For power cords, the user is most concerned with several metrics that characterize their performance. These indicators include attenuation, near-end crosstalk, impedance characteristics, distributed capacitance, and DC resistance.
attenuation
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.
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 a UTP link. For UTP links, NEXT is a key performance indicator and one of the most difficult to measure accurately. As the signal frequency 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. Today's testers are equipped with appropriate equipment so that the NEXT values ​​at both ends can be measured at one end of the link.
DC Resistance
TSB67 does not have this parameter. The DC loop resistor consumes a portion of the signal and converts it into heat. It refers to the sum of a pair of wire resistances,
The DC resistance of the twisted pair of the 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 means poor contact and the connection point must be checked.
Characteristic impedance
Unlike the loop DC resistance, the characteristic impedance includes the resistance and the impedance of the inductor and the impedance of the capacitor from 1 to 100 MHz, which is related to the distance between the 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.
Attenuation crosstalk ratio (ACR)
In some frequency ranges, the proportional relationship between crosstalk and attenuation is reflected in cable performance.
Another important parameter of the power cord. ACR is sometimes also expressed in terms of signal-to-noise ratio (SNR: Signal-Noice ratio).
It is calculated from the difference between the worst attenuation and the NEXT magnitude. The ACR value is large, indicating that the anti-interference ability is stronger. General system requirements are at least greater than 10 decibels.
Cable characteristics
The quality of the communication channel is described by its cable characteristics. SNR is a measure of the strength of a data signal, taking into account interfering signals. If the SNR is too low,
This will cause the receiver to be unable to resolve the data and noise signals when the data signal is being received, eventually causing data errors. Therefore, in order to limit data errors to a certain range,
A minimum acceptable SNR must be defined.
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