Twisted pair, effect on non balanced/differential systems? - Page 1
Forum section: electronics / projects

author: MFX
author_rank: Regular Contributor
timestamp: June 11, 2020, 03:58:54 pm
content: 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?Cheers.Martin.

author: pwlps
author_rank: Frequent Contributor
timestamp: June 12, 2020, 10:10:15 am
content: I found this picture in my archives unfortunately I can't find the book reference. As far as I understand floating loads mean differential (balanced) input, but A,B,C represent single-ended inputs. For these there is apparently no much difference between a coax and a twisted pair. I find it a bit strange because I thought a twisted pair should reduce the magnetic flux much better than a coaxial cable. I'm curious to understand it too.

author: David Hess
author_rank: Super Contributor
timestamp: June 12, 2020, 12:12:51 pm
content: With a common mode connection only on one end, twisted pair or coax reject magnetic interference very well. As shown above however if both ends are connected, rejection is very low. In that case other steps are necessary like common mode filtering, transformer isolation, or an instrumentation amplifier.The pair of wires could also just be bundled together; magnetic flux caught between them is what causes problems. The idea is to minimize the "loop area".

author: Marco
author_rank: Super Contributor
timestamp: June 12, 2020, 03:22:01 pm
content: Quote from: MFX on June 11, 2020, 03:58:54 pmHow is that non balanced? The currents are exactly balanced.

author: MFX
author_rank: Regular Contributor
timestamp: June 12, 2020, 03:44:23 pm
content: Quote from: Marco on June 12, 2020, 03:22:01 pm"Balanced" is the term used in audio world for what is basically a differential system. Rather than signal and ground you have signal and inverted signal.

author: MFX
author_rank: Regular Contributor
timestamp: June 12, 2020, 03:56:57 pm
content: Quote from: pwlps on June 12, 2020, 10:10:15 amThanks, it's more the difference between just two straight wires and exactly the same system with those wires twisted I'm interested in.

author: Bassman59
author_rank: Super Contributor
timestamp: June 12, 2020, 06:05:31 pm
content: Quote from: MFX on June 12, 2020, 03:44:23 pmTrue, but for quite some time now there has been a push to "educate" the audio folks (of which I am one!) to explain what is really meant by "balanced." It started when some manufacturers started using what they called "impedance balanced" outputs, which they knew was redundant. These outputs use one driver through a build-out resistor to the XLR pin 2, and pin 3 connects to ground via same-value build-out resistor. That balanced the line impedance so you get all of the benefits of balanced drive at half the price of a differential driver.The differential drive gives you double the maximum voltage swing, which is in most cases not interesting because a "nominal" line level of 1.23 Vrms (+4 dBu) is well under the maximum which is determined by output driver rails.Also, the differentially-driven balanced outputs always have the build-out resistors to set the line impedance.The point of setting the line impedance is not because it is a transmission line -- it's not -- it's to ensure that the currents in the signal and the return are matched.

author: MFX
author_rank: Regular Contributor
timestamp: June 12, 2020, 06:12:49 pm
content: Quote from: Bassman59 on June 12, 2020, 06:05:31 pmYou'll never get them to change, just like you'll never stop lighting people using 3 pin XLR for DMX so it's pointless trying. Anyway this is drifting away from my question.

author: floobydust
author_rank: Super Contributor
timestamp: June 12, 2020, 06:19:45 pm
content: Quote from: MFX on June 12, 2020, 03:44:23 pmActually it goes back to telephony in the early 1800's, electric streetcars making interference on the phone lines. Look up "Longitudinal Balance".

author: MFX
author_rank: Regular Contributor
timestamp: June 12, 2020, 07:28:02 pm
content: This is all very interesting but could we get back to the question?

author: MFX
author_rank: Regular Contributor
timestamp: June 12, 2020, 11:52:31 pm
content: I give up. Looks like getting a simple answer is much harder than I thought. Thanks anyway.

author: SuperFungus
author_rank: Contributor
timestamp: June 13, 2020, 06:28:08 am
content: 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,yesthere 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.

author: MFX
author_rank: Regular Contributor
timestamp: June 13, 2020, 04:00:00 pm
content: Quote from: grouchobyte on June 13, 2020, 03:04:51 amIt 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.

author: duak
author_rank: Super Contributor
timestamp: June 13, 2020, 08:40:14 pm
content: 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.

author: bson
author_rank: Unknown
timestamp: June 14, 2020, 01:51:36 am
content: Quote from: grouchobyte on June 13, 2020, 05:40:30 pmWell, he didn't ask for it quantified, it was only asimplequestion and he only wanted asimpleanswer. No reason to start thinking.

