How Do Ferrite Beads Suppress High Frequency Signals?

[likephysics]

I think we have all seen them on usb cables or some other cable. I know they act as a resistor for high frequency signals. How can they attenuate the HF signals when they are not electrically in the circuit (like a RC filter).

 

[berkeman]

They are acting as a 1-turn ferrite core inductor when they are just put over a cable like that. The AC RF current in the conductor generates a magnetic field that circulates around the conductor, and the ferrite ring acts as an inductive and lossy impedance to that field.

The ferrite materials that are used for RF EMI supression beads, baluns and chokes are lossy at RF frequencies. So instead of the impedance versus frequency characteristic climbing inductively up to the LC resonant frequency (with the parasitic capacitance) and then coming back down capacitively, the impedance characteristic climbs inductively, and then flattens out at the loss value (measured in Ohms), where it stays through much of the medium frequency RF range, where EMI is typically a problem (say, 100MHz-300MHz).

Lots more info here: http://fair-rite.com/