Characterizing High Impedance Cable? - Page 1
Forum section: electronics / projects

author: Andrew_K
author_rank: Contributor
timestamp: July 01, 2022, 03:10:40 am
content: Hi All,I am trying to characterize some specialty cable. It is NOT coax, which is making research on this topic very difficult for me. (I can only find things about transmission lines / coax)As for the cable, it is about 10kOhm per foot. Each wire consists of a spiral of wire around a plastic core (for the high R).There are 6 individual wires in the cable, separated by insulation only. There is no twisting of pairs. Differential measurements are taken from each pair.This is great for DC, but there is unknown impedance at higher frequencies.My end goal is to make a model of this wire for use in LT spice. I'm not sure if this can be considered a lossy Tline in terms of modeling, but R is per-length.I do have measurements from a S21 measurement (high impedance active probe), as well as measurements from an impedance analyzer.I have S21 mag and phase (from VNA), as well as Z mag/phase (from impedance analyzer). However, I'm not sure how to apply these for a model, other than using matlab tfest to get a transfer function.Has anyone done similar?Any recommendations on textbooks or app notes that would help me better understand this?

author: Terry Bites
author_rank: Super Contributor
timestamp: July 01, 2022, 09:28:23 am
content: Its likey going to bee seen as an open circuit by the vna.Terminate the line with a known impedance and measure the Reflection Coefficient, Г.You need a scope, a function generator and some basic algebra.You want a load of the same order of magnitude.SeeMetamaterials - 2008 - Munk - Appendix C How to Measure the Characteristic Impedance and Attenuation of a Cable.pdf(91.41 kB - downloaded 79 times.)

author: mag_therm
author_rank: Frequent Contributor
timestamp: July 01, 2022, 11:57:39 am
content: If 10kOhm /ft it must be made of some resistive material ?For a transmission line to form and energy to travel over wavelengths, the primary constants per length have to bejWL >> RjWC >> Gif R >> jWL, the wire will just look like a resistor.

author: Andrew_K
author_rank: Contributor
timestamp: July 01, 2022, 02:23:15 pm
content: It's like a super long wire wound resistor. Which makes me think it should be inductive.There's definitely some complex impedance that I'm measuring

author: gf
author_rank: Super Contributor
timestamp: July 01, 2022, 09:27:57 pm
content: Mybe this helps:https://www.edaboard.com/threads/extracting-rlgc-values-from-network-analyzer-s2p-files.168869/( I don't feel able to assess the correctness of this information, though. )

author: Benta
author_rank: Super Contributor
timestamp: July 01, 2022, 09:52:59 pm
content: Sounds like an ignition cable for a petrol engine spark plug.

author: Andrew_K
author_rank: Contributor
timestamp: July 01, 2022, 11:59:07 pm
content: Quote from: Terry Bites on July 01, 2022, 09:28:23 amI ended up using an impedance analyzer to get the measurements. I did find a way to measure characteristic impedance with the short/open method.Quote from: gf on July 01, 2022, 09:27:57 pmInteresting! Unfortunately, I can't really get *full* 2-port measurements, as the impedance is not anywhere near 50 ohms.And making a balun sounds painful.That all being said, I do have the characteristic impedance and a few other parameters (Alpha, Beta).Given that this is a lossy T-line, not a lossless one,I'm really struggling to find equations where I could solve for the per-length C and L given what I have.Maybe I need to take additional measurements...

author: T3sl4co1l
author_rank: Super Contributor
timestamp: July 02, 2022, 05:10:02 am
content: How much does it matter, anyway? What are you using it for?It sounds like the resistance will be so high, inductance is utterly negligible, and capacitance can be measured at low frequencies, and using a lossy (RC) t-line model should suffice. And indeed, making a balun for that impedance is only going to work in a narrow bandwidth -- over which you probably won't be able to measure much inductive reactance.Tim

author: Bud
author_rank: Super Contributor
timestamp: July 02, 2022, 05:24:35 am
content: There is no point to measure L because R will dominate in the impedance.