author: bsdphk
author_rank: Regular Contributor
timestamp: June 14, 2020, 08:02:34 am
content: Quote from: duak on June 13, 2020, 08:40:14 pmFor more info, see:https://archive.org/details/bstj15-4-1/page/n7/mode/2up

author: MFX
author_rank: Regular Contributor
timestamp: June 14, 2020, 05:28:39 pm
content: Quote from: grouchobyte on June 14, 2020, 02:56:41 am"(roughly)" generally implies rough finger in the air estimate, sorry if that wasn't clearer.

author: SuperFungus
author_rank: Contributor
timestamp: June 15, 2020, 04:39:11 am
content: The "rough" general rule of thumb is: the closer you bring a current to it's return (twisted pair) the less it radiates and the less susceptible it is to interference.Further quantifying the benefit is no longer "simple" because it can be incredibly sensitive to every implementation detail. I know it's not satisfying, but to give you an idea of what I'm talking about I've tightened a nut from "snug" to "tight" on a bulkhead connector and gotten ~10dB better radiated EMI performance (which itself is only true because the whole thing was very quiet to begin with). In comparison the variability inherent in a twisted pair vs non twisted pair is almost infinite. How tight is the twist? How thick is the insulation? How do the wires run in the "non-twisted" comparison? What frequency are you making the comparison at? You could compare a specific twisted pair to a specific different wire configuration (generally by building both and measuring), but anyone who you find who thinks they can give you a number for this that applies "in general" is fooling themselves.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: June 15, 2020, 08:11:02 am
content: It helps even on single ended signals, when the aim is towards fair signal quality, rather than RF emissions for example.Consider a short length of twisted pair over ground plane. The transmission line has two modes, CM and DM. Because it's used unbalanced, the modes are coupled to some extent. With one wire grounded at both ends, and the other wire the signal, that coupling is about 50%. That is, the common mode is simply (V(signal) + V(GND)) / 2.Since the TL is short, it won't be exposed to too much interference, nor radiate too much itself; if it were long, CM would in turn couple with ambient fields, i.e. a lot of signal evaporates into space, bad news. Evidently, about half, which isn't good news even for digital signals.Note that TL length is relative to the signal, its analog bandwidth or digital edge transition rate (not the clock rate!).So, since CM loss depends on length and balance, if we have a short, unbalanced stretch, it's not a big deal, and we won't lose a huge amount of signal.For example, 1dB of signal loss is merely 11% amplitude, likely no problem for a digital signal. If the loss is entirely radiated, that 11% lost is -20dB to the signal, and if the signal is some 120dBuV (i.e., about a volt), that's 100dBuV of emissions -- not a great start when your limit is, whatever, 60dBuV or so!In simpler terms: if you lose say 1% of a signal, that's 99% still making it through, which isn't much bother to the intended signal path. And it's only 1% leaking away, who cares, right? Trouble is when the signal is large enough, relative to some emissions limit, that even that 1% causes further problems.For a mechanical analogy perhaps -- consider the power output of a car engine. 100kW say comes out the drive shaft, and, Idunno, 200kW comes out of the exhaust pipe thermally and another 50kW is soaked up by the coolant, and so on. Well, heat isn't a bother, it's fine as long as it has somewhere to go (hence the tailpipe and the radiator). But acoustic energy is a problem. It's literally illegal (in many places) to operate a vehicle without a muffler, it's irritating for bystanders and it's damaging to the driver! It might only be a few hundred or thousand watts of actual acoustic power -- a tiny fraction of the total power, but one that, if not dealt with, becomes a big problem, for everyone!The engine is probably perfectly fine either way, with or without a muffler; it won't make much difference to overall efficiency, or maximum power output (analogous to signal quality, as it were). The pipes themselves, might need to be tuned for best operation -- particularly true of two-cycle engines; a more direct signal quality analogy. But after that, what happens to the noise leaving the vehicle doesn't matter much to what's inside, so you can get away with various things; heh, well, I'm not sure there's an analogy for coax vs. twisted pair in terms of acoustics. But in any case, in electronics, we get concerned when the signal quality must be especially good, and when the emissions must be low.Tim

author: David Hess
author_rank: Super Contributor
timestamp: June 15, 2020, 05:09:05 pm
content: Analog Devices discussed the subject in their application note 347 - Shielding and Guarding:https://www.analog.com/media/en/technical-documentation/application-notes/41727248AN_347.pdf

author: wizard69
author_rank: Super Contributor
timestamp: June 15, 2020, 05:27:13 pm
content: Demanding a simple answer when it doesn't exists and then abusing people for trying to answer the question, is not cool at all. I just logged on and frankly I'm shaking my head here after reading through this thread.Quote from: bson on June 14, 2020, 01:51:36 am

author: MFX
author_rank: Regular Contributor
timestamp: June 15, 2020, 07:29:51 pm
content: Quote from: wizard69 on June 15, 2020, 05:27:13 pmWhere exactly have I "demanded" anything or "abused" anybody? I may have been a bit direct in trying to get the thread back on track, sorry if that offended anybody.