Twisted pair wiring - Page 1
Forum section: electronics / projects

author: ezalys
author_rank: Frequent Contributor
timestamp: February 09, 2021, 06:39:25 pm
content: I’ve noticed that most cryostats are wired with twisted pair DC wiring. I’m a big fan of twisted pairs! The thing I don’t understand is how they frequently seem to be used by physicists in circuits where the circuit has seemingly no common mode rejection, which makes no sense to me. We have the following circuitWhere V_exc and V_ref come in on high frequency coaxial lines. Is there any “best” way to connect twisted pairs to this circuit or will anything I do basically be the same?

author: bob91343
author_rank: Super Contributor
timestamp: February 09, 2021, 06:45:25 pm
content: I don't get what this is supposed to do, or where you want twisted pair.Twisted pairs work well for low frequencies. If the signal is single ended, you must make sure that the right wire goes to the right destination. For balanced operation it doesn't matter unless phasing is important.The purpose of twisted pair is to cancel most capacitive coupling but it only is approximate and only works at low frequency.

author: tooki
author_rank: Super Contributor
timestamp: February 09, 2021, 06:51:52 pm
content: What counts as “low” frequencies to you, given that Ethernet, with its signaling in the hundreds of MHz, uses twisted pairs? USB 3, HDMI, and DisplayPort use twisted pairs, and they’re insanely fast signals, in the GHz.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: February 09, 2021, 07:31:38 pm
content: Well, the usual application for twisted pair is where signal quality needs to be "good enough".ECL logic is a good example: the outputs are typically balanced, while the inputs are unbalanced. A gate might connect to a twisted pair, which is terminated at the end, referenced to ground. Say, a 100 ohm twisted pair, terminated with two series 50 ohm resistors, the midpoint of which are grounded. Then one side or the other is connected to gate input(s) as needed.At least, I think that's a typical use case? Can't say I was around for the heyday of wire wrapped ECL backplanes.Anyway, the transmitter is balanced, and has a fairly low impedance; the receiver is unbalanced, so is sensitive to common mode noise. As long as common mode is a relatively small fraction of the signal level, it's okay. (And other assumptions to do with logic timing and precision. Don't have to worry about jitter if it's going into a clocked flip-flop, but do have to worry about it on the clock itself, or anything else where timing is critical. The sum of CM + DM signals will cross the receiver input threshold voltage sooner or later, incorporating that CM noise as timing errors.)Also, the common mode impedance is typically higher than DM in these situations, so it can have relatively higher voltages, and still meet the above assumptions (albeit, not by much). For example, if we consider the twisted pair to be a single conductor above a ground plane, then at typical cable diameters and routing heights, it might be 70 to 200 ohms, say. All of which is higher than the normal-mode termination resistance (50 ohms), so there is that tiny amount of advantage.Put another way: if we can afford say signal loss to 70% of nominal, and 30% of interference, and still get reasonable logic levels at the receiver, we're fine (that's still 40% of reliable logic signal). If we routed the single wire, we'd suffer from uncontrolled impedances (who knows where it's going to be 70 or 200 ohms, in a random stack of wire-wrapping), but with the twisted pair, we have just enough control to keep a clean signal.The flip side of this problem is emissions and susceptibility. If the missing 30% of signal goes into the common mode, that's a lot of radiation, and coupling to nearby wires. A few such signals, switching at the same time, might add up to that 30% of interference on an unlucky neighbor. So, that sets the functional susceptibility limit: how much the system can interfere with itself, and still operate (within some margin of reliability, anyway).The regulatory emissions and susceptibility limits, however, are much broader. For commercial purposes, emissions need to be in the fractional millivolts, more or less (exactly what, depends on waveform and frequency). The computer with the ECL backplane, most likely needs metal shields to cover up all that nastiness, reducing the ~100s mV of noise down to acceptable levels. For a sensitive physics experiment, it might need to be orders of magnitude lower (like, ~nV to measure quantum states or something?). Susceptibility needs to withstand some volts of nearby interference (say from a mobile radio); inside a physics experiment, all this is under control, it just needs to be well enough shielded at some point to keep out ambient fields, including say, techs' cellphones.So, there can be some value in using twisted pair in unbalanced mode, it's just that the CMR and CM/DM coupling is pitiful (say 6dB), so the application needs to be tolerant of that.Down asymptotically to DC, ambient electric fields are rejected only by, well, half, compared to a lone wire in otherwise the same geometry. I mean, easy enough to see, there's nothing blocking electric field lines from touching either conductor, and half the time they do exactly that.In the given schematic, it seems most signals are bypassed anyway, so it doesn't make much difference how they're delivered. If external magnetic fields must be kept low, then DC currents can be delivered via twisted pair, assuming the currents are balanced properly -- this can itself be a difficulty when the apparatus is all grounded metal, so the preferred ground return path probably ends up through the structure instead of through the wires. In that case, an isolated DC supply might have to be used, so the return current is guaranteed through the twisted pair.The near field around a twisted pair, is probably not as good as coax. Twisted pair locally makes a dipole field, which due to the twist, cancels out at a distance proportional to the twist rate (on top of the asymptotic 1/x^3 dipole field). Whereas coax is simply a Gaussian surface of perfect current cancellation (again, assuming currents are balanced), even at DC where the shield is not diamagnetic.These are... all things that a good physicist might be expected to know, or at least be able to reason about -- but there is a lot of domain-specific knowledge in EE, that the average physicist just isn't aware of. The EM fields inside real geometries, are very complicated, not easily solved with the physicist's toolbox of ideal planes and loops.Apparently, physicists are notorious for making rather boneheaded circuits, "low noise" amplifiers with >20dB worse than ideal performance for example, or complete ignorance of thermal issues (like an opamp mounted on a Teflon board, in complete vacuum -- where's the heat going to go?). And chemists are even worse, or something?Tim

