Triaxial Cables for Keithley 236

[sixtimesseven]

Hi

I finally deceided to get a Keithley 236 SMU. But now, as so many before I'm looking into a reasonably priced Triaxial Cable option. Interestingly I haven't found a detailed Triax + Connector discussion. I might be wrong though in which case I kindly ask to point me to it.

So far I have found these PL75-9 and PL75-24A connectors which cost about 10$ each:


https://www.ebay.com/itm/Trompeter-PL75-9-Twinax-Triax-Connector-NEW/142352663506?epid=1245507767&hash=item2124e143d2:g:mYgAAOSwzqFY9S-S

https://www.ebay.com/itm/Trompeter-PL75-24A-Triax-Triaxial-Cable-Connector-Military-NSN-5935010754440/142261005467?epid=520802263&hash=item211f6aac9b:g:AUoAAOSwopRYjTfZ


I was looking for some Triax cable as well but couldn't really find something suitable. What I found so far are:

001 101 TRIAXIALKABEL LEMO / Low Noise / PTFE / 16Euro/m --> A bit pricy and too small in diameter. Maybe with the -24A though?
Source: https://www.koax24.de/online-shop/article/001-101-triaxialkabel-lemo-low-noise-d-340-mm-ptfe-temp-55c-bis-200c.html
Datasheet: https://www.koax24.de/fileadmin/download/datasheet/de/050206_Datenblatt_001101_Triaxialkabel_Lemo.pdf


Huber Suhner G_02330_HT --> Low (triboe.) noise, only 5.36Euro/m BUT with PE Dielectric - Should fit with some heatshrink / fiddling onto the PL75-9 or the PL75-24A
Source: https://www.koax24.de/online-shop/article/g-02330-ht-low-noise-triaxialkabel-50-ohm-d-53-mm-pe-temp-25c-bis-85c.html
Datasheet: https://www.repic.co.jp/pdf/g02330ht.pdf


On first glance I thought of going with the G_02330_HT. Isolation resistance is specified as 10^6 MOhmm. Although I'm not completely clear on what this number represents, is this from the center conductor to the surface of the outside jacket or from the center conductor to the second conductor  ?
Isolation voltage is with 1500VRMS also a lot higher than on the Lemo option.

What is your take on this? The resistance numbers I found so far on PE (lot of varieties) seem to be only slightly lower than on PTFE. Plus the PE dielectric is larger in diameter. The question is, what will the practical difference be on a 1-2m cable assembly?

Another option would be a normal, non carbon lubricated triaxial cable with PTFE or FEP shield.


Thank you for your thoughts
Andreas

[alm]

This topic has definitely come up, these are two I remember:
https://www.eevblog.com/forum/testgear/low-noise-triax-cable-conductive-insulation-layer/
https://www.eevblog.com/forum/testgear/cheap-triax-connector/

For the connectors, make sure it has the correct number of lugs (all modern products, and I believe most 236/237/238s, use three lugs, some older instruments and cables are two lugs), and that it is TRB (there are larger and smaller versions that definitely do not fit).

The Keithley Low Level Measurements handbook has some discussion of the performance of different dielectrics with regards to triboelectric noise etc.

[sixtimesseven]

Hi Alm

Thank you for your quick replay. I have seen those two posts already as well as Robrenz's videos on his measurement box which was very helpful. And the first post together with the Keithley Guide pointed me towards PTFE / Low Noise carbon lubricated cables.
Yes, the PL75 variant is a 3LUR TRB connector which should fit onto the Keithley 23x series.

[VintageNut]

Hi Alm

Thank you for your quick replay. I have seen those two posts already as well as Robrenz's videos on his measurement box which was very helpful. And the first post together with the Keithley Guide pointed me towards PTFE / Low Noise carbon lubricated cables.
Yes, the PL75 variant is a 3LUR TRB connector which should fit onto the Keithley 23x series.

If you build your own, you should use the 236 to characterize the cables. Force voltage and measure current with the cable connected and the termination open. Make sure to strap the LO to chassis at the instrument rear so that the shield of the 236 is at LO.

I have seen in person a DIY cable, built at a university, have horrible leakage from HI to LO. Probably dirt/grime/fingerprint on the insulator between HI and SHIELD at the connector. The cure was to cut off the connector and re-solder a new connector. The technician was advised to wear gloves and make sure everything was clean before and wile soldering the new connector. The new cable was leak-free.

I have created a demo for measuring insulation resistance of a piece of solid hookup wire with a SMU where the resistance is in the neighborhood of 25 T ohms. If you place a grimy fingerprint on the piece of wire, the measurement is destroyed. Wipe off the fingerprint and the measurement is restored.

This is one reason that I use authentic Keithley cables. They just work. They also have a conductive lubricant to reduce triboelectric effects.

 

[HighVoltage]

We had more discussions here in my Triax box thread

https://www.eevblog.com/forum/metrology/my-triax-cable-shielded-box-project-for-keithley-instruments/

I bought a few original Keithley TRIAX cables and modified them to my needs.

[martinr33]

Modifying ready made cables seems like the best approach. There are some on eBay at a reasonable price. For example, 4m for about $50. Not sure, though, if they have the carbon lubrication in the cable. I doubt it.

Or maybe, rather than cutting them up, make a Triax connector for the other end, with flying leads.