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Low noise triax cable conductive insulation layer?

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ConKbot:

--- Quote from: TerraHertz on July 03, 2014, 12:44:57 pm ---

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.
--- End quote ---
#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.



--- Quote ---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.

--- End quote ---
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. 




--- Quote ---
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?

--- End quote ---

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:

--- Quote from: ivaylo on July 02, 2014, 03:48:12 am ---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!

--- End quote ---
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.  |O

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).  :phew:


*) 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....  :palm:

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

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