On Mar 2, 12:40*am, "N_Cook" wrote:
When such a ferrite cylinder is slid over shielded cable ...

Mostly, these are inductors to suppress common-mode RF
currents, but they are also made lossy (like a resistor
coupled to a secondary winding), so they dampen
ringing.

Even if the cable is already shielded, the shield is
a potential antenna. Damping the antenna current is
sometimes necessary.