices.