What is Cross-talk?

Cross-talk is a perturbation produced by the electric or magnetic fields of a telecommunication signal impacting on a signal in an adjacent circuit. In a telephone circuit, cross-talk could result in your hearing part of a voice conversation from a different circuit. The phenomenon that produces cross-talk is known as electromagnetic interference (EMI). It can happen in microcircuits within computers and audio equipment as well as within network circuits. The term is also utilized to optical signals that interfere with each other.

How does the coaxial cable reduce the cross-talk?

Even low-power signals traveling along with a wire produce electromagnetic fields. These fields, out in the open, are the origin of cross-talk. But, as coax (coaxial cable, where both conductors share the same long axis) has one conductor within the other, those fields produced stay inside the wire and do not exit. Likewise, an adjacent coaxial is also separated from any nearby fields.

No matter whether its coaxial wire or UTP, copper-based cabling is sensitive to cross-talk. Cross-talk takes place when the signal from one cable becomes mixed up with the signal in a different cable. This can occur when wires run too closely together. Some cables implement shielding to help decrease the influence of cross-talk. If shielded cable is not applied, cables should not be run immediately near each other.

In contrast with UTP, the coax noise and interference surroundings are moderately simpler. For example, the problems of alien cross-talk and near-end cross-talk do not occur in coaxial systems. The cross-talk between adjacent wires is so minuscule that for conventional digital local-area-network applications, it could be totally ignored. Also, the near-end repetition problem, which usually produces so much concern in bidirectional Unshielded twisted-pair systems, rarely shows up in coaxial applications. That’s because most applications employ coaxial in a unidirectional fashion.

How does the coaxial cable reduce the cross-talk?

Coax: Radio Frequency Interference

Coaxial wires have a reasonably good natural resistance to external noise, due to the physical symmetry of the signal current conductor and the returning current conductor (the concentric shield). This symmetry eliminates, to first order, all consequences of external electromagnetic fields. Any extra sensitivity in a coaxial cable comes from imperfections in its shield.

At frequencies, up to a few MHZ, coax susceptivity is equivalent to the resistance of the wire shield. The end-to-end level of resistance of the shield, when excited by the large common-mode currents that can be produced by Radiofrequency interference generates a small residual voltage from end to end all over the cable shield.

This residual voltage appears to be to the receiver as an origin of noise. Susceptibility issues due to shielding resistance happen most frequently in the below-30-MHz band. To get over low-frequency susceptibility problems, use a thicker, lower-resistance outer braid or switch to a larger cable (which has a bigger, lower-resistance braid).

Higher-frequency electromagnetic fields can leak directly as a result of the holes in the braid. To overcome high-frequency susceptibility problems, designate a cable with a heavy braid plus a solid foil shield. The solid foil shield is often covered around the dielectric, just beneath the heavy braid. The combination of foil shield and heavy, low-resistance braid works exceptionally well for fighting external noise, even though a thin aluminum foil will somewhat increase the skin-effect resistance of the shield, slightly deteriorating the high-frequency attenuation.

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Jessica Cardona