author: ezalys
author_rank: Frequent Contributor
timestamp: February 09, 2021, 07:56:24 pm
content: Heh, yes, chemists definitely do some strange things. That said, physicists do truly awful things with chemicals so it's probably mutual.

author: penfold
author_rank: Frequent Contributor
timestamp: February 09, 2021, 08:30:28 pm
content: The case may be purely practical as means of keeping track of many many "pairs" and avoid half the tangling problems running them separate, also helps keep things consistent in terms of stray inductances.As for your actual question.. what did you have in mind in terms of possible ways of connecting the twisted pairs? I'd suggest doing it "neatly"!Don't be too afraid of untwisting them quite a way if you need to in order to negotiate a strange route round the box, the resistances of 100k and 10k are significantly higher than any parasitic inductance you'd introduce by doing so and I guess the currents are way too small to cause any coupling effects.

author: ezalys
author_rank: Frequent Contributor
timestamp: February 09, 2021, 11:03:48 pm
content: I was wondering which connections should be paired. Do I pair V_d1 and V_g3, and then V_d3 and GND, and so on?

author: penfold
author_rank: Frequent Contributor
timestamp: February 09, 2021, 11:24:00 pm
content: Quote from: ezalys on February 09, 2021, 11:03:48 pmUsual good practice would be to pair any "signal" with its return path (presumably ground). So V_d1 and ground, V_g3 and ground... etcYou could theoretically get away with pairing any arbitrary combination so long as they were purely stable and noise free DC sources, if they're not then you'd end up coupling any noise that's present on one to the other... hence why you would normally pair with ground.

author: JohnnyMalaria
author_rank: Super Contributor
timestamp: February 09, 2021, 11:37:59 pm
content: I'm a physical chemist so I'd better not show you what I do thenLet's just say that what I build shouldn't even work according to the experts but it works better than anything else out there.

author: penfold
author_rank: Frequent Contributor
timestamp: February 10, 2021, 12:42:23 am
content: Quote from: JohnnyMalaria on February 09, 2021, 11:37:59 pmDo show! I do tend to make a lot of jokes about things I've seen scientists do with electronics (apart from using a soon as a screwdriver) but they're rarely serious. It is interesting though to see how problems are approached from either an engineering or scientific perspective, to the opposing discipline it can look horribly hack-ish or just darn perverse.When I worked an an electronics designer in a mass-spec company, there were definitely a lot of "produce-ability" problems stemming from physicists not considering the more practical side of grounding (I'm not attacking physicists, its just a very unbalanced anecdote), like assuming a 6 foot stainless tube is equipotential even when its used as the return for a very fast pulse... the grumbling looks you get when you say "I can fix that if ya want... it'll even get you a faster rise time on that pulse... and it won't change so much when the weather does.. and your resolution specs will be somewhere closer to what you said they were in xyz journal!"

author: David Hess
author_rank: Super Contributor
timestamp: February 11, 2021, 01:33:25 pm
content: A twisted pair, parallel pair, or coaxial line all perform the same function here of minimizing the loop area between the signal and return path which reduces external coupling. I have built point-to-point circuits on perforated board using the same idea which operated at 100 MHz and above without a ground plane.