BNC Style Connectors

[veedub565]

Hi,

I picked up a pair of DC-4ghz 10db attenuators from ebay. They are supposed to be be BNC but don't appear to be compatible. The female end mates with BNC ok but not the male end. Can anyone offer any suggestions as to what they might be ?

http://www.ebay.co.uk/itm/272226802723?_trksid=p2057872.m2749.l2649&ssPageName=STRK%3AMEBIDX%3AIT


I did think it strange to be honest that they should go that high as your never gonna get 4ghz down a BNC cable.

[Andy Watson]

BNCs are available in both 50 and 75 ohm versions - are you using the matching pairing of plug and socket? (I think the pin in a 75 ohm is larger than the 50 ohm).

looks like they should be 50 ohm:
http://www.parttarget.com/5985-01-286-6643_5985012866643_NK0947-0204-10.html/-ABB418D7-40B0-46E9-B281-4FA7C2BCDCFD

[helius]

Amphenol seems to think BNC connectors (mated with the appropriate cables) can carry 0-4 GHz and 0-500 Vp.
http://www.amphenolrf.com/connectors/bnc.html

"appropriate" is some version of LMR cable: not RG-58 of course!

The pictures of that item look like regular 50 ohm BNC. Check if the metal collars are bent.

[veedub565]

Yeah I wondered if they were 75ohm or not, usually the pin is thicker. In these though it looks as though the insulator is thicker compared to a normal BNC.

I guess the connector itself might carry 4ghz, perhaps that's why the insulator seems thicker? When I say you'll never get 4ghz down a BNC lead I am of course thinking of the usual boggo RG58 patch cable. The collars don't appear to be bent at all.


It doesn't really matter if they aren't normal BNC, it's always useful to have odd bits and pieces. seemed a good price for that frequency range, most of the cheap (ish) RS attenuators only run to 1ghz

[edpalmer42]

50 and 75 ohm BNC connectors look different inside*, but they mate safely.  N connectors have different center pins and can be destroyed by mixing them.

I've had problems with tolerances on some cheap Chinese BNC connectors.  Sometimes they work, sometimes not.

For all BNC connectors:
- check both male and female sides to make sure they're round.  Use something like a vernier caliper or better.
- check the center socket on the female side to make sure it isn't mangled.
- check the fingers around the insulator on the male side to make sure one of them isn't bent out a bit.
- check the center pin on the male side to make sure it isn't out of line.

Ed

* http://www.cmple.com/skins/LearningCenter/images/blog/75-ohm-and-50-ohm-BNC-Connectors.gif

[veedub565]

I saw that pic while I was googling, they certainly look like 50ohm at both ends, however they won't connect to each other. I can't for instance connect them together to make 20db.

I can connect a 50ohm scope probe into one end, but the other end wont connect to a 50ohm BNC socket.  So something odd going on, nothing looks bent, will have to get the verier calipers on it tomorrow.

[helius]

I just looked into the male end of a terminator, and compared it to the pictures in the first link. The dielectric does seem thicker in the picture!

No idea why that is.

[veedub565]

Just confirmed with vernier calipers. The dielectric is indeed thicker. So seems standard 50ohm BNC at the female end, but something else at the male end.

The plot thickens !

[edpalmer42]

What did you measure?

I just googled for "bnc connector" dimensions and it looks like the inside diameter of the insulator on the male side should be 0.190".  Some diagrams show that as a minimum.  A random survey of connectors here shows that to be reasonable.

Ed

[veedub565]

Measures about 0.202 on the ones I've bought, and 0.207 on my scope probe. I've only got the manual verniers at home though and I'm not too confident of reading them correctly. Will have to get the digital ones on them tomorrow.

I know that if I measure the scope probe BNC and lock the verniers they then won't fit inside the Dielectric of the ones I've bought.

Maybe they are just manufactured to a tighter tolerance, but then odd that they wont fit each other.

[edpalmer42]

So the problem isn't the inside diameter, it's the outside diameter on the male side?

I checked a couple of HP brand scope probes and a couple of Mini-circuits filters.  The outside diameter of the fingers on the male side (I can just get my calipers into the slots) is about 0.295".  For comparison, the internal diameter of the female side is about 0.325.

Ed

[veedub565]

Had a bit more of a closer look today and they do appear to be 50ohm BNC, tried them on a couple of pieces of equipment and seemed to fit. Just a very tight fit.

A friend of mine kindly checked them on a network analyser for me, flat from DC to 3.8ghz then they drop about 0.1db to 4Ghz so quite pleased for that price.Hopefully they'll loosen up a bit with use.

Thanks for all the info and suggestions