2
\$\begingroup\$

Why does cross talking happen between different cables? Is it because any electrical cable has the capability to act as an antenna?

And most important, why do twisted pair of cables have less cross talk between them?

\$\endgroup\$
  • \$\begingroup\$ Why not research the subject yourselves first, start here: en.wikipedia.org/wiki/Crosstalk Coupling cables are not seen as Antennas so forget about that, focus on capacitive and inductive coupling. The reason twisted pairs are used is related to the crosstalk being the same for each pair so that it can be eliminated easily. \$\endgroup\$ – Bimpelrekkie Aug 11 '17 at 17:42
4
\$\begingroup\$

why do twisted pair of cables have less cross talk between them

The dominant reason for twisting wires is to cancel out interfering magnetic field induction. If two wires are not twisted, the wire closest to the interfering source has induced into it a greater voltage than the wire further away. So, if the wires are repeatedly crossed, the net induction into each wire is the same and, that interference generates a "common mode" signal that is much more easily dealt with than a differential interference signal.

Also, twisted pairs, in turn, produce less magnetic interference to other wires because the net field at some distance tends to zero due to the twists.

The other mechanism is electric field interference and twists do help but the better solution is a cable shield that is grounded.

\$\endgroup\$
  • \$\begingroup\$ @yoyo_fun if you are satisfied with this answer please do consider formally accepting it (or another) OR maybe you have some comment that can be cleared up? \$\endgroup\$ – Andy aka Aug 25 '17 at 14:41
3
\$\begingroup\$

Cross talk is caused by capacitive and inductive coupling between lines which run close together... The schematic shown below is typical for a two wire cable. As you can see, any AC signal or transient on one wire will be coupled into the other. How much depends on the geometry of the wires, the insulation type and the signal frequency. This is called cross talk for the obvious reason the signal crosses from one wire to the other.

schematic

simulate this circuit – Schematic created using CircuitLab

Twisted pair cables actually have MORE crosstalk not less since the coupling effect is greater.

However, I have a feeling you are talking about noise pickup, not cross talk.

Twisted pair cables are used to pass a differential signals. Whether that be a driven plus / minus signal or a single ended signal with a reference ground makes little difference. The benefit to using twisted pair here is that any noise or crosstalk source in the vicinity of the twisted pair tends to be picked up equally by both wires and as such becomes what we call common mode noise which cancels itself out at the differential receiving end.

\$\endgroup\$
0
\$\begingroup\$

Yes, all cables can act like antennas, both to transmit and receive. If two cables are poorly insulated/shielded and sufficiently close to each other, especially if they're within near-field distance, the electromagnetic field from one will affect the current in the other, and vice versa.

Twisted-pair cable is done for a few reasons - a large one is that if the pair is used for the transmission of a differential signal, noise immunity will be improved, and the antenna effect will be reduced. Conceptually this is caused by each twist acting to cancel the antenna effect of the twist opposite to it. But as @Trevor mentioned, crosstalk between the two conductors of a twisted pair is actually worse, not better.

\$\endgroup\$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.