Ferrite toroids are commonly used on data cables to suppress common mode current in the cable assembly, suppressing electro-magnetic interference or noise.
Ferrite toroids have been threaded onto or placed onto bulk cable and attached as close to the noise source as possible. Congestion at or near the connection interface between the cable and the computer or display or other electronic component sometimes creates the need in the cable for a sharp bend radius where the ferrite toroid would be placed; and other physical spatial requirements such as size and the toroid size and/or shape of adjacent connectors may dictate the location of a toroid several inches from the connector. Cables are efficient radiating elements and frequently are a significant source of electro-magnetic interference (EMI), noise or erratic signals on data lines and tend either to disturb the purity of the data signals or other signals sent over the cables or otherwise to create undesirable radiation of electro-magnetic interference or noise into the surroundings, thus creating interference for other closely located electronic devices such as other computers, radios and television receivers.
A ferrite toroid suppresses the noise by acting as a series impedance element both to the common mode current typically found on the data lines and the cable shield of a data cable without suppressing the differential current on the data lines used to convey the data.
In prior practices, a ferrite toroid typically has been attached by one of various techniques to the bulk portion of the cable and, particularly, to the cable exterior. One technique previously used includes the use of wire ties or adhesive to attach the toroid to the exterior of the cable insulation as well as to confine the toroid within shrink tubing; the tubing is caused to shrink to confine the toroid in a desired location relative to the bulk insulated cable.
An alternative approach has been to remove or lay back the insulation and braid of the cable shield and to place the toroid under the braid of the cable shield and then to re-braid the cable shield. The braid then must be externally re-insulated. This technique has proven to be undesirable due to the cost involved.
Typical cables used for cabling computer components are made up of a plurality of electrical conductors, typically wires made of copper, which are insulated individually and then gathered together in a bundle and surrounded by a metal foil and a braided metal wire shield. The braided wire shield or cable shield sleeve typically is further surrounded or is wrapped around a thin metal foil, forming the cable shield. To complete the formation of the bulk cable, the cable shield and insulated electrical conductors and the foil wrap are encased in a vinyl, rubber or similar polymeric insulating sleeve.
The prior approaches to electronic noise suppression using ferrite toroids placed the ferrite toroids on the exterior of the outer insulating sleeve or cable jacket and then additionally confined their movement by utilizing one of the techniques described above. Placement on the exterior of the insulating outer sleeve, while generally effective, is not as efficient as the placement of the toroid in closer proximity to, or in contact with, the wire braid cable shield; the most efficient EMI suppression dictates that the diameter of the hole in the toroid be as small and as close as possible to the data conductors. An outer insulation sleeve of substantial thickness diminishes the effectiveness of the toroid. The ferrite EMI or noise suppression element is preferably placed as close to the end of each of the electrical conductors as is possible and, similarly, as close to the electrical conductors themselves as possible. The positioning of the ferrite toroid on the exterior of the bulk cable is generally considered at best to be a compromise dictated by cable size and the constraints imposed by the congestion near the cabling interface with the electronic module.