Crosstalk

In Telecommunication, it is said that crosstalk exists between two circuits, called in English Crosstalk (XT), when part of the signals present in one of them, considered disturbing, appears in the other, considered disturbed. That is, an electromagnetic disturbance produced in a communication channel by its coupling with another or other neighbors.

Crosstalk, in the case of twisted pair cables, generally occurs due to magnetic couplings between the elements that make up the disturbing and disturbed circuits or as a consequence of admittance imbalances between the wires of both circuits.

Crosstalk is measured as the attenuation between the disturbing circuit and the disturbed circuit, which is why it is also called crosstalk attenuation.

Crosstalk Intelligibility

According to how disturbing signals are perceived, generated in a circuit as a consequence of crosstalk, this can be intelligible or not intelligible.

• Intelligible typhoon is the one in which the disturbed circuit is heard and the conversation is understood by the disturbing circuit. This type of diaphony is extremely harmful because, in addition to the disturbance itself, it poses a risk to the secret of communications that the telephony operators are obliged to protect, so it is recommended by ITU that the difference between the level of the useful signal and the level of the diaphony between the different pairs of the same cable should not be less than 58 dB for 90% of the combinations of two circuits.

• There are cases where the diaphony is inintelligible, which in the disturbed circuit is only perceived as noise. This happens when the nature of the signals transmitted by both circuits are different, for example: analog in one and digital in the other or when multiplexation is used by frequency division and the carriers of the transmission systems used in each of the circuits do not match.

In fact, at the time when this type of multiplexing was widely used, it was resorted to having several versions of the same system with the carriers displaced 1 or 2 kHz with respect to each other, with which the crosstalk was always unintelligible.

In the case of digital signals in both circuits, the objective is that the crosstalk is kept within such limits that it cannot be recognized as a useful signal by the receiving end.

Far-end crosstalk and near-end crosstalk

For crosstalk measurement, a signal of a known level is sent through the disturbing circuit, and the level received in the disturbed circuit is measured. However, depending on whether the measurement is made at the same end from which we are sending the signal or at the distant end, we will have two different values. Thus we arrive at the concepts of near-end crosstalk or near-end crosstalk (NEXT) and far-end crosstalk or far-end crosstalk (FEXT).

Near Crosstalk

Figure 1.- Provision of elements for paradiaphony measurement.

The arrangement of elements for near-end crosstalk measurement, called Near end crosstalk (NEXT), is shown in Figure 1, in which both circuits appear terminated in their characteristic impedance Z_c at the far end, while at the near end the emitter is connected to Circuit 1 and the meter in Circuit 2.

A measure of the ratio in decibels between the emitted and received signals, obtained with this arrangement of measuring instruments, is what is called near-end crosstalk attenuation (α_p), whose value is equal to:

α α p=20× × lorgV1V2{displaystyle {alpha _{p}}=20times log{frac {V_{1}}}{V_{2}}}}}}}} (dB)

Power Sum Near End Crosstalk (PSNEXT) measures the cumulative effect of NEXT across all wire pairs in the cable. PSNEXT is computed for each wire pair by the effects of NEXT on the other three pairs. The combined effect of crosstalk from multiple simultaneous transmission sources can be very detrimental to the signal. This PSNEXT test is currently required for TIA/EIA-568-c certification.

Fart Crosstalk

Figure 2.- Provision of elements for the telephone measure.

The arrangement of elements for the measurement of far-end crosstalk, called Far end crosstalk (FEXT), is shown in Figure 2, where we see how in Circuit 1 we have a generator, which sends a V₁ level signal, at one end, while the other end is terminated with an impedance Z_c equal to the characteristic impedance of the circuit. Power Summation Far-end Crosstalk (PSFEXT) measures the cumulative effect of FETX on all wire pairs in the cable. PSFEXT is computed for each wire pair by the effects of FEXT on the other three pairs

The Circuit 2, is loaded at the sending end with Z_c and at the far end we have a level meter (R) in which we will measure a certain level signal V₂ corresponding to crosstalk.

The relationship in decibels between V₁ and V₂ is called far-end crosstalk attenuation (α_t) and its value is equal to:

α α t=20× × lorgV1V2{displaystyle {alpha _{t}}=20times log{frac {V_{1}}}{V_{2}}}}}}}} (dB)