Although ferrite beads and sleeves are a common sight on cables, the technique for reducing both outgoing and incoming RF interference is the least understood. To study ferrites, and to do some comparative frequency domain measurements, one needs actual ferrite samples, a specially designed test jig, a spectrum analyzer, and a tracking generator.
Any current flowing through a metal conductor will create a magnetic field around it. The inductance of the conductor transfers the energy between the current and the magnetic field. A straight wire has a self-inductance of about 20 nH per inch. Any magnetically permeable material placed around the conductor helps to increase the flux density for a given field strength, thereby increasing the inductance.
Ferrite is a magnetically permeable material, and the composition of the different oxides making it up control its permeability, which is frequency dependent. The composition is mainly ferric oxide, along with nickel and zinc oxides. Furthermore, the permeability is complex with both real and imaginary parts. Therefore, the line passing through the ferrite has both inductive and resistive components added to the impedance.
The ratio of these components varies with frequency. The resistive part dominates at higher frequencies, and the ferrite behaves as a frequency dependent resistor. Therefore, the assembly shows loss at high frequencies, with the RF energy dissipating in the bulk of the material. At the same time, there are few or no resonances with stray capacitances.
Cables are usually in the form of a conductor pair, carrying signal and return, or power and return. Multi-way cables may carry several such pairs. The equal and opposite return current in each circuit pair usually cancels the magnetic field from the current in the forward line. Therefore, any ferrite sleeve place around a whole cable will have zero effect on the differential mode currents in the cable. This is true as long as the sum of differential-mode currents in the cable is zero.
However, for currents in the cable in common mode, with conductors carrying current in the same direction, the picture is different. Usually, such cables produce ground-referred noise at the point of connection or have an imbalance of the impedance to ground, causing a part of the signal current returning to ground through paths other than through the cable.
For instance, a screened cable, improperly terminated, may carry common-mode currents. As their return paths are essentially uncontrolled, these currents have a great potential for interference, despite being of low levels. Sometimes, the incoming RF currents, although generated in common mode, convert to differential mode and so affect circuit operation. This happens due to differing impedances at the cable interface.
As common mode currents in a cable generate a magnetic field around it, placing a ferrite sleeve around the cable increases the local impedance of the cable and operates between the source and load impedances.
When interfacing cables, low source impedance implies the ferrite sleeve is most effective when adjacent to a capacitive filter to ground. Since the length and layout of a cable will usually vary, engineers take the average value of the cable impedance as 150 ohms.