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Name: Ann
Status: student
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Question:
In twist pair cables how does twisting reduce cross talk, electromagnetic interference and noise?



Replies:
Hi Ann

The simplest way to think about this is geometrically. Consider a pair of untwisted wires encountering an incoming wave of electromagnetic energy broadside across the length of the wires. If one wire is closer to the source of the wave, that wire will experience an effect that is a tiny bit stronger and happens a tiny bit sooner than that experienced by the wire that is downstream from the wave. This will result in the generation of a small signal that could cause interference in the otherwise clean signal running through the wires. Now, if we twist the wires into a kind of double helix, the impinging wave strength and time delay in each wire tends to null out, as the wave hits both wires within the pair approximately evenly when averaged over the length of the wires. The result is that potential for interference is greatly reduced.

Before television signals were supplied through cable or satellite, analog television sets were wired to TV antennas with a flat cable that was made up of a pair of wires separated by about a centimeter with an insulator. This was called 300 ohm twin-lead. In order to prevent ghosting (the repeat of a faint image displaced slightly from the original), the twin-lead had to be twisted a full turn every 30 centimeters or so in its route from the antenna to the TV set.

Hope this helps.

Bob Froehlich



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