IDC cable as a transmission line?  - Page 1
Forum section: electronics / rf-microwave

author: LM21
author_rank: Regular Contributor
timestamp: August 05, 2023, 10:04:36 pm
content: How about using a pair of conductors from IDC cable as a transmission line. I think some have 100 ohms as impedance.

author: TimFox
author_rank: Super Contributor
timestamp: August 05, 2023, 10:10:20 pm
content: Some ribbon cables are sold with specified characteristic impedance.If you need to measure a sample, take a long length and measure the capacitance with the other end open, and the inductance with the other end shorted.Use a length long enough to get a measurable inductance on your LCR meter at its highest frequency.Z0= (L/C)1/2.This ignores resistance; see the literature for more details.If you have an impedance meter, there is a better formula.

author: LM21
author_rank: Regular Contributor
timestamp: August 05, 2023, 11:55:25 pm
content: First cable at Mouser had 83 ohms. I'll check later if there are any closer to 100 ohms. Twisted pairs of LAN cable should also have 100 ohms. I have a NanoVNA. So I'll just build a 4:1 balun and see how it works. Both have thin wires so they are probably not good for transmitting, but if I get 100 ohm transmitting line without problems, I'll be happy.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 06, 2023, 01:08:54 am
content: Quote from: LM21 on August 05, 2023, 10:04:36 pmA bit higher than that.Quote from: LM21 on August 05, 2023, 11:55:25 pmBe careful to check how it's measured. If they don't say, either run away (it's undefined!), or at least look for an appnote or something that does finally stipulate how it was measured.Typically ribbon cable is in the 120-150 ohm range differential (-SS- or GSSG), 80-100 single ended (GSG). "GSSG" means flank the signal pair (SS) with grounds. They could also be open ("-") but this allows more crosstalk between pairs, and you generally want more signals in less cable, and repeating GSSGSSG... is a reasonable enough way to go. They make ribbon cable for digital purposes, of course, not RF signal transmission; "low enough" crosstalk is say 10%. The impedances don't need to be especially tight either, just good enough to match say an LSTTL output, or RS-422 pairs (indeed, both were used in the archaic ST-506 hard drive interface!).The impedance also comes down if the cable is wrapped in shield (SATA cable is ribbon at heart, but has lower impedance for this reason).If you want just completely isolated single pair for undefined RF use, CAT5 is probably better for you, and is 100 ohm.You can also make somewhat thinner pairs from wire-wrap wire, if you need to save space (good for transformer winding, signal baluns, EMI networks, etc.). Thinner still from enameled / magnet wire, but the insulation is so thin, the Zo tends to be low (50-80 ohm).Tim

author: langwadt
author_rank: Super Contributor
timestamp: August 07, 2023, 04:18:07 pm
content: you can also get twisted pair ribbon cable, and there was also the half pitch ribbon cable with special connectors to ground every other wire used for ATA66

author: coppercone2
author_rank: Super Contributor
timestamp: August 16, 2023, 07:23:07 am
content: I am kind of thinking that for some runs of ribbon cable you could make a shield braid out of mesh thats spot welded.Probobly not a great shield but fine mesh can be had and its actually really easy to spot weld. Not many options without buying a few thousand dollars worth of wide braid.