Common mode noise suppression

You probably need something like this

or this

martin

The cables probably provide a return path for parasitic currents that follow the basic capacitive current relation:

I =3D C dv/dt

Your common mode ferrite probably forces the parasitic currents to take some other less harmful path. More effective remedies usually involve reducing dv/dt, reducing capacitance, or shunting parasitic currents through low impedance structures, i.e., shields.

There is lots of dv/dt associated with your clocked circuitry and/or any switchmode power supplies. You'd like to have such currents flow in as short a path as possible, well away from any sensitive 'victim' circuits. One way to accomplish this is to add an additional shield around the radiating circuitry, connecting that shield to the local ground. Another remedy is to reduce dv/dt by slowing down rise and fall times and eliminating ringing.

Paul Mathews

Use a twisted pair balanced line driver system to start with.. also, use a shielded cable with grounded shield at one end only..