Patent Publication Number: US-5298680-A

Title: Dual twisted pairs over single jacket

Description:
BACKGROUND OF THE INVENTION 
     This application relates in general to electronic cables utilizing twisted pair technology for the transmission of balanced or unbalanced signals. 
     A twisted pair arrangement consists of two insulated conductors twisted about each other to form a two conductor group. When more than one twisted pair group is bunched or cabled together, it is referred to as a multi-pair cable. A problem encountered when using a multi-paired cable is that data transmitted on one pair can often interfere with the transmission of data on another pair within the multi-pair cable. This is due to the fact that all insulated conductors or pairs carrying data in the form of current will radiate an electric and magnetic field. This is commonly referred to as electro-magnetic noise interference, or EMI. The further away one wire is from another wire generating this interference, the less susceptibility there is to its effects. 
     One prior art method of distancing elements, in this case twisted pairs, is discussed in U.S. Pat. No. 4,873,393, assigned to AT&amp;T. In &#39;393 individual pairs have different nonsequentional laylengths. Laylength is a term referring to the axial distance required for one conductor to complete one revolution about the axis of another conductor. It is known that when adjacent twisted pairs employ the same twist frequency they tend to nest into each other, making each pair more susceptible to the other&#39;s emissions. By varying the laylengths, this nesting effect is reduced. However, problems still arise with this solution. An example illustrating the problem is realized when a twisted pair, whether varied in twist frequency or not, can have as many as six adjacent pairs surrounding it. Therefore, one pair can have up to six noise sources directly surrounding it. 
     SUMMARY OF THE INVENTION 
     The present invention is a cable for electronic transmission of data comprised of twisting together a first and second pair to form a first group, and twisting together a third and fourth pair to form a second group. In another embodiment, the first and second groups are twisted together forming an overall multi-group cable. More than two groups may also be used. 
     Using this technique, the maximum adjacent pair count is one, greatly reducing any one pair&#39;s susceptibility to interfere when compared with those cables whose pairs have multiple adjacent elements. By limiting the number of adjacent pairs to one, hence limiting noise coupling, subsequent data transmission over the pairs within said cable is improved. Additionally, by using the dual twisted pair concept more air is introduced into the cable, lowering the overall dielectric properties of the cable. Air is the lowest dielectric next to a vacuum. This will further lower the cables attenuation which is a favorable result. All of these beneficial effects can also be obtained by pulling the dual twisted pair groups in parallel with each other to form a multi-group cable rather than twisting the groups together. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a perspective view of a twisted pair cable as known in the prior art. 
     FIG. 2 is a cross sectional view along lines 2--2 of the prior art cable shown in FIG. 1. 
     FIG. 3 is a perspective view of a twisted pair cable according to the present invention. 
     FIG. 4 is a cross sectional view of the cable shown in FIG. 3 along lines 4--4. 
    
    
     DETAILED DESCRIPTION OF THE DRAWINGS 
     Referring now to FIG. 1 there is shown a multi-pair data transmission cable designated generally by the numeral 10. Individual conductors 12 are comprised of conducting material 13 and insulating material 14. Conductors 12 are twisted together to form twisted pairs 16. Pairs 16 are enclosed in an insulating jacket 17 to form multi-pair cable 10. Shown in a cross sectional view in FIG. 2, it is seen that many of the individual conductors lie adjacent to other conductors in different pairs, causing interference as discussed above. 
     FIG. 3 shows a multi-media cable 3 according to the present invention. Insulated conductors 12 are the same as conductors in prior art and consist of a conductor 13 in an insulated jacket 14. The conductors 12 are twisted together to form a twisted pair 16. A second twisted pair 18 is combined with first twisted pair 16 to form a twisted pair first group 32. In a similar manner, twisted pair 20 and third twisted pair 22 are twisted together to form a second group 34. Likewise, twisted pair 24 and 26 are twisted together to form twisted pair group 36. These are encased in a jacket 16. 
     In the example shown in FIG. 3, first twisted pair 16 is twisted counter clockwise, and twisted pair 18 is twisted clockwise. These are joined together to form first group 32 by twisting counter clockwise. It is not necessary for the purpose of this invention that twisted pairs 16 and 18 be twisted in opposite directions, but this further reduces EMI. 
     Second group 34 is also comprised of two oppositely twisted pairs 20 and 22. However, the group 34 is twisted in a clockwise direction. It is not necessary for the purpose of this invention that groups 32 and 34 be twisted in opposite directions, but it preferable due to reduction in EMI. 
     While this invention has been illustrated and described in accordance with a preferred embodiment, it is recognized that variations and changes may b made therein without departing from the invention as set forth in the claims.