Miscellaneous SI Topics

Cable Twisting

One advantage of twisting the cable is that it reduces the effective separation between the cable pairs. If one of the pairs is signal and other is ground, the reduced separation helps in achieving lower effective impedance of the signal. Reduced effective separation also leads to lower crosstalk with the adjacent pair. This effect is similar to that we discussed in the cross talk section where reducing the power plane to signal layer separation reduces the cross talk.



Figure- Twisted and non twisted pair – Cross talk.

Twisted pair reduces the crosstalk on the adjacent signal victim wire. Let us assume that the signal pair consists of a wire and a returning ground pair. This is shown in the first section of the above figure. The signal flows in the left wire and returns from the right wire. The magnetic field produced by the signal carried in the left wire is equal and opposite to the magnetic field produced by the right wire. Since the victim is closer to the right wire, the net crosstalk induced by the right wire will exceed that of the crosstalk induced by the left wire.

If we, however, twist the pair, the separation of the two wires from the victim signal wire alternates. The fields due to the two wires alternate in direction and therefore cancel each other.

Twisting also makes the twist pair immune to magnetic coupling. Since the twisting reverses the direction of the loop, the external magnetic field can couple only to a small twist area and not to the cable as a whole. This provides some kind of inherent magnetic shielding.


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