Performance index of twisted pair (1)

For twisted-pair cables, 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 the link. Attenuation is related to the length of the cable. As the length increases, the signal attenuation also increases. Attenuation uses "db" as the unit, which represents the ratio of the signal strength from the source transmitting end to the receiving end. Since 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 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 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 its 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. Most testers are equipped with corresponding equipment so that the NEXT value at both ends can be measured at one end of the link. Refer to the following table for the results of the NEXT test:

The attenuation limit at various frequencies when various twisted pair connections are the maximum length
Frequency (MHz)	Maximum attenuation (20℃)
	Channel (100 meters)			Link (90 meters)
Media category	Class 3	Class4	Class5	Class3	Class4	Class5
1	4.2	2.6	2.5	3.2	2.2	2.1
4	7.3	4.8	4.5	6.1	4.3	4.0
8	10.2	6.7	6.3	8.8	6	5.7
10	11.5	7.5	7.0	10	6.8	6.3
16	14.9	9.9	9.2	13.2	8.8	8.2
20		11	10.3		9.9	9.2
25			11.4			10.3
31.25			12.8			11.5
62.5			18.5			16.7
100			24			21.6

NEXT attenuation limit at a specific frequency
Frequency (MHz)	Minimum NEXT
	Channel (100 meters)			Link (90 meters)
Media category	Class 3	Class 4	Class 5	Class 3	Class 4	Class 5
1	39.1	53.3	60.0	40.1	54.7	60.0
4	29.3	43.3	50.6	30.7	45.1	51.8
8	24.3	38.2	45.6	25.9	40.2	47.1
10	22.7	36.6	44.0	24.3	38.6	45.5
16	19.3	33.1	40.6	21	35.3	42.3
20		31.4	39.0		33.7	40.7
25			37.4			39.1
31.25			35.7			37.6
62.5			30.6			32.7
100			27.1			29.3

The above two indicators are the main content of the TSB67 test, but some types of testers can also provide indicators such as DC resistance, characteristic impedance, and attenuation crosstalk ratio.