BNC connectors - 50 or 75 Ohm ? - Page 1
Forum section: electronics / general

author: fmunson
author_rank: Newbie
timestamp: March 13, 2018, 12:14:30 pm
content: Hi everyone,My first post here, so bear with me. I have recently started working with video a little more serious, and what do you know, I entered the world of BNC cables - and connectors. So in order to get the 2K video from my Blackmagic card to the HDLink Displayport converter properly, I got hold of two 3ft cables. One cable clearly reads RG58/50Ohm and the other RG59/75Ohm, but the connectors are very similar.Basically, my question is - how to tell if these particular connectors are 50 or 75 Ohm - and whether it matters for the described purpose (at only 3ft length).Many thanks.Cheers

author: capt bullshot
author_rank: Super Contributor
timestamp: March 13, 2018, 12:36:15 pm
content: For me, they all look 50 Ohm.I've got a 75 Ohm RG59 cable, its connector has a larger inner diameter and the teflon insulation is missing. See photo, left: 75 Ohm, right: 50 OhmI don't know if this is applicable as a general rule

author: Brumby
author_rank: Unknown
timestamp: March 13, 2018, 01:25:05 pm
content: There are the connectors - and there is the cable.The writing on the cable will be the impedance of the cable. Hopefully, the connectors will match:(As I understand it.)

author: Howardlong
author_rank: Super Contributor
timestamp: March 13, 2018, 01:30:34 pm
content: Quote from: Brumby on March 13, 2018, 01:25:05 pmThat picture is exactly my understanding too. They will mate with each other with no mechanical problems, although electrically there is of course an impedance mismatch. This is unlike some other connectors, N type for example, where you will physically ruin the connectors if you attempt to mate them.

author: Mr. Scram
author_rank: Super Contributor
timestamp: March 13, 2018, 02:23:35 pm
content: Why not get out your multimeter and measure what's what? That seems to work well enough and has the added bonus of detecting defects.

author: drussell
author_rank: Super Contributor
timestamp: March 13, 2018, 02:29:24 pm
content: Quote from: Mr. Scram on March 13, 2018, 02:23:35 pmHow do you propose that he examine the characteristic impedance of the line with just a multimeter?

author: Andy Watson
author_rank: Super Contributor
timestamp: March 13, 2018, 02:31:18 pm
content: Quote from: Mr. Scram on March 13, 2018, 02:23:35 pm! UhCan you elucidate?@drussel Snap!

author: Bud
author_rank: Super Contributor
timestamp: March 13, 2018, 02:34:21 pm
content: Quote from: drussell on March 13, 2018, 02:29:24 pmSeems to work well enough ..?

author: Bud
author_rank: Super Contributor
timestamp: March 13, 2018, 02:36:57 pm
content: Quote from: capt bullshot on March 13, 2018, 12:36:15 pmif it has a larger inner diameter then it would not mate reliably with a 50 Ohm one i think.

author: Mr. Scram
author_rank: Super Contributor
timestamp: March 13, 2018, 02:37:58 pm
content: Quote from: drussell on March 13, 2018, 02:29:24 pmI admit I was thinking of terminators, but that certainly works well enough. Regardless, testing is preferred over making assumptions based on how something looks.

author: glarsson
author_rank: Frequent Contributor
timestamp: March 13, 2018, 02:43:33 pm
content: Quote from: Mr. Scram on March 13, 2018, 02:37:58 pmYes, but very few amateurs have the equipment and knowledge required to make this test. It is much easier to look at the physical properties that determines the impedance.

author: capt bullshot
author_rank: Super Contributor
timestamp: March 13, 2018, 02:54:32 pm
content: Quote from: Bud on March 13, 2018, 02:36:57 pmSorry for confusing you, what I wanted to mention is the missing dieletric - which leads to an larger inner diameter surrounding the pin. The metallic shield contact appears to have the same diameter on both plugs. Anyway, I don't have any 75 Ohm sockets to see if one can plug a 50 Ohm into them, but these 75 Ohm plugs plug into 50 Ohm sockets without mechanical issues.

author: edpalmer42
author_rank: Super Contributor
timestamp: March 13, 2018, 05:07:44 pm
content: The connector shown in the first three pictures is, as others have stated, a plain, ordinary 50 ohm connector.The last three pictures show a connector that I have never seen IRL. I have seen it in another message thread a few months ago. It looks like it has been ruggedized by folding the external sleeve over the end of the insulator. The insulator is still there which suggests a 50 ohm impedance, but the smaller diameter of the folded section will change the impedance. I have no idea what the effect of such a change would be. Someone with a VNA would have to make some measurements.Ed

author: helius
author_rank: Super Contributor
timestamp: March 13, 2018, 05:34:41 pm
content: Analog video cabling is supposed to be 75 ohm, but 50-ohm BNC connectors are almost always used as the reflections are invisible in the noise that is already present. For digital video (SDI, HD-SDI), true 75-ohm BNC connectors are required.

author: Howardlong
author_rank: Super Contributor
timestamp: March 13, 2018, 09:52:14 pm
content: Quote from: helius on March 13, 2018, 05:34:41 pmFor analog you will risk seeing ghosting effects with mis-terminated cables.

author: dmills
author_rank: Super Contributor
timestamp: March 14, 2018, 12:12:23 am
content: The really scary thing is when you realise that 12G SDI over copper uses BNC, and it even allegedly works!Granted only the US truck companies seem to be going there, Japan and Korea seem to favour 12G over fibre and Europe is jumping on the IP bandwagon, largely driven by weight and volume savings.Regards, Dan.

author: Howardlong
author_rank: Super Contributor
timestamp: March 14, 2018, 12:05:45 pm
content: Quote from: dmills on March 14, 2018, 12:12:23 amInteresting: coming from RF land, at that bit rate I was expecting some sort of complex modulation and encoding, but Wikipedia is saying it's simple NRZI with LFSR scrambling for zero/one balance and clock extraction. It's also unidirectional so any training on the receive equaliser will need to be autonomous without the benefit of any training signals other than the actual live signal itself. I don't believe there's any FEC either, just CRC. I guess the rationale is that being a point to point cable with well-defined and reasonably constant properties makes this a reasonable proposition.To be fair, at 12Gbps that's a 6GHz maximum phase rate change, but I see they also have a proposed 24Gbps standard which is a bