Low noise triax cable conductive insulation layer? - Page 1
Forum section: electronics / testgear

author: ivaylo
author_rank: Frequent Contributor
timestamp: July 02, 2014, 03:48:12 am
content: So I buy some of this triax cable -http://www.ebay.com/itm/10m-33-ft-Low-noise-Triax-Cable-50-Ohms-RF-Medical-aerospace-camera-NEW-/201098156225. All looks good and everything, low-noise, teflon, blah, blah, blah... Tried mounting them a few different ways (triax connectors, crimping some pins to them, soldering directly to a PCB, etc.) until I notice that on some of my connections the resistance between the inner shield and the center wire measures ~50-100KOhm. Drove me crazy... I disassemble the connection and all is back to insulating properly.Turns out the inner most insulator (around the center wire) consists of two layers - black semi-conductive one and a white proper insulator (see pictures). And the black stuff is not a tubing kinda thing which you can just cut and slip off, it's rather molded on top of the real (white-ish) insulation bellow.So my question is - what would be the proper technique to strip/mount these cables? I had a moderate success by sanding off the black stuff with a Dremel felt tip tool (second picture) but that is clumsy. How would you strip the center wire such that it's guaranteed the black stuff never touches it (or the connectors attached to it)? Or how would you strip the black stuff only? Asked the seller for a spec or application note for this cable but still no reply...Appreciate your help!

author: skipjackrc4
author_rank: Regular Contributor
timestamp: July 02, 2014, 03:54:37 am
content: I don't know, but I'm interested in finding out.

author: robrenz
author_rank: Super Contributor
timestamp: July 02, 2014, 04:10:40 am
content: Here is a method I used in this video. This link starts at the point where I am cutting the cablehttp://youtu.be/o13e5LWsPTI?t=21m47s

author: ivaylo
author_rank: Frequent Contributor
timestamp: July 02, 2014, 04:33:31 am
content: Oh, I know that video by heartThanks for posting it btw, I am doing a similar project for a two channel SMU, will hopefully post results soon. My cable is much thinner though. Center wire with the insulation (black+white under) is only 0.9mm, with the black stuff peeled off it's 0.85mm. So how do I get rid of the black thing only? What I missed on your video is that your cable also has this black layer and I am going to guess now that it was conductive too (did you measure it by any chance?).

author: Richard Crowley
author_rank: Super Contributor
timestamp: July 02, 2014, 05:36:21 am
content: It is that black conductive coating on the inner dielectric that is the major contributor to the "low noise" performance.Do you have a commercial brand/part number for the cable? Can you Google recommended compatible connectors?You can work from there how they intended to terminate this stuff.

author: robrenz
author_rank: Super Contributor
timestamp: July 02, 2014, 11:44:03 am
content: Quote from: ivaylo on July 02, 2014, 04:33:31 amI did not measure it but I don't think it was conductive. There are some cables that use a graphite coating to lube the inner shield against the inner conductor insulation. This needs to be removed thoroughly with methanol when terminating according to the Keithley low level measurements handbook.

author: ConKbot
author_rank: Super Contributor
timestamp: July 02, 2014, 12:12:53 pm
content: Quote from: Richard Crowley on July 02, 2014, 05:36:21 amYup, its what makes the cable low triboelectric noise. The conductive layer is tightly bonded to the insulator, so no TE noise can be generated between that and the insulation. Since the braid and the conductive layer are both conductive (obviously...) no TE noise is generated when the cable is flexed, and the braid and insulation move slightly relative to each other.PCB piezoelectric uses liquid graphite (as mentioned by robrenz) on their low TE noise charge mode gauge cableshttp://www.pcb.com/spec_sheet.asp?model=030#.U7P0dLGN_LAThe gauges have a micro/pico coulomb per engineering unit output usually, so the capacitance needs to be stable. No extra shield as its not down at the fA sensitivity level like SMU's

author: ivaylo
author_rank: Frequent Contributor
timestamp: July 02, 2014, 04:11:26 pm
content: This is what the seller says:QuoteThis will test my patience for sureStill there must be a more "commercial" way of doing this where they do this a thousand times a day. But it's probably some big ass stripping machine or something (or a factory floor with Chinese children)... Seller did not reply with manufacturer/part number.

