Twisting wire pairs useful or not. - Page 1
Forum section: electronics / rf-microwave

author: snarkysparky
author_rank: Frequent Contributor
timestamp: January 30, 2018, 11:57:39 pm
content: Anybody have the book " Grounds for grounding" ?Section 7.4.5 takes on the practice of twisting wire pairs to reduce magnetic coupling when one line of the twisted pair is grounded at both ends. The argument seems to be that the coupling loop exists not between the twisted conductors but between the pair and whatever reference ground structure is nearest. This is of course true. But the authors reach the conclusion."In Figure 7.63(a) the circuit is unbalanced due to the circuit’s multipoint groundingconnection to a reference ground. At lower frequencies, most of the return current willflow through the reference structure, yielding poor flux cancellation or large magneticfield pickup into the loops. Twisting will not offer any significant flux cancellationwhen the current flowing through both conductors is not equal and opposite, makingthe twisting virtually useless. "So we been wringing our fingers numb for years for no purpose ?I take issue with the conclusion of the last sentence. Even though the currents are not equal and opposite it does not mean no useful shielding is provided. The wire in the twisted pair that connects to ground on each end will carry current induced by the magnetic field passing through the loop area. This magnetic field will couple somewhat tightly to the other non grounded conductor inducing a similar voltage in it turning the interference into a common mode voltage to a certain extent.This is how I would try to explain the suppression supplied by twisting.In Henry Ott's book he outlined and experiment to test magnetic field shielding by twisting wires with a common ground. He found 13 db reduction of just a single wire occupying the same loop area. Grounds for grounding shows the same test but with only 2 db improvement.What do you think about wire twisting. A lot of it still is done these days for it to be so useless.

