Cable extension for reducing EMI emissions

1. An apparatus for reducing cable emissions comprising:

a conductive cable core that extends longitudinally from a first end to a second end;

a filter coupled to and encasing the conductive cable core, the filter extending longitudinally only a portion of the conductive cable core length and positioned between the first end and the second end; and

a cable shield coupled to and encasing the conductive cable core and the filter, the cable shield extending longitudinally from the first end over the filter and terminating prior to the second end.

2. An apparatus according to claim 1 further comprising:

a shielded connector coupled to the first end of the conductive cable, the shielded connector having a shield electrically coupled to the cable shield.

3. An apparatus according to claim 1 further comprising:

a connector coupled to the second end of the conductive cable, the second end connector being electrically isolated from the cable shield.

4. An apparatus according to claim 1 further comprising:

an insulative coating coupled to the cable shield and encasing the cable shield, the filter, and the conductive cable core.

5. An apparatus according to claim 1 wherein:

the filter is a rigid-walled ferrite filter.

6. An apparatus according to claim 1 wherein:

the filter is a flexible structure formed of a ferrite-filled elastomeric matrix.

7. An apparatus according to claim 1 wherein:

the length of the conductive cable core and the distance from the first end to the filter arc sufficient to reduce EMI emissions to a predetermined standard.

8. An apparatus according to claim 1 wherein:

the conductive cable core is approximately five feet or longer and the distance from the first end to the filter is in a range from one inch to four meters.

9. A cable extension comprising:

a partially shielded cable comprising interior conductive elements and cable shielding overlying the interior conductive elements, the partially shielded cable extending longitudinally from a proximal end to a distal end, the cable shielding beginning at the proximal end and extending only part of the longitudinal cable length;

a ferrite filter coupled between the interior conductive elements and the cable shielding and longitudinally positioned between the proximal end and the distal end;

a shielded connector coupled to the proximal end and having a connector shield electrically coupled to the cable shielding; and

a second connector coupled to the distal end and electrically isolated from the cable shield.

10. A cable extension according to claim 9 wherein:

the interior conductive elements comprise a plurality of twisted data cable pairs.

11. A cable extension according to claim 9 further comprising:

an insulative coating coupled to and encasing the partially shielded cable.

12. A cable extension according to claim 9 wherein:

the ferrite filter is a rigid-walled ferrite filter.

13. A cable extension according to claim 9 wherein:

the ferrite filter is a flexible structure formed of a ferrite-filled elastomeric matrix.

14. A cable extension according to claim 9 wherein:

the length of the partially shielded cable and the distance from the proximal end to the ferrite filter are sufficient to reduce EMI emissions to a predetermined standard.

15. A method of manufacturing a cable extension comprising:

coupling a ferrite filter to an unshielded portion of a cable that is partially or completely unshielded, the cable extending longitudinally from a proximal end to a distal end, the ferrite filter being coupled at a selected position between the proximal end and the distal end;

encasing at least the ferrite filter in a cable shield, the cable shield combining with any shielding of the cable to form a substantially continuous cable shielding from the proximal end, overlying the ferrite filter to a position prior to the distal end;

coupling a shielded connector to the proximal end of the table,

coupling a shield of the shielded connector to the ethic shield at the proximal end; and

coupling a second connector to the distal end of the cable the second connector being left electrically uncoupled to the cable shield.

16. A manufacturing method according to claim 15 further comprising:

supplying an unshielded cable;

coupling the ferrite filter to the selected position on the unshielded cable;

covering the unshielded cable and the ferrite filter with the cable shield from the proximal end to the position prior to the distal end.

17. A manufacturing method according to claim 15 further comprising:

supplying a shielded cable;

removing shielding from the shielded cable as the selected position and at the position prior to the distal end;

coupling the ferrite filter to the selected position on the unshielded cable;

covering the ferrite filter with the cable shield; and

reconnecting the cable shield covering the ferrite filter to the cable shield on the cable adjacent the ferrite filter.

18. A manufacturing method according to claim 15 further comprising:

supplying a partially shielded cable with shielding removed at the selected position and at the position prior to the distal end;

coupling the ferrite filter to the selected position on the unshielded cable;

covering the ferrite filter with the cable shield; and

reconnecting the cable shield covering the ferrite filter to the cable shield on the cable adjacent the ferrite filter.

19. A manufacturing method according to claim 15 further comprising:

selecting the cable length and the position of the ferrite filter to reduce EMI emissions to a predetermined standard.

20. A cable extension comprising:

a cable extending longitudinally from a proximal end to a distal end and comprising a plurality of interior conductive elements;

means coupled to the interior conductive elements and positioned at a selected location along the cable for increasing common mode impedance of a noise signal;

means overlying the common mode impedance increasing means the cable and extending from the proximal end to a position prior to the distal end for containing the noise signal;

means electrically coupled to the containing means for coupling the cable proximal end in proximity to a communication device; and

means coupled to the cable distal end for coupling.