author: gf
author_rank: Super Contributor
timestamp: July 02, 2022, 01:38:03 pm
content: Quote from: Andrew_K on July 01, 2022, 11:59:07 pmA RLGC transmission line model is completely described by the parameters gamma(f) and Zc(f), where gamma(f) = alpha(f) + j * beta(f).So it seems you have everything you need.For a lossy TL, alpha is nonzero and Zc is not real, but complex.If you need R,L,G,C instead of gamma and Zc, then I found a conversionhere. If I'm not mistaken, this should be equivalent toR(f) = real(gamma(f) * Zc(f));L(f) = imag(gamma(f) * Zc(f)) / (2*pi*f);G(f) = real(gamma(f) / Zc(f));C(f) = imag(gamma(f) / Zc(f)) / (2*pi*f);when numLines=1, i.e. for a single two-port transmission line.

author: Andrew_K
author_rank: Contributor
timestamp: July 02, 2022, 06:31:14 pm
content: Quote from: T3sl4co1l on July 02, 2022, 05:10:02 amI am trying to characterize this cable because the parasitics cause distortion. Changing the cable is a last resort, and might not even work.If I can characterize it properly, and simulation reasonably matches, I can determine if compensation is feasible.Some experimentation shows that compensation may work, but I'd rather not brute-force a design by trial-and-error on the bench if I can help it.I'm not 100% positive that inductance would be negligible, since that is not the case for wire-wound resistors.Quote from: gf on July 02, 2022, 01:38:03 pmThis looks promising, and this is what I was having trouble finding. Thank you very much!

author: Stray Electron
author_rank: Super Contributor
timestamp: July 02, 2022, 07:37:29 pm
content: Quote from: Benta on July 01, 2022, 09:52:59 pmThat was my first thought too. But not with six conductors. And with six conductors insulated from each other then it must be for some kind of low voltage service or it would arc over.

author: mag_therm
author_rank: Frequent Contributor
timestamp: July 02, 2022, 07:46:33 pm
content: When all else fails you can calculate inductance L/metre directly, as you can see the spiral.dia of spiral d Area of spiral A [metre^2] = pi*(dia^2)/4Turns /metre can be countedL/metre = 4*pi*1e-7 *(N^2) A/ 1 [Henry/metre]Very sure you will find R/metre , which you already measured is about 30000 Ohm/metre, will be >> | jW L |As a <GUESS> ran <20 T/cm> = 200 T/mspiral dia = <6mm> so A = 28.27e-6 m^2L = 1.4 microHenry /metre@ 1 MHz jWL = 8.79 Ohm/metre ........100 MHZ jWl= 879 Ohm/m 1000MHz jWL=8790 Ohm/m..... but check my math

author: T3sl4co1l
author_rank: Super Contributor
timestamp: July 02, 2022, 09:46:14 pm
content: Quote from: Andrew_K on July 02, 2022, 06:31:14 pmDistort what? And "distortion" in what sense? To me, it's a linear element which therefore contributes no distortion, in the unqualified sense.Tim

author: mag_therm
author_rank: Frequent Contributor
timestamp: July 02, 2022, 10:32:52 pm
content: Sometimes the R and G primary constants were referred to as "parasitic transmission line losses"I just saw reference to it in skin effect loss of microstrip lines.The losses did cause distortion 100 years ago when thay were directly transmitting key pulses and baseband audio, because the distortionwas, in phase and attenuation, frequency dependent. Telegrapher's Equation solves this.Distortion like this was not such a problem when baseband was converted up on a carrier with relatively narrow bandwith.Maybe this will arise again with the new spread spectrum direct sequence modes. -high frequency data over a wide bandwidth.

author: Andrew_K
author_rank: Contributor
timestamp: July 02, 2022, 11:18:26 pm
content: Quote from: T3sl4co1l on July 02, 2022, 09:46:14 pmIf I put a pulse through the circuit, it looks like there was a LP filter applied to it. But modeling a simple 1st order filter does not match what is observed in the TD.The frequency response also looks different.Looking at my data, the phase of the Z0 changes dramatically with frequency. It makes some of the parameters- R' C' L' G', change rapidly - positive and negative, throughout the sweep. They do settle at high frequencies, but the values obtained don't give good results in Spice.I'm thinking I may need to change my measurement procedure.An impedance analyzer is 1-port only. VNAs are 2-port, but at Z0=50.Is there any sort of 2-port network analyzer that is meant for high-impedance with other parameters? ABCD for example?

