2.2 Dependence of crosstalk on the length

As shown by measurements in the SL, the average near-end crosstalk ratio in multi-pair cable does not depend on the length and defined crosstalk at the near-end of the first building length.

The average values ​​of crosstalk at the far-end, and immunity at the far-end of multi-pair cables are depend on the frequency and length of subscriber lines and are determined by these formulas:

;

,

where A_{i BL} – immunity at the far-end of the SL km of length l, dB;

A_{i BL} – immunity at the far-end of one building length of cable, dB;

A_l SL – far-end crosstalk of SL, dB;

a – cable attenuation coefficient in dB / km at a frequency ƒ;

L_BL – length of the construction of cable length, km.

Analysis of the above given formulas for A_i and A_l shows that with increasing length of the SL at the far-end of the immunity decreases and the far-end crosstalk at first decreases and then begins to increase due to increase self-damping αl.

2.3 The dependence of the crosstalk at the near-end and far-end immunity on the frequency

Mean values ​​of crosstalk between the pairs at the near-end of cable type of TП on the frequency range from 10 kHz to 2 MHz can be determined as follows based on an analysis of the results of measurements conducted by the authors [1] the approximate formula:

_,

where А₀₁ – value of near-end crosstalk ratio at the frequency 1 MHz (see Table 2.1); f - frequency, MHz.

The main reason is the electromagnetic coupling between the pairs for reducing the crosstalk at the near-end when the frequency increases .

The mean values of crosstalk at the near-end and their standard deviations at 1 MHz for cable type TП are shown in Table. 2.1.

The average value of immunity at the far-end of cable-type TП length of 280 m between pairs located within a close helix at frequencies above 10 kHz, approximately determined by using an expression based on the results of measurements:

,

where f – frequency, kHz.

In that case when the building length of the cable l_BL different from 280 m, the value of immunity at the far-end is changed to an amount equal to minus 10•lg(l_BL/280).

The value of A_{i BL1} at the far-end is not between adjacent pairs (between non-adjacent pairs of a helix, or between pairs of different helix), a construction of cable length is determined by the formula:

,

where dА_i – increase security at the expense of mutual recession affecting and those affected by steam, established on the basis of the results of measurements of the authors and given in Table. 2.2.

Table 2.1 - The mean values of crosstalk at the near-end and their standard deviations for the cable type ТП with twisted construction of the core at a frequency of 1 MHz

Location pairs in the cable core	The average value of near-end crosstalk attenuation, dB	The standard deviation in dB
Adjacent to the lay	53	4,7
Over a 1 pair in lay	65	5,5
Over a 2 pairs in lay	73	7,4
Over a 3 pairs in lay	74	7,2
Over a 4 pairs in lay	75	5,2
In the relative lay	68	5,6
Over a one layer	75	6,3
Over a 2layers	79	6,8
Over a 3layers	81	6,2
Over a 4layers	81,5	6,9

Table 2.2 – Increment of security at the far end (dА_i) construction of cable length type ТП with twisted construction of the core at different relative positions of pairs

Location pairs in the cable core	The average value of far-end crosstalk attenuation, dB
Over a 1 pair in lay	6
Over a 2 pairs in lay	10
Over a 3 pairs in lay	11
Over a 4 pairs in lay	12
In the relative lay	8
Over a one layer	13
Over a 2layers	15
Over a 3layers	16
Over a 4layers	16,5

Analysis of the data shown in Table. 2.1 and 2.2, proves that the location pairs away from each other the mutual influence between them decreases.