author: RandallMcRee
author_rank: Frequent Contributor
timestamp: January 31, 2018, 01:04:54 am
content: My understanding (from reading Ott's book) is that the paragraph you quoted is correct.Twisted pair is only useful if the return current flows in the other pair. Or, to the extent that it does, gates the effectiveness.Hence, in industry the usefulness ofshieldedtwistedpair. The shield can work regardless of the current flow, but of course a different noise source (emi, not magnetic) is being targeted.All of this points out why engineers are so obsessed with proper grounding. If you do the grounding right that usually means the currents are then going to flow correctly (e.g. as intended by the test equipment designer). Incorrect grounding subverts the designers intent and creates noise (or rather, noise that would otherwise be canceled is not). It all goes back to currents not flowing as intended.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: January 31, 2018, 03:39:36 am
content: At what impedance? In what environment?People get hung up on these things because they want a simple "do or not" result without understanding anything. You don't go far in EMC without an understanding. (You may eventually get there, but throwing on ferrite beads and ground shields willy-nilly is a very slow process indeed.)So let's construct a hypothetical case study to reason about.Suppose you have a common ground, a chassis. Three boards. A signal leaves one board, in a wiring harness. The harness crosses in front of a noisy board, maybe a power inverter or something. The harness terminates at another board.The fields around the noisy board couple EMF into any nearby wires. For magnetic fields, there is an EMF induced in series with the wires in the harness, proportional to strength and the loop area between the harness and chassis. (If the chassis isn't even present beneath the harness, well, all the more area, huh?) For electric fields, there is parasitic capacitance between the board and wires in the harness, forming a potential divider, from the offending source (say, a heatsink with switching voltage on it), through the capacitance of space, to the wire, through the wire's self-impedance and driven impedance, to ground.If the harness is just signal wires (no signals with low impedances on both ends: typically these are power or ground), then 100% of possible induction occurs to the harness.If the harness contains signals with low impedances at both ends, then that EMF is at least partially shorted out. A diamagnetic loop is formed, through chassis ground and linked by those wires in the harness. The signal lines, in the harness, will not fully share flux with those wires, but as long as the harness is smaller (those wires are closer together) than the harness is to ground, there will be a dominant shielding effect.This always works both ways. For modest speed digital signals, signal quality will be improved. There might still be, say, 20% loss and ringing on the rising edge, but that's not enough to violate the logic threshold, and signal quality is acceptable. Conversely, if induced EMF is well below the logic threshold, you're fine.There are actually four ways this works, not just two. So, conversely conversely, so to speak: suppose the chassis is not 100% enclosed, but say there's a plastic cover on top of this thing.Now you have to worry about radiation to space (RFI), as well as susceptibility (space radiating to wires).There are always four sides of the dice:1. Intentional signal source and load. Signal quality. This is what your schematics are drawn to reflect.2. Internal interference. At the absolute minimum, a device must deal with itself!3. Unintended radiation to space. This is the only part that is regulated*.4. Unintended radiation from space*.*In the US, the FCC regulates emissions. Susceptibility testing is recommended, but not required. For the CE mark, both are required.With the cover off, now we must worry about:- Radiation from the noisy board (obviously)- Radiation from the harness (and, well, in general, from everything)- Radiation to the harness (and everything else!)Whereas before, we might've lost, say, 20% of the rising edge and still achieved acceptable signal quality; now that 20% is what's radiating out of the machine, and that's basically 100% full signal, when you have to be orders of magnitude lower to meet regulations!It's always the same thing both ways, it's always symmetrical. At least, you'd have to work very hard indeed to build a system where its EMC considerations are non-reciprocal! (Not impossible, just that I suspect you'd have to contrive a system where this is the case.)What's not symmetrical is the standards, and with good reason: emissions should be in the millivolts or less, while susceptibility should be in the volts or more.This gives the case that, any two random devices, will probably have a ~60dB noise margin on their signals, when placed side by side. More importantly though, this provides an overhead to avoid disturbances fromlicensedor otherwise legitimate users of radio spectrum -- those whoareallowed to blast out volts at a time: whether it's free CB or FRS radio, or a HAM operator with a portable passing by, or you're in a car driving under a radio tower. And, likewise, a margin to avoid interfering with those authorized users, without being painfully strict (emissions in the ~60dBuV range are quite clearly sensible to any radio tuner, but if there's some distance between the two, it'll disappear into the noise floor very easily).Other environmental sources are even more intense, if infrequent. ESD is the sharpest: a risetime of nanoseconds, with a supply of many kilovolts and a peak current over 10A! If you have signals which must maintain the absolute lowest possible bit error rate, you will find it is fiendishly difficult to avoid hiccups: even a well shielded harness may afford 60dB of attenuation, but that still leaves several volts of ESD blip on your signal!Continuing our example, suppose we wrap the harness with foil or braid, so that it is reasonably well shielded for the run. What do we ground the shield to? Chassis? Board? One end or both?Well, more ground is usually better. Since we have a known offender in this situation, we might ground the shield on either side of the noise source, at a reasonable distance away from it. That could be done by routing the bare harness through shielding (like a conduit, cable tray or raceway), or by grounding the braid with something like these:https://www.icotek.com/us/product-catalog/emc-cable-clamps/That way, the induced fields are shorted out, in the loops near the noisy board. The current and EMF is prevented from reaching the signal boards. Or, at least, attenuated.Let's reflect on that a bit. Wewantground currents to flow, so that the currents are confined around the noise source.It's noteworthy that fields can only be shielded, never insulated*. (*Meta-materials can effectively act as insulators (index of refraction = 0), but only at tuned frequencies. So it's not apracticalmethod.)We definitely donotwant those currents to go up onto the signal boards. Or if it does reach them, that they are well grounded, and the signals picked up beyond those grounds, to avoid the ground loop voltage thus developed.Note that any voltage drop between shield-ground and circuit manifests as voltage in series with the signal. You definitely don't want to run a shielded cable and ground it with a floppy wire link. Ideally, you want that shield terminated into a solid ground plane, so that outside currents are shunted away from the signals.Another example:It's a surprisingly often repeated mistake, how to terminate a USB connector. Many appnotes show them DC-grounded through a ferrite bead. Precisely the diametric opposite of what you want! This puts 100% of shield RF currents into the inner wires, and that current will be distributed among the VCC, GND and D+/D- lines. Well, most of it goes to VCC/GND, obviously, but those in turn develop a voltage drop, which acts in series with D+/D-. So, guess what, you getbasically 100%of interference into your circuit (namely, at frequencies where those lines resonate). USB is 3.3V logic level, so you only have a volt or two of headroom to work with, not much when immunity is usually tested at 3V (or 3V/m).We do have our choice of how to deal with certain currents, or voltages. We can use an isolation barrier to reduce the current that an induced voltage would develop. This works great at low frequencies, where the few pF of an optoisolator or transformer is inconsequential. But be careful, this is not a general method: it will always fail at some ever-higher frequency!Which, by the way, can at least be extended. This is what Ethernet does: a transformer provides isolation (from DC to some 10s of MHz, probably), and a common mode choke extends that the rest of the way (to the high-100s or low-1000s of MHz that the receiver might still be able to comprehend).One final note, to avoid confusion:RF grounding is very different from galvanic grounding. It need never be exclusive. Though it often is easier to do one with the other. Example: a solid metal enclosure can serve as both galvanic (when eventually grounded to a safety earth ground) and RF grounding. Conversely, a 1m ground wire makes an excellent RFinsulatorat ~120MHz (or less, depending on what's hanging off it).Every time I say grounding here, I mean RF grounding. It's perfectly okay to have, say, a PCB sitting on top of a metal panel, with metal screws/standoffs, and to use capacitors to ground the circuit to those screw holes. Indeed, this is usually better, because the galvanic isolation means your DC supply currents will flow through the harness ground wires, like they're supposed to. And, in the event you do need galvanic ground -- nice thing about this kind of construction is, you always have it handy.Tim

