triaxial probes for electrometers? - Page 1
Forum section: electronics / testgear

author: sarepairman2
author_rank: Frequent Contributor
timestamp: August 23, 2015, 04:28:32 pm
content: I have a EM with a triaxial connector on it. I made one ghetto probe using triaxial cable, teflon strain relief block and some teflon wires with aligator clips (so the shield of the triax starts to float inside of the teflon strain relief block).The keithley electrometer probes I see on ebay use some kind of UHF connector, meaning you would need a triax to UHF converter, losing your triaxial capabilities.so, whats the deal with electrometer probes?edit: i found the entire keithley manual, I had a condensed version before for my unit.I made the top one using the teflon block cut in half with some groves filed into it + screws holding it together. Easy enough to do for around 30-40$ (teflon block 8$, triax teflon cable like 10$, triaxial connector 15-25$, teflon wire.. 2$, aligator clips, 2$)Where the hell do the low leads connect to on ANY of these probes though. Is it like a oscilloscope probe with a little conductive band that you clip a C clamp thingy to?When is it appropriate to use the triaxial probe vs the UHF "bnc" type probe?What input impedance is the conventional one good for (vs the guarded 10^14th probe)

author: robrenz
author_rank: Super Contributor
timestamp: August 24, 2015, 01:24:04 am
content: You might get some ideas here from the triaxial lead system I made for my SMUhttp://youtu.be/o13e5LWsPTI

author: Richard Crowley
author_rank: Super Contributor
timestamp: August 24, 2015, 01:43:22 am
content: The inside conductor is the input (or output) to the instrument. The outer shield is ground. The inner shield is a driven guard. At the same potential as the inner conductor, but driven with a low impedance to minimize leakage in the inner insulation/dielectric.I developed an IC probe card with driven probes using the same principle.https://www.google.com/patents/US4983907

author: vindoline
author_rank: Unknown
timestamp: August 24, 2015, 05:27:05 am
content: Hi Sarepairman2, What model of electrometer do you have? I ask because I picked up a Keithley 616 a while back and the triax cable issue has been a PIA that's kept me from using it much so far. The images you posted look like they're right out of the 616 manual. There are a couple of things to know. First off, at some point in the past, the triax connectors changed from a "2-lug" design (like regular BNC) to a "3-lug" design. Most of the cables and connectors I see on eBay are the "modern" 3-lug variety. They won't work on the 616. Secondly, on the 616, the triaxial connections aren't as robrenz and Richard suggest. The outer shield is connected to the "chassis ground." The inner shield is called "Lo" in the manual and connects to the power supply ground. The electrometer circuitry is "floating" and not tied to the chassis or referenced to mains earth (unless you tie it together on the back terminals). The center conductor is obviously the high impedance input. I'd love to see some pictures of your homemade probe.

author: TimFox
author_rank: Super Contributor
timestamp: August 24, 2015, 11:34:35 pm
content: I believe Keithley abandoned the "two-lug" triax connectors because too many university students forced a BNC connector onto the panel jack and ruined the guard connection.Keithley made a UHF to two-lug adapter (often available on eBay) which only works if the guard is grounded.

author: Jim Narem
author_rank: Contributor
timestamp: August 28, 2015, 12:54:11 am
content: On my Keithley 616 I use a triax-M to triax-M cable, a pomona 5299triax-F to BNC-M adapter and a pomona 3788 BNC-F to clip leads. The triaxcable was surplus and 3-lug so I simply filed off one of the lugson the Keithley triax input(!). In the back of the 616, I have the LO toGround connected via the shorting strip. I generally don't do measurementsthat need a guarded enclosure........jim (narem at narem dot com)

author: sarepairman2
author_rank: Frequent Contributor
timestamp: September 03, 2015, 06:18:23 am
content: my probe is some ghetto shit and I will only take pictures if I get a milling machine and make a nice one, my instrument is the 616.any idea what the UHF connector probe (shielded, unguarded) is good for (impedance wise)?I did notice that in the datasheet as you said the LO is just the ground of the dmm, rather then an active guard.Do more modern electrometers use mains earth reference on the outermost shield ? Do they have quadaxial cable, earth ground outer, return, active guard, probe?I'm guessing that the active guard allows more digits (the 616 is not particularly high resolution)? Or do modern electrometers have much higher input impedances? IIRC I checked and there is no particular difference.