Twisted pair performance index

- Jan 31, 2019-

For twisted pairs, the user is most concerned with several indicators that characterize their performance. These indicators include attenuation, near-end crosstalk, impedance characteristics, distributed capacitance, and DC resistance.


Attenuation is a measure of signal loss along the link. Attenuation is related to the length of the cable, and as the length increases, the signal attenuation increases. The attenuation is expressed in units of "db", which represents the ratio of the signal strength of the source transmitter to the signal strength of the receiver. Since the attenuation varies with frequency, the attenuation over all frequencies within the application range should be measured.

2. Near-end crosstalk

Crosstalk is divided into near-end crosstalk and far-end crosstalk (FEXT). The tester mainly measures NEXT. Because of the line loss, the influence of the magnitude of FEXT is small. Near-end crosstalk (NEXT) loss is the measurement of signal coupling from one pair of wires to another in an unshielded twisted pair link. For unshielded twisted pair links, NEXT is a key performance indicator and one of the most difficult to measure accurately. As the frequency of the signal increases, the measurement difficulty will increase. NEXT does not represent the crosstalk value produced at the near endpoint, it simply represents the crosstalk value measured at the near endpoint. This magnitude will vary with cable length, and the longer the cable, the smaller its value becomes. At the same time, the signal at the transmitting end is also attenuated, and the crosstalk to other pairs is relatively small. Experiments have shown that only the NEXT measured within 40 meters is more realistic. If the other end is an information outlet that is more than 40 meters, it will generate a certain degree of crosstalk, but the tester may not be able to measure the crosstalk value. Therefore, it is best to perform NEXT measurements at both endpoints. Most testers are equipped with appropriate equipment so that the NEXT values at both ends can be measured at one end of the link.