Which of the following can cause shielded cables to radiate or receive interference?
Why A shielded cable only stays quiet when the currents in the center conductor and the return are equal and opposite, so their fields cancel. Common-mode current is current flowing in the same direction on the shield and inner conductors, usually returning through the ground system or through the air, and that unbalanced current turns the cable jacket into an antenna that both radiates and picks up signals. Ferrite chokes or common-mode chokes on the cable raise the impedance to that common-mode path and are the standard cure.
Watch out The choice about differential-mode current names the normal, balanced condition where fields cancel, which is exactly what you want, not a cause of interference. Braided shielding and low-inductance grounding at both ends generally improve shielding rather than degrade it.
Differential mode = fields cancel, quiet. Common mode = the cable becomes an antenna. Fix it with a ferrite choke.