author: ivaylo
author_rank: Frequent Contributor
timestamp: July 02, 2014, 09:57:42 pm
content: More on the origin of the cable:QuoteMaybe it's not the right cable for this project to begin with

author: TerraHertz
author_rank: Super Contributor
timestamp: July 03, 2014, 04:09:16 am
content: Ah ha! Very informative. I've never had any encounters with triax cable, but I do need some for a project.The main problem I have is I don't know where to find mating connectors for the weird ones on the back of the bit of gear I have.It's a HP 3567A 3567A FFT analyzer, with 35652B plugins. Pic of the connector:Anyone?That same cable is available from the same seller in original manufacturer's 300m rolls, for $120. Ebay 191216654228You can read the details on the roll label, which seems to be a reseller's:Quick Wire PaCNMC Company, Inc.Po #94651 Lot #143093 Mfg.12/09E29SOL-1CSTJSFJ-LO NOISE "ETCHED"2009/95/EC RoHS P/N 3CABLEWTotal 305 Mt.

author: ConKbot
author_rank: Super Contributor
timestamp: July 03, 2014, 05:51:05 am
content: The little nubbin on the center there looks almost like a 10-32 connector that PCB uses for charge mode gauges (see above), though the threads look a bit coarse. Ive also heard it called a micro-dot connector

author: TerraHertz
author_rank: Super Contributor
timestamp: July 03, 2014, 07:55:05 am
content: Quote from: ConKbot on July 03, 2014, 05:51:05 amHere's a better pic.You're probably right, for standard coax. But it looks like it's also possible to use triax, with an outer shell screwing onto the large thread. Maybe.Of the four 35652B modules, one appears to have never been used - since it still has a shorting cap and plastic dust cover on top of that. There is no way the previous owners of this unit wouldn't have lost those too, if they ever removed them. The person who gave me this unit told me his company was just about to bin the entire thing. In fact they actually already had binned the manuals, but he fished them out of the dumpster (as loose pages) when he found out that for some reason I'd like them. Fortunately not one missing page, and I got the original binders too. I spent an unusual afternoon in my car in a park in Ballarat sorting out the pages into the right binders. Too windy outside to do it on a park bench, and I wasn't going to drive off till I knew whether it was all there.The original 720K system software floppy disks were included, and much later the Win 3.1 PC with the HPIB interface card turned up too. Also on their way to the bin. So I had all the manuals except for the Installation Guide - but later was able to buy one.In all the 5 volume set of manuals, despite having multiple pics of the back of the machine with those connectors visible, there's not one mention of what they are.Perhaps that's in other manuals for the modules. Still searching...Edit:Sigh. Despite having all the manuals mentioned in this:Just found this pic of manuals in a listing of a 3567A for sale:Middle one on the left; HP 35??? System Reference. Don't have that one. Pity the numbers are illegible.Dammit! It's only 20 year ago, why is this stuff so lost?

author: ConKbot
author_rank: Super Contributor
timestamp: July 03, 2014, 11:44:42 am
content: http://www.pasternack.com/10-32-female-standard-thru-hole-pcb-connector-pe44360-p.aspxI take that back about them looking too coarse.And given that the dust cap only covers the center bit, I'd be sure that the thread on the outside are just for the jam-nut. The shield of the coax floating prevents ground loop issues as its only grounded at one end, and the jam-nut lets you remove the bulkhead mount jacks without having to unhook them from the PCB.If youre using charge mode gauges, definitely get the low-noise cable. And remember that low TE noise isnt low-loss or low EM/RFI noise if the issue ever comes up. At work we were using a ICP/IEPE mode gauge that had a 10-32 connector on it in a electrically noisy environment, and someone though the low-noise cable would be the perfect solution for that. Nope. Did a comparison test, and it was worse than normal RG-58, and LMR low loss coax blew both out of the water.

author: TerraHertz
author_rank: Super Contributor
timestamp: July 03, 2014, 12:44:57 pm
content: Quote from: ConKbot on July 03, 2014, 11:44:42 amYep that's them!~searches '10-32 connector' on ebay~Heh, and now I know why they are also called microdot, not '10-32 connectors'. Because there is a common plumbing connector called '10-32'.Goodness me, microdot != micro-price.QuoteApparently I have a lot to learn about grounding triax. I'd have grounded the outer braid at the receiving end. Um... never mind, I'll work it out.QuoteI can imagine. When did three parameters ever have an optimal solution of all three together?QuoteAt least in my intended application I don't have to learn about accelerometers and piezos.Ha ha I love it when google fails. OK, so LMR is just a cable class spec like RG? Or is it an acronym for something?

author: ivaylo
author_rank: Frequent Contributor
timestamp: July 03, 2014, 04:34:38 pm
content: Very nice eBay seller actually. Asked him if he doesn't have the full cable spec if he thinks cable is suitable for SMU cables:QuoteNot sure what dimensionality "1 x 10(-14)/m" is in V/m, A/m, something else? And rate of what is that?

