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In another answer here on the SE, this statement was made:

Usually, my goal is to have all EM noise appear to a good amplifier as common mode, and use twisted pairs to achieve this."

(emphasis added)
This seems to suggest that using twisted-pairs vs independent, single wiring will change how EM noise will alter a signal.

My question is in regards to the part I've highlighted. How does twisted-pair wiring affect noise on the signal? Is there something special about the twisting that facilitates this effect? Or is it just the proximity that's important. At which point, could I just bundle the individual wires and zip-tie them (or similar) together?



(see here) for the answer from which I'm quoting. Taken from 2nd paragraph.

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Adam Head
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2 Answers2

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It's actually magnetic pickup.

If you go all the way back to laws of physics, electromagnetic fields passing through a loop of conductor generate currents, and its going to increase with the area of the loop. Twisted pairs minimize the area of the loop, and signals generated in successive twists tend to cancel. (Horowitz and Hill, 2nd edition, page 456).

That's probably the main effect. I like to think that much of remaining EMI on each lead of the twisted pairs will be similar (and thus common mode), as the close proximity of the wires exposes them to very similar EMI profiles. That's probably not as big an effect as just minimizing differential EMI induced noise, as above.

Practically speaking, an old trick to make twisted pairs out of plain old stranded hookup wire is to run two lengths of wire next to each other, tie one end down, chuck up the free end in a power drill, and use the drill to wind the pair. Go a little more than you think you should, as it will open up a bit on release.

Scott Seidman
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    Also, whatever EMI pickup is received is roughly picked up by both lines at the same rate because of the interleaving, where-as a ribbon cable might have slightly different common mode noise between adjacent lines. – helloworld922 Aug 26 '13 at 15:45
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We can see two general ideas in this arrangement - balancing and twisting. These techniques solve different problems: the balanced pair solves the problem of compensating while the twisted pair - of homogenizing. Actually, the twisted pair solves both the problems; it is a "twisted balanced pair".

So the main idea of the twisted pair is: the two twisted wires form a homogeneous structure in which each wire is exposed equally on average to the noise radiation.

This simple but genius 19th century Bell's idea makes the impossible possible - as though it places the two wires at the same place. If we generalize this powerful idea, we can see (and apply) it in our routine in situations where we have to expose two long or large objects to an equal radiation. To solve the problem, we divide the objects into many equal parts and intermix (shuffle) them.

Imagine you are in the place of Bell and you have to solve the problem of induced noises. So, you first "balance" the line (place the return wire in parallel to the direct one) and second, you "homegenize" them (by twisting)...

Circuit fantasist
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