BNC into triax - Page 1
Forum section: electronics / general

author: jrmymllr
author_rank: Regular Contributor
timestamp: August 23, 2022, 02:29:36 am
content: Dumb question here. I have a test instrument with a 2 lug triax jack, but no mating plug. What happens if one plugs BNC into a triax jack, mechanically?The outer and inner shell are already connected with banana jacks so if those short it's no problem. Is it an electrical problem if these are plugged together, or a mechanical issue? This is the first device I've had with triax and I seem to recall reading that you don't plug BNC into it, but I cannot find the reason why online.

author: martinr33
author_rank: Frequent Contributor
timestamp: August 23, 2022, 07:22:16 am
content: One approach is to try it. I would start with a 75 ohm cable, because it does not have the dielectric spacer sleeve that 50 ohm connectors have. The unit may also have some configuration options to switch between the outer shield and inner shield.

author: alm
author_rank: Super Contributor
timestamp: August 23, 2022, 08:20:47 am
content: Here's a picture of a two-lug triax connector and a 50-Ohm BNC connector side by side. Left is triax, right BNC. The center pin of the BNC connector looks substantially larger. I wouldn't want to plug that into something designed for triax.That's the reason why they later moved two triax with three lugs: to prevent people from destroying triax connectors by plugging in BNC plugs.The one modification you can do if you have only a three lug male connector is file off one of the lugs so the female connector fits both two and three pin. Obviously this is destructive, and you may experience intermittent contact.There's cheap triax connectors and cables from China on eBay these days. Performance may not be great. Possibly only three lug, I haven't checked.

author: MegaVolt
author_rank: Frequent Contributor
timestamp: August 23, 2022, 08:59:43 am
content: Chinese connectors are compatible with each other but are not compatible with proprietary ones from Keithley or Keysight. If you do your own thing - Chinese connectors are a good choice. If you want compatibility, you have to pay.

author: gamalot
author_rank: Super Contributor
timestamp: August 23, 2022, 09:44:47 am
content: Quote from: alm on August 23, 2022, 08:20:47 amThat's exactly what I‘ve done with my Keithley 6167 guarded adapter, buy a cheap Chinese triax cable, find a file, and Bob's your uncle.

author: alm
author_rank: Super Contributor
timestamp: August 23, 2022, 09:55:43 am
content: Quote from: MegaVolt on August 23, 2022, 08:59:43 amThe connectors used by Keithley and Keysight are not proprietary, they are standard TRB connectors as made by Trompeter and other major RF connector manufacturers. If the Chinese connectors are not a good match to the standard TRB connectors, then that's a problem. I have not used them, so I can't comment on the quality.

author: ch_scr
author_rank: Frequent Contributor
timestamp: August 23, 2022, 10:04:19 am
content: Quote from: alm on August 23, 2022, 09:55:43 amI beg to differ. I bought NOS Trompeter Triaxial plugs on Ebay quite a while a ago. They fit an old HP fixture well but are too hard to push on the Keithley 2636. Somehow the tolerances seem to be off. There was discussion about this with regard to Keithley units before, IIRC.

author: gamalot
author_rank: Super Contributor
timestamp: August 23, 2022, 10:13:28 am
content: Quote from: ch_scr on August 23, 2022, 10:04:19 amSometimes big names are not a guarantee of quality, I bought different batches of Pomona BNC cables of the same model at Digikey, and their connectors were some too tight and some too loose!

author: jrmymllr
author_rank: Regular Contributor
timestamp: August 23, 2022, 01:16:08 pm
content: This is on a Keithly, but only has 2 lugs. I have a bag of NOS RF connectors, lots of BNC, and I might try taking just the BNC pin of a connector and see how it fits into the triax jack, or if it's too tight.

author: mendip_discovery
author_rank: Super Contributor
timestamp: August 23, 2022, 04:36:54 pm
content: You can get triax to bnc converters and I think that would be a better option vs trying to get a bnc to fit directly onto it.https://www.digikey.co.uk/en/products/detail/pomona-electronics/5300/1762488I have bought a bunch of triax connectors as I plan to make a test fixture plus some fixed resistor setups for the next time one appears in the lab.

author: MiDi
author_rank: Frequent Contributor
timestamp: August 23, 2022, 06:16:36 pm
content: Quote from: jrmymllr on August 23, 2022, 01:16:08 pmDo not recommend trying that, it will widen or even damage the triax receptacle.

author: jrmymllr
author_rank: Regular Contributor
timestamp: August 23, 2022, 07:15:53 pm
content: Quote from: MiDi on August 23, 2022, 06:16:36 pmI was planning to do this as a way to determine if the center pin is bigger on BNC, but if you (or someone else) knows that it is for sure, I won't bother.

author: DavidKo
author_rank: Frequent Contributor
timestamp: September 01, 2022, 10:08:26 am
content: You can buy 2-lug triax on ebay for $9.9 + shipping. I have not tested them yet, but looks OK. It seems to me that only difference between 2lug and 3 lug triax is the locking ring, which looks like "crimped".https://www.ebay.com/itm/284917758481