author: ConKbot
author_rank: Super Contributor
timestamp: July 03, 2014, 05:15:25 pm
content: Quote from: TerraHertz on July 03, 2014, 12:44:57 pm#10-32 is a common screw size in the US, so I'm sure the results will be littered with irrelevant things. Quite poor choice in naming the connector.QuoteThe microdot cables (or the ones IVe seen) are generally only coax, not triax. But stuff gets a bit different when youre going 10's of meters rather than all on the same bench. If the shield of the coax were hard grounded to the chassis, and you hooked it to a an accelerometer attached to a motor housing, and the ground potential varied by a few hundred mV then you get a few mA flowing back and forth in the cable, possibly coupling into the center conductor, and also, your accelerometer output will have the 50/60 hz superimposed on it, and current flowing in and out of the center conductor as the outer conductor voltage changes, etc... a real trainwreck, ruining your low noise.Since the HP instrument pictured supports ~4V of floating the shield of the coax above ground, if you were hooking the same way, then the floating input varies just fine, and the only current flowing in the shield of the cable is caused by the capacitance of the instrument. It gives it a pseudo differential input (if its not an actual differential input)If you're doing an extended run above that, then switching to triax could be warranted, and you'd hard-ground the shield to the chassis on one end only to provide some EMI/RFI shielding. We used triax that wasnt low TE noise for ICP gauges that ran for 100+ meters. Though in the RF/EMI testing I did the LMR performed better than the triax, however stuff could be different frequencies, etc.QuoteLMR is times microwaves brand (and I think other competitors use it too? not 100% on that) of low-loss double shielded coax. It comes in different sizes, LMR-100 is usually a drop in replacement for RG-174/RG-316, LMR-195 is usually a drop in replacement for RG-58 (and by usually I mean depending on your connector design/tolerances) There is LMR-200 if you want the benefits of it being slightly better than LMR-195, but using completely different connectors, etchttp://www.timesmicrowave.com/cms/products/cables/lmr/

author: guenthert
author_rank: Frequent Contributor
timestamp: January 10, 2017, 02:27:12 am
content: Quote from: ivaylo on July 02, 2014, 03:48:12 amI wished I'd have seen this thread earlier (or rather wished I had looked for it). I made the exact same experience -- won't admit how long it took me to recognize that the black stuff is conductive.robrenz offered of course the right solution (*), but for those which are not quite as skilled, patient and well equipped the quick and dirty way is to leave it in place and just cover it with a piece of the 2nd insulation layer (the white stuff -- PTFE presumably). Make sure, that this piece protrude a bit further than the insulation of the innermost conductor with the conductive surface in order to avoid contact between that conductive surface and the connector. Works for me (>200GOhm for a 6 foot cable).*) Others suggested methanol. I haven't tried that but the black conductive stuff (graphite?) doesn't seem to dissolve in isopropyl alcohol.

author: 1audio
author_rank: Frequent Contributor
timestamp: January 10, 2017, 06:51:16 am
content: The black stuff is conductive rubber. its extruded onto the PE and usually sticks really well sometime you can score it and peel it off. Mostly it bonds to the PE (white stuff) and you need to abrade it off. The dremel tool may be the most practical for a few cables. its a real PITA but it works so people put up with it.I suspect the HP may use a driven shield for the inner shield to reduce the capacitance. The outer shield would be the real ground. The usual Triax connectors are quite different from those Microdot connectors.

author: Assafl
author_rank: Frequent Contributor
timestamp: January 10, 2017, 09:16:32 pm
content: Oy vey...Now I understand why the electrometer guard kept zapping me all day long....

author: 3roomlab
author_rank: Frequent Contributor
timestamp: May 25, 2017, 04:34:06 pm
content: i ran into this webpage describing tribo effects and charges ... very interestinghttps://www.trifield.com/content/tribo-electric-series/captured the pic for ref

author: Smith
author_rank: Frequent Contributor
timestamp: May 25, 2017, 05:21:18 pm
content: As far as I have seen all good triax has this graphite stuff or whatever it is against the triboelectric effect. Even the almighty Keithley triax cable, which is really great stuff. We recently used some Belden triax (the thick yellow stuff, cant recall the number) and it performs terrible.If anybody ever needs Keithley triax, instead of demolishing there ready-made cables you can also buy it from them per inch. I use it for nA to fA stuff.

author: BD1QMP
author_rank: Contributor
timestamp: April 30, 2024, 07:34:39 am
content: Today， I got 2 meters low noise triax cable with 2 black layers. not easy to deal with.

author: MegaVolt
author_rank: Frequent Contributor
timestamp: June 14, 2024, 09:52:13 am
content: Quote from: BD1QMP on April 30, 2024, 07:34:39 amWhat brand is it?