BNC Socket Without Centre Insulator? - Page 1
Forum section: electronics / projects

author: dexters_lab
author_rank: Unknown
timestamp: March 17, 2015, 11:55:06 am
content: Just bought some BNC panel mount sockets and noticed there is no insulation/support on the centre conductor, anyone seen this before?Pic below, what i would call a normal BNC on the left with the one missing the insulation on the right.

author: andtfoot
author_rank: Unknown
timestamp: March 17, 2015, 12:00:35 pm
content: Chances are one is 50 ohm impedance, and the other is 75 ohm impedance (with the one missing the insulation being the 75 ohm).

author: dexters_lab
author_rank: Unknown
timestamp: March 17, 2015, 12:11:04 pm
content: ha! never realised or noticed that before. I think all my BNC things are 50ohm which is why i never saw a 75ohm before.The ironic thing is the panel socket (on the right) was sold to me as a 50ohm

author: T3sl4co1l
author_rank: Super Contributor
timestamp: March 17, 2015, 01:27:29 pm
content: eBay special?Want to say the proper 75 ohm ones have a different dimension sleeve, not that it's missing altogether (which would be shit for mechanical stability, eh?).Tim

author: dom0
author_rank: Super Contributor
timestamp: March 17, 2015, 01:32:29 pm
content: Quote from: T3sl4co1l on March 17, 2015, 01:27:29 pmUh, even the Radiall ones don't have any dielectric.

author: nixfu
author_rank: Unknown
timestamp: March 17, 2015, 01:36:10 pm
content: Yep.. the 75 Ohm ones are used for video and quite common in that industry for video production, television etc, and the traditional 50 Ohm are used in RF, as well as in test/measurement/lab devices.If you are a radio guy. or using it for lab equipment you have to be very careful which one you are getting because that video production stuff is quite common in the surplus market.

author: dexters_lab
author_rank: Unknown
timestamp: March 17, 2015, 01:52:21 pm
content: thanks for clearing that up guysyes this was off ebay, but i think it's just a mistake on the sellers part

author: dexters_lab
author_rank: Unknown
timestamp: March 17, 2015, 02:41:14 pm
content: well interestingly the seller is saying they have checked with their supplier who has stated the connector is 50ohms even though its missing the extra bit of dielectric over the pin like a 75ohm would be.

author: miguelvp
author_rank: Super Contributor
timestamp: March 18, 2015, 06:21:34 am
content: My 75 Ohm terminators with purple caps do have the insulation/support sleve.But you can get out of doubt if they are 50 or 75 Ohms by measuring the resistance with your DMM.

author: aargee
author_rank: Frequent Contributor
timestamp: March 18, 2015, 06:51:22 am
content: ? I'd like to see that...

author: rs20
author_rank: Super Contributor
timestamp: March 18, 2015, 07:10:54 am
content: Wikipedia states that judging the impedance of the BNC based on how the dielectric is shaped is only a 90% true rule, not specification or anything like that. So it's still possible that they're both 50 ohm.Quote from: miguelvp on March 18, 2015, 06:21:34 amI assume you're kidding

author: miguelvp
author_rank: Super Contributor
timestamp: March 18, 2015, 07:13:26 am
content: Oh, sockets! my badI saw the terminators and assumed

author: macboy
author_rank: Super Contributor
timestamp: March 19, 2015, 02:31:17 am
content: Quote from: dexters_lab on March 17, 2015, 02:41:14 pmThe supplier is mistaken. Those are 75 ohm.I once bought some "silicone" banana leads which melted nicely on contact with a soldering iron (silicone won't melt of course). My seller's supplier insisted that the leads were in fact silicone, just as yours insists those are 50 ohm BNC.Insulation that melts isn't silicone, and a BNC with air as dielectric can't be 50 ohm, the physics doesn't pan out.

author: wraper
author_rank: Unknown
timestamp: March 19, 2015, 02:50:13 am
content: Quote from: rs20 on March 18, 2015, 07:10:54 amBNC cable that came with my Rigol DS2072 was without dielectric. Should be 50 ohm though. Coaxial cable itself is 50 ohm rated 3D-FB.

author: wraper
author_rank: Unknown
timestamp: March 19, 2015, 03:15:42 am
content: It seems that only 100% reliable method to check is to destroy the cable and check pin diameter at the end where it meets central wire of the cable. 50 ohm pin is much thicker at that end as central wire is much thicker compared to the 75 ohm cable.EDIT:BTW, thinking about this. It would be almost impossible to attach 75 ohm central pin on 50 ohm cable. Therefore if cable itself is 50 ohm rated, then connector should be 50 ohm too.

author: Alex Eisenhut
author_rank: Super Contributor
timestamp: March 19, 2015, 03:42:49 am
content: Quote from: wraper on March 19, 2015, 03:15:42 amA 25ps rise-time signal should sort it out too.

author: dexters_lab
author_rank: Unknown
timestamp: March 19, 2015, 09:58:14 am
content: Seems odd there is nothing in the design that identifies them, i am guessing the difference could be critical for some usersfor me, i am just making cables for my geiger counters so it's not a big issue

author: dom0
author_rank: Super Contributor
timestamp: March 19, 2015, 10:00:20 am
content: Quote from: dexters_lab on March 19, 2015, 09:58:14 amI heard the HDTV/SDI folks were quite pissed when they found that many installations used 50 ohms connectors all over the place. Not an issue with analog TV, apparently, but SDI links would fail.

author: wraper
author_rank: Unknown
timestamp: March 19, 2015, 10:41:58 am
content: Quote from: dom0 on March 19, 2015, 10:00:20 amFrom this document:http://www.amphenolconnex.com/media/downloads/5648/bnc.pdfQuote