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 А ₀₁ – 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		4,7
Over a 1 pair in lay		5,5
Over a 2 pairs in lay		7,4
Over a 3 pairs in lay		7,2
Over a 4 pairs in lay		5,2
In the relative lay		5,6
Over a one layer		6,3
Over a 2 layers		6,8
Over a 3 layers		6,2
Over a 4 layers	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
Over a 2 pairs in lay
Over a 3 pairs in lay
Over a 4 pairs in lay
In the relative lay
Over a one layer
Over a 2 layers
Over a 3 layers
Over a 4 layers	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.

Home task

4.1Give the answers the key questions.

4.2 Solve the following problem.

a) Determine the near-end crosstalk attenuation ratio A₀ (f _e) between pairs on the estimated frequency f _e for a given their relative positions in the TП cable with layer core construction (twisted construction of the core).

Table 4.1 – Initial data for 1-st problem

Variant
Relative positions of pairs	Over a 1 pair in lay	Over 2 pairs in lay	Over 3 pairs in lay	Over 4 pairs in lay	Over a one layer	Over a 2 layers	Over a 3 layers	Over a 4 layers
f e, МHz	0,25	0,5	0,75	1,25	1,5	1,75		0,2	0,4	0,6	0,8	1,2	1,4	1,6	1,8	0,1

b) Determine the far end crosstalk attenuation A _{l SL} (f _e) between pairs on the estimated frequency f _e for a given it relative positions in the TП cable with layer core building of the length l.

Table 4.2 – Initial data for 2-nd problem

Variant
Relative positions of pairs	Over a one layer	Over a 2 layers	Over a 3 layers	Over a 4 layers	Over a 1 pair in lay	Over 2 pairs in lay	Over 3 pairs in lay	Over 4 pairs in lay
f e, МHz	0,2	0,3	0,4	0,5	0,6	0,7	0,8	0,9	1,2	1,3	1,4	1,5	1,6	1,7	1,8	1,9
l, m

4.3 Prepare a record form in accordance with laboratory task.

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