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Twisted pair, effect on non balanced/differential systems?

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MFX:
I give up. Looks like getting a simple answer is much harder than I thought. Thanks anyway.

SuperFungus:
I'll give it a shot. "Twisted pair" is about reducing loop area, ie: making the supply and return currents travel as closely together as possible.  When you do this, there are at least two effects relevant to your question:

1)  You reduce the chance that the field lines from your circuit's currents are shared with the field lines from some external current and vice-versa.  ie: you are less susceptible and radiate less.
2) Both wires experience about the "same" influence from outside interference, so interference tends to couple in as a "common-mode" signal.

#2 is only an advantage if you set up a "balanced" or differential signal that can provide additional rejection to outside interference through the common mode rejection of the drivers and receivers in the system.  #1 can be an advantage for even a "single-ended" signal.  The same principal is at play when you use a board with plane layers for better susceptibility and emissions.  The plane layer more tightly couples the return current to the supply current (reduces the loop area). 

I think it's worth noting, these are AC features of the system.  At DC, twisted or not twisted, big loop area or small loop area makes no difference, and the slowly changing switch signal gets down the wire just the same.  That said, it can still be a very good idea to use twisted pair for a wired switch since it provides greater immunity to external interferes like mains, cellphones, motors etc.  which might couple in and false out the switch.

So to answer your question, yes there is a susceptibility benefit to even a single ended signal to using twisted pair (or coax).  Quantifying "how much" depends on a whole host of factors and isn't possible to just state generally.  How far away are the wires if they aren't twisted?  Are you sharing one ground wire with multiple signal wires (can be an issue even at DC!)?  How long are the wires, and what environment are they in?  What is the impedance of the circuit connected to the wires?  etc. etc.

MFX:

--- Quote from: grouchobyte on June 13, 2020, 03:04:51 am ---
--- Quote from: MFX on June 12, 2020, 11:52:31 pm ---I give up. Looks like getting a simple answer is much harder than I thought. Thanks anyway.

--- End quote ---


I know the principle behind using twisted pair cable for differential (RS485, Ethernet, USB etc. etc.) and balanced (Microphones etc.) systems but occasionally I see people suggest that using twisted pair can reduce noise pickup problems on non-balanced systems, E.G. a microswitch connected to a microcontroller with just a 5 I know the principle behind using twisted pair cable for differential (RS485, Ethernet, USB etc. etc.) and balanced (Microphones etc.) systems but occasionally I see people suggest that using twisted pair can reduce noise pickup problems on non-balanced systems, E.G. a microswitch connected to a microcontroller with just a 5v and Gnd wire between them. Does using twisted pair have any noise pickup reduction benefit in this case and if so how much (roughly) benefit compared to a proper differential system?


You want a simple answer.....if you want to connect a switch to 5 volts or ground on a microcontroller I/O pin and are worried about noise immunity then the simple answer to your initial question is.....it doesnt matter whether or not you string a mile  of circuitous clip leads to your switch or use shielded twisted pair routed through the noisiest radio transmitter or electric generator. Now if its something else you have in mind that you want to use twisted cabling for than thats another matter altogether!

@grouchobyte

--- End quote ---

It isn't about whether I care about noise the point is OTHER PEOPLE suggest using twisted pairs on such non-differential systems and I don't know whether there really is any significant benefit in that, whether there is a SLIGHT benefit or whether they're talking rubbish. If I had the facilities then I'd do tests (I don't) I've searched Google (nothing I can find that's clear on the issue, as soon as you search for "twisted pair" a load of differential systems come up.) Hence asking.

duak:
If I may add to SuperFungus' point 2 above.  Even though twisting the conductors minimizes the loop area, some voltage can be induced into the circuit by an external EM field.  Each half twist will have some voltage induced in it but successive half twists will have a polarity opposite to its immediate neighbor.   One full twist should pretty much cancel out the net induced voltage over that twist.

A twisted pair will also radiate EM so that adjacent pairs can couple into each other and cause crosstalke.  Commercial flat cables should alternate the twist direction for adjacent pairs. ie.,  odd numbered twisted pairs should be twisted one way while even numbered twisted pairs should be twisted the other way.  In addition to alternating the twist direction, the twist pitch can also be varied so that adjacent pairs of pairs can be of different pitches, ie. half or double that of their neighbors.  See the attached image for an example.

The ultimate in twisted pairs was worked out by the Telcos.  I remember driving along an interstate in New Mexico that was parallel to an old telephone line that used open wires strung on doorknob insulators on crossbar poles.  I'd say there were maybe 16 pairs in some sections.  If you followed each pair you would see there usually only two doorknob insulators but sometimes there were four where the wires crossed over.  Not all pairs crossed over at same pole but there was a pattern to it.  If memory serves it's a Walsh function.

bson:

--- Quote from: grouchobyte on June 13, 2020, 05:40:30 pm ---Bottom line: YES, twisted pair to connect to something like a switch does provide marginally better noise immunity. Quantifying that is a little more involved.

--- End quote ---
Well, he didn't ask for it quantified, it was only a simple question and he only wanted a simple answer.  No reason to start thinking.

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