For twisted pair, users are most concerned about several indicators that characterize its 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 a link. The attenuation is related to the length of the cable, and as the length increases, the signal attenuation also increases. Attenuation uses "db" as the unit, and represents the ratio of the signal strength of the signal at the source transmitting end to the signal strength at the receiving end. Since attenuation varies with frequency, attenuation should be measured at all frequencies within the application range.
(2) Near-end crosstalk
Crosstalk is divided into near-end crosstalk and far-end crosstalk (FEXT). The tester mainly measures NEXT. Due to the existence of line loss, the influence of the magnitude of FEXT is small. Near-end crosstalk (NEXT) loss is a measure of signal coupling from one pair of wires to another in a UTP link. For UTP links, NEXT is a key performance indicator, and it is also one of the most difficult to measure accurately. As the signal frequency increases, its measurement difficulty will increase. NEXT does not indicate the crosstalk value produced at the near end point, it just indicates the crosstalk value measured at the near end point. This value varies 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 also be attenuated, and the crosstalk to other wire pairs will be relatively small. Experiments show 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 generate 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 testers are 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 dissipates a portion of the signal and turns it into heat. It refers to the sum of the resistance of a pair of wires, and the DC resistance of the twisted pair of 11801 specification shall not be greater than 19.2 ohms. The difference between each pair should not be too large (less than 0.1 ohms), otherwise it indicates poor contact and the connection point must be checked.
(4) Characteristic impedance
Different from the loop DC resistance, the characteristic impedance includes resistance, inductive impedance and capacitive impedance with a frequency of 1 to 100 MHz, which is related to the distance between a pair of wires and the electrical properties of the insulator. Various cables have different characteristic impedances, while twisted pair cables are available in 100 ohms, 120 ohms and 150 ohms.
(5) Attenuation Crosstalk Ratio (ACR)
In certain frequency ranges, the proportional relationship between crosstalk and attenuation is another important parameter that reflects cable performance. ACR is sometimes expressed as a signal-to-noise ratio (SNR: Signal-Noice ratio), which is calculated by the difference between the worst attenuation and the NEXT value. The larger the ACR value, the stronger the anti-interference ability. General system requirements are at least greater than 10 decibels.
(6) Cable characteristics
The quality of a 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, when the data signal is received, the receiver cannot distinguish the data signal and the noise signal, which will eventually cause data errors. Therefore, in order to limit data errors within a certain range, a minimum acceptable SNR must be defined.
