Triax RAQ

[MaxFrister]

I just finished repairing a pair of Keithley 236 SMUs (https://www.eevblog.com/forum/repair/keithley-236s-repair/).  As such I’d like to test, compare, and calibrate them.  To do so, obviously, I need to connect to them.  Therein lies the problem.

Like many precision current devices these machines these machines require triax cables.  Each takes 3 cables, 6 total, and at $150+ each we’re starting to talk real money.

So I’ve been trying to figure out triax.  This is the document I wish I found when I started.

Keep in mind that I’ve been a triax “expert” for about 3 days.  Feel free to jump in with corrections and clarifications.

Triax Rarely Asked Questions

Q: What is triax cable?
A:  There are variations but the important one has a center conductor surrounded by an inner shield and then another outer shield.  Sort of like coax twice.  The innermost conductor is the signal, the inner shield is the “guard”, and the outer shield is typically earth.

Q: Why triax?
A: When trying to source or measure in the sub-nA ranges, electrical noise in the environment becomes a big problem.   Triax cables are engineered to have nearly complete shielding coverage and with high quality insulation and outer jacket to minimize noise pickup.  In addition to shielding from EM interference, it also minimizes charge transfer in the cable when it is moved (triboelectric effect).

Q: What is the guard signal?
A: On the 236 the guard is a current-limited and isolated copy of the voltage on the signal conductor.


Q: How does the guard shield work?
A: First it is just another shield for any remaining em noise that gets past the outer shield.  Second, and more importantly, by surrounding the signal with a shield at the same potential, it protects the signal from capacitive pickup from the outer shield.

Q: Is there a cheap and easy source for triax cables?
A: No.

Q: Why so expensive.
A: Triax cable and connectors are low volume and precision manufactured out of the highest quality materials.  They are also mostly used in military and aerospace applications where price is rarely mentioned.

Q: So can I just use BNC cables?
A: Yes, but with some important caveats.  The inner pin on a bnc plug is larger than that on a triax plug.  Even if it seems to fit it will distort and potentially ruin the expensive triax socket.  You can buy an adapter that adapts from Triax M to BNC F and then use BNC cables.   
These adaptors typically short the inner and outer shields.  On the 236 that means that the guard is shorted to earth.  That’s why it is important that the guard signal is current limited (to +/-2mA on the 236).  To use BNC cables short the signal low to earth as well.

N.B., you lose all the careful engineering to minimize noise and will not be able to accurately source or measure the lower ranges of current.

Q: So earlier you implied that there were different kinds of triax cables.  What are they?
A: Early triax connectors have two lobes and look at first glance like a BNC connectors.  However connecting to BNC ruins the triax parts.  So later triax connectors have 3 lobes to distinguish them. 

Some triax cables used in other applications are actually “twinax”, a twisted pair of inner conductors and a single shield.  That isn’t what is needed for test equipment but they look physically identical from the outside.

There are the usual quality differences.  The inner pin on a triax is even smaller than BNC and delicate.  On good quality connectors it is a bronze alloy.  On cheaper ones it is made of recycled coke cans.  Good quality cable has nearly complete shields and high quality insulation, cheap ones don’t.

The connectors come in crimp and bolt-on versions.  Obviously the bolt-on ones will be preferable for DIY construction.

Finally there is the usual impedance issue with 50 and 75  ohms being common.  For test equipment 50 ohms is recommended. [The 236 is basically a DC machine and I’m not sure how important impedance matching is.]

[alm]

Q: Why triax?
A: When trying to source or measure in the sub-nA ranges, electrical noise in the environment becomes a big problem.   Triax cables are engineered to have nearly complete shielding coverage and with high quality insulation and outer jacket to minimize noise pickup.  In addition to shielding from EM interference, it also minimizes charge transfer in the cable when it is moved (triboelectric effect).

Some triaxial cables are constructed with dielectric and lubrication to minimize triboelectric/piezoelectric noise, but not all. Triax is also commonly used for RF, like Belden 9222, with no such features. This is less important if you can prevent any motion in the cable by for example sticking it to the bench.

The second thing is that the second shield is mostly for safety. Inside instruments coax is commonly used, with the shield at guard potential. For example the Keithley 236 uses a coax cable with an SMB connector internally to the triax jacks. But this means the shield may go up to 110V (1000V for Keithley 237), so for safety purposes a second shield at ground level is added externally.

Q: So can I just use BNC cables?
A: Yes, but with some important caveats.  The inner pin on a bnc plug is larger than that on a triax plug.  Even if it seems to fit it will distort and potentially ruin the expensive triax socket.  You can buy an adapter that adapts from Triax M to BNC F and then use BNC cables.   
These adaptors typically short the inner and outer shields.  On the 236 that means that the guard is shorted to earth.  That’s why it is important that the guard signal is current limited (to +/-2mA on the 236).  To use BNC cables short the signal low to earth as well.

There are various versions of adapters by Keithley and Trompeter. Some short inner and outer shield, some connect BNC shell to outer shield and leave the inner shield open, and others connect BNC shell to inner shield and leave the outer shield open. The latter one should be used with care for the same reasons as mentioned above that the full guard potential will be exposed, but should have a lower leakage than either of the other options. I have personally only used the version with shields shorted for instrument with a passive guard.