Low noise triax cable conductive insulation layer?

[ivaylo]

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!

[skipjackrc4]

I don't know, but I'm interested in finding out.

[robrenz]

Here is a method I used in this video. This link starts at the point where I am cutting the cable
http://youtu.be/o13e5LWsPTI?t=21m47s

[ivaylo]

Oh, I know that video by heart Thanks 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?).

[Richard Crowley]

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.

[robrenz]

Oh, I know that video by heart Thanks 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?).

I 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.

[ConKbot]

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.

Yup, 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 cables http://www.pcb.com/spec_sheet.asp?model=030#.U7P0dLGN_LA  The 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

[ivaylo]

This is what the seller says:

That's the low noise treatment. I make cables with this almost everyday at work and use a scalpel. You should use something similar and not try to cut it with strippers. It's a very thin layer, so be careful not to cut into the insulation underneath.

I make a line with a knife around the base of the coating - barely apply pressure - and then a vertical line to the tip and peel it off. Just make sure there aren't any small or loose pieces left on the cable.

This will test my patience for sure  Still 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.

[ivaylo]

More on the origin of the cable:

The manufacturer is Wirecraft in Boston. My company has it made to our specs. This lot, however, had a higher leakage rate than we can use. It IS within the specs in the listing, though.

Maybe it's not the right cable for this project to begin with 

[TerraHertz]

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 191216654228
You can read the details on the roll label, which seems to be a reseller's:
 Quick Wire Pa
 CNMC Company, Inc.
 Po #94651  Lot #143093  Mfg.12/09
 E29SOL-1CSTJSFJ-LO NOISE "ETCHED"
 2009/95/EC RoHS  P/N 3CABLEW
 Total 305 Mt.

[ConKbot]

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

[TerraHertz]

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

Here'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?

[ConKbot]

http://www.pasternack.com/10-32-female-standard-thru-hole-pcb-connector-pe44360-p.aspx

I 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.   

[TerraHertz]

http://www.pasternack.com/10-32-female-standard-thru-hole-pcb-connector-pe44360-p.aspx

I take that back about them looking too coarse.

Yep 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.

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.

Apparently 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.

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.

I can imagine. When did three parameters ever have an optimal solution of all three together?

  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.

At 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?

[ivaylo]

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:

The rate is 1 x 10(-14)/m, so it should be fine. I just looked up the SMU and this cable will definitely work. The problem we had with it, is that it takes 5-10 minutes to settle down for a correct measurement for our specific products. They are much more sensitive than the Keithley source meter.

Another tip - I don't know what triax connector you're using, but if the nut is larger than the diameter of the cable, put a small piece of shrink tubing on it. Put the nut on the cable first, then the heat shrink, then the first gasket. After you cut the outer layer of insulation, slide the shrink just beyond the the insulation and shrink it. That will help prevent the braid wires from breaking off and makes the connector less likely to pull/move around.

Not sure what dimensionality "1 x 10(-14)/m" is in V/m, A/m, something else? And rate of what is that?

[ConKbot]

Yep 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.

#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.

Apparently 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.

The 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. 

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?

LMR 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, etc
http://www.timesmicrowave.com/cms/products/cables/lmr/

[guenthert]

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!

I 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.

[1audio]

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.

[Assafl]

Oy vey...

Now I understand why the electrometer guard kept zapping me all day long.... 

[3roomlab]

i ran into this webpage describing tribo effects and charges ... very interesting
https://www.trifield.com/content/tribo-electric-series/

captured the pic for ref

[Smith]

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.

[BD1QMP]

Today， I got 2 meters  low noise triax cable with 2 black layers. not easy to deal with.

[MegaVolt]

Today， I got 2 meters  low noise triax cable with 2 black layers. not easy to deal with.

What brand is it?