Pair-to-Pair Crosstalk & Power-Sum Crosstalk

Pair-to-Pair Crosstalk

For both near-end crosstalk and far-end crosstalk, one way of measuring crosstalk is the pair-to-pair method. In pair-to-pair measurement, one pair, the disturber, is energized with a signal, and another pair, the disturbed, is measured to see how much signal transfer occurs. The following six combinations are tested in a four-pair cable:

• Pair 1 to pair 2
• Pair 1 to pair 3
• Pair 1 to pair 4
• Pair 2 to pair 3
• Pair 2 to pair 4
• Pair 3 to pair 4

The test is repeated from the opposite end of the cable, resulting in 12 pair-to-pair combinations tested. The worst combination is what is recorded as the cable's crosstalk value. See Figure 1.

 

Figure 1: Cutaway of a UTP cable, showing pair-to-pair crosstalk

Power-Sum Crosstalk

Power-sum crosstalk also applies to both NEXT and FEXT and must be taken into consideration for cables that will support technologies using more than one wire pair at the same time. When testing power-sum crosstalk, all pairs except one are energized as disturbing pairs, and the remaining pair, the disturbed pair, is measured for transferred signal energy. Figure 2 shows a cutaway of a four-pair cable. Notice that the energy from pairs 2, 3, and 4 can all affect pair 1. The sum of this crosstalk must be within specified limits. Because each pair affects all the other pairs, this measurement will have to be made four separate times, once for each wire pair against the others. Again, testing is done from both ends, raising the number of tested combinations to eight. The worst combination is recorded as the cable's power-sum crosstalk.

 

Figure 2: Power-sum crosstalk