author: Bud
author_rank: Super Contributor
timestamp: July 02, 2022, 11:37:06 pm
content: Quote from: Andrew_K on July 02, 2022, 06:31:14 pmThat is true for low Ohm resistors, up to couple hundred Ohm. But you have 10K resistance. It will swamp any inductive impedance.

author: Andrew_K
author_rank: Contributor
timestamp: July 03, 2022, 12:06:42 am
content: Quote from: Bud on July 02, 2022, 11:37:06 pmYou and Tim are probably correct, then.Any suggestions on actually measuring this? The Open/Short method didn't seem to give good results.Would I get a good measure of C given that it is so resistive?

author: ejeffrey
author_rank: Super Contributor
timestamp: July 03, 2022, 12:17:00 am
content: For a transmission line you need signal and return. Are you using a separate return wire / plane or are some of the 6 wires used for the return?

author: Andrew_K
author_rank: Contributor
timestamp: July 03, 2022, 01:11:28 am
content: I used the open/closed method. I took two wires of a "pair". Measured across one "port", kept the other side of the cable open/short.For reference, this is in section 5.9 of the impedance measurement handbook by Keysight.I understand that this is technically a "balanced" cable, and I'm taking an imbalanced measurement. But I'm not sure how significant that is, given the impedance mismatch of the "UNKNOWN" ports would be very small w.r.t. the impedance of the cable being tested.I looked at wide frequency baluns, but they're all 50 ohm impedance.For reference, these are the measurements and obtained parameters.AndThe angle of the measured Z has a fairly large range. And it jumps by a significant amount, even when "settled". Units are degrees...

author: T3sl4co1l
author_rank: Super Contributor
timestamp: July 03, 2022, 04:14:42 am
content: Even if the cable has a large height above ground plane, it will still be in the vicinity of low 100s ohms, common mode. That is, driving one line, alone or even with multiple remaining lines grounded, will end up with the common mode driving over GND, while the differential mode(s) within the cable decay more rapidly due to the higher capacitance between them.So, you have a situation that varies with frequency and position/length. High frequencies are absorbed quickly, so that only the nearest length behaves as a differential transmission line, and the remaining fraction of the input signal (which divides between the driven line and ground line(s) according to their relative impedances) follows the common mode path, which similarly decreases over distance, but should be slower than DM as mentioned, so you should observe two cutoff frequencies for DM and CM. And since DM decays faster, you can measure the CM on any wire at the far end, driven or otherwise.At low frequencies, at some point, DM will be perceptible at the far end, and CM will be less significant. Obviously, at DC it's straight through, GND is GND and driven is driven.A single pole RC should not give a good fit, but a chain of arbitrary length, with equal values of R and C, should give a good match with some adjustment. It will diverge at high enough frequencies, as a lumped-element RC is only order N, while your cable is distributed, giving I think a cutoff probably something like exponential with frequency, the roll-off point depending on length.Remember to model capacitance between wires in the cable, and from the cable (from each wire) to GND. GND can be assumed ideal. Set R from the DC measurement (resistance of the length, divided by the number of segments being modeled), and adjust the C's until the frequency response matches observation.Your plots look rather noisy, which suggests using too long a length and too high a test frequency range. s12 should be sufficient, or you can do a 4-port with ports 1 and 2 as +/- source and 3 and 4 as +/- load, measuring s13, s14, s23, s24.Tim

author: David Hess
author_rank: Super Contributor
timestamp: July 03, 2022, 04:29:54 am
content: Are the conductors all wound in the same direction? Delay line cable has wound conductors but they are wound in opposite directions. I am thinking it might be low frequency delay line cable which is not going to behave sanely at frequencies above its cutoff.