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For twisted-pair cables, users are most concerned about several indicators that characterize their performance. These indicators include attenuation, near-end crosstalk, impedance characteristics, distributed capacitance, DC resistance, etc.
(1) Attenuation
Attenuation is a measure of signal loss along the link. The attenuation is related to the length of the cable. As the length increases, the signal attenuation also increases. The unit of attenuation is "db", which represents the ratio of the signal strength of the source transmitting end to the receiving end. Since the attenuation varies with frequency, the attenuation at 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 magnitude of FEXT has less influence. Near-end crosstalk (NEXT) loss is a measurement of signal coupling from one pair of wires to another pair of wires in a UTP link. For UTP links, NEXT is a key performance index, and it is also the most difficult index to accurately measure. As the signal frequency increases, its measurement difficulty will increase. NEXT does not mean the crosstalk value generated at the near end point, it just means the crosstalk value measured at the near end point. This value will vary with the length of the cable, the longer the cable, the smaller the value becomes. At the same time, the signal at the transmitting end will be attenuated, and the crosstalk to other wire pairs will be relatively small. Experiments have proved that only the NEXT measured within 40 meters is more real. If the other end is an information socket farther than 40 meters, then it will produce a certain degree of crosstalk, but the tester may not be able to measure this crosstalk value. Therefore, it is best to perform NEXT measurements at both endpoints. The tester is equipped with corresponding equipment so that the NEXT value at both ends can be measured at one end of the link
(3) DC resistance
TSB67 does not have this parameter. The DC loop resistance consumes part of the signal and converts it into heat. It refers to the sum of the resistance of a pair of wires. The DC resistance of the 11801 twisted pair must not be greater than 19.2 ohms. The difference between each pair should not be too large (less than 0.1 ohm), otherwise it means poor contact, and the connection point must be checked
(4) Characteristic impedance
Different from the loop DC resistance, the characteristic impedance includes resistance and the frequency of 1～100MHz inductance impedance and capacitance impedance, which is related to the distance between a pair of wires and the electrical properties of the insulator. Various cables have different characteristic impedances, and twisted-pair cables have 100 ohms, 120 ohms and 150 ohms.
(5) Attenuation crosstalk ratio (ACR)
In some frequency ranges, the proportional relationship between crosstalk and attenuation is another important parameter reflecting cable performance. ACR is sometimes expressed in terms of SNR (Signal-Noice ratio), which is calculated by the difference between the worst attenuation and the NEXT value. A higher value of ACR indicates stronger anti-interference ability. General system requirements are at least greater than 10 decibels
(6) Cable characteristics
The quality of the communication channel is described by its cable characteristics. SNR is a measure of the strength of the data signal taking into account the interference signal. If the SNR is too low, it will cause the receiver to be unable to distinguish between the data signal and the noise signal when the data signal is received, which will eventually cause data errors. Therefore, in order to limit the data error within a certain range, a minimum acceptable SNR must be defined.
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