author: snarkysparky
author_rank: Frequent Contributor
timestamp: January 31, 2018, 12:48:32 pm
content: T3,Agree the devil is in the details. But that isn't what the authors said. They said twisting wires when one wire has a low impedance to ground at both ends is virtually useless.The blanket nature of that statement is my issue. What I propose to say about the issue is.Twisting a signal pair will virtually never cost any signal degradation and will in most cases offer a significant reduction in magnetic field coupled noise over allowing the pair to lay in a bundle according to where they just happen to land in the wiring process.The authors are wrong on this point and it isn't a minor omission.Looking for comment though. This EMC stuff can be very subtle.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: January 31, 2018, 08:19:19 pm
content: They're right from one side, but wrong from the other.The improvement in signal quality is clear. 20% vs. 80% of lost edge and ringing, say.The improvement in radiated is not: if it's down by 10dB, that's still another ~50dB you need to meet regulations!I suspect it might've been in the latter sense, or in the extreme limit (for long cables), that the author had meant.The absoluteness of the statement is certainly undeserved, but it could also be that the author did not intend an absolute; it often happens that people say "always" when they mean "99% of the time" or something like that. People are notoriously bad at statistics.Tim

author: duak
author_rank: Super Contributor
timestamp: January 31, 2018, 09:23:32 pm
content: Paraphrasing Tim, it all depends on what you have and what you need to do.If you look at most twisted pair flat cable, the alternate pairs are twisted in opposite directions. This is primarily to minimize crosstalk between adjacent pairs. Some cables even have varied twist pitches. Power transmission lines and old open-wire telephone land lines use Transposition (see wikipedia) to swap the conductors relative locations to average out coupling effects. I remember driving thru New Mexico and looking at the telephone lines on the poles by the highway and seeing the pattern of the wires crossing over. There were like 16 or 20 pairs and had a period of a few dozen poles. If memory serves, it's a Walsh code.The impression I get from the original article is when a twisted pair with both ends grounded is close to a common ground. This can be a problem if there's an interfering field that can induce significant current in the common conductors. An example is an unshielded transformer in a gizmo that uses a twisted pair or even a coax cable to transfer a low level signal from one module to another. If the signal return line forms a large enough loop area with the overall common (ie. chassis or power distribution) a Ground Loop can be formed that has current flowing thru it that then causes an interfering voltage to be developed across the series impedance of the signal pair. I remember having trouble with hum in my home audio system in the 80's when I connected a VCR to it. The solution was to put two back-to-back balun transformers in series with the cablevision to break the low frequency ground loop formed by the power line, the audio cables between the components and the cablevision distribution system. I seem to remember measuring about 5 VAC at up to 100 mA loop current.For what it's worth,

author: Kalvin
author_rank: Super Contributor
timestamp: February 01, 2018, 12:08:17 pm
content: Does this also imply that using a normal coaxial cable (one signal conductor within a shield) will also create similar problem to the twisted pair, as the outer conductor (shield of the coax cable) will also induce EMI and will be affected by EMI in similar fashion?Using a twisted pair or coax will maintain better impedance matching at higher frequencies and reduce EMI by reducing the loop area of the signal. One can reduce problems of the circulating ground currents by AC coupling at the other end of the coax shield to the signal ground by a capacitor (hybrid cable shield grounding) . This hybrid grounding can also be applied to the twisted pairs. Using twisted pair will also reduce EMI susceptibility of the circuit as the interference signal due to magnetic coupling will be reduced.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: February 01, 2018, 01:59:24 pm
content: Just as a twisted pair cable carries two independent currents, or equivalently, two current modes (CM and diff), so too does the coax cable carry two currents: inner and outer, or CM (outer / shield current) and diff (inner / signal current).The difference is that the coax shield is solid, preventing external fields from inducing EMF upon the diff mode.Which is to say, it's just the same for a twisted pair, when balanced. When not balanced, you get CM-diff conversion, and the original concern.Coax can have this problem as well, when the shield is poorly terminated (outer shield current creates a voltage drop across the termination, which acts in series with the signal).Is one better than the other? No, of course not; they are equivalent. It does happen often enough, in practice, that unbalanced is easier than balanced (only needing a single amplifier, no balun), but both have their place.Tim