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I read that the twisted-cable wiring is used for eliminating the common mode noise. As far as I know, in differential signalling the two wires carry their mirrored images so any common mode noise like EMI will cancel at the differential amplifier and the signal will remain since a-(-a) = 2a.

But does that mean using twisted pair wiring makes sense only for differential signalling? If not why? And I read Alexander Graham Bell invented this type of wiring to for telephone wires. But were telephone wires differential signals that time? I'm confused.

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    \$\begingroup\$ Twisted pair wiring makes sense for many types of signals. If it is not a differential signal, one of the wires can be GND, and the other can be the signal. This will improve signal integrity for signals that must travel over longer wires. \$\endgroup\$ – mkeith Mar 4 '17 at 18:08
  • \$\begingroup\$ I still don't understand what is the reason using twisted pair in single-ended signalling. What is wrong with this type of wiring for long distances they are also very close each other: av2buy.co.uk/image/cache/data/… There is no twisting and I dont think there will be capacitive coupling since they are enclosed in same shell. I guess it is so hard to explain \$\endgroup\$ – user1245 Mar 4 '17 at 18:21
  • \$\begingroup\$ Two wires close together also have great benefits. Ribbon cable with alternating GND wires works very well, even if the wires are not twisted. I think the main benefit is from placing the wires close together, but I am not 100% sure. To understand this better, you really need to understand transmission lines a little bit. It is hard to explain without going into transmission lines. \$\endgroup\$ – mkeith Mar 4 '17 at 18:30
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OK so the key is in your question, COMMON MODE NOISE...

Consider the following image..

enter image description here

Without twisted pair the line closest to the noise source gets more noise than the other line. So the noise is no longer common mode.

By twisting the wires you negate, or significantly reduce this effect.

Telephone systems are, or were, all transformer based, which is effectively differential.

So what of single ended..

Well arguably, there is no such thing. You are always passing the difference between something... but I won't open that can of worms...

enter image description here

In a simple system, you are not only passing the signal but also passing a reference point... usually we call it ground.. Again. if there is more noise in the signal line than the ground line, that gets picked up by your receiver.

Of course, that all goes out the window if there is some other ground paths or the grounds are shared with other signals.

Ultimately, it depends on the application. If it's single ended... where there are only two wires connecting between each device, and the receiver is battery operated or powered from the communication line, twisted pair helps a lot. If there are multiple connections or both the transmitter and receiver signal grounds are grounded locally to the mains... not so much.

In fact, in some instances over-use of twisted pairs can make things worse...

For example, running single ended signals through twisted pair ribbon cable is generally a bad idea.

enter image description here

The fact that you are bringing your signal wire closer to the neighboring signal wires increase the cross coupling between them. It is better to use a standard flat ribbon cable with ground wires between each signal acting as a kind of shielding.

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  • \$\begingroup\$ I saw some guys twisting the wires when they use single-ended signals. Are they wasting their time? How does twisting have a good effect when the signalling is single ended? First part amazing explanation but I still dont get how in a single ended system twisting has a positive effect. Ground is always ground what ever the noise imposed on it. I would be glad if you can expound on this. \$\endgroup\$ – user1245 Mar 4 '17 at 17:46
  • \$\begingroup\$ :) I probably added that answer while you typed that @user134429 \$\endgroup\$ – Trevor_G Mar 4 '17 at 17:47
  • \$\begingroup\$ I actually read it before but I dont get the conclusion here. Better twisted pair even it is single-ended? \$\endgroup\$ – user1245 Mar 4 '17 at 17:49
  • \$\begingroup\$ It depends on the application. If it's single ended like a phone... where there are only two wires connecting between each device, and the receiver is battery operated or powered from the line, twisted pair works great. If there are multiple connections or the receiver is grounded to the main line... not so much. \$\endgroup\$ – Trevor_G Mar 4 '17 at 17:51
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    \$\begingroup\$ As an aside, the voice signal in a POTS system isn't single-ended, but floating. DC is provided via a transformer or twin-coil relay, and possibly a baretter. This circuit was referred to as a transmission bridge. \$\endgroup\$ – peterG Mar 4 '17 at 18:53
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Even for single-ended signals, the twist means that magnetic coupling from adjacent wires will cancel, as every couple of inches it will reverse polarity.

Where a number of pairs will be in close proximity, they will often be arranged to have a different twist rate (number of inches per twist) to minimize coupling. (Wikipedia - twisted pairs)

As you say, capacitative coupling will not cancel if the signal is not differential but that is less of a factor at low frequencies with low impedances as used with phone lines.

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    \$\begingroup\$ This is potentially mistaken - twisting only works for cancellation if the return current (or other mirror image) is paired with each signal. That may or may not be true in a single -ended system - it depends on the details of connection at each end. \$\endgroup\$ – Chris Stratton Mar 4 '17 at 17:31
  • \$\begingroup\$ I copy my comment here: I saw some guys twisting the wires when they use single-ended signals. Are they wasting their time? How does twisting have a good effect when the signalling is single ended? I still dont get how in a single ended system twisting has a positive effect. Ground is always ground what ever the noise imposed on it. I would be glad if you can expound on this \$\endgroup\$ – user1245 Mar 4 '17 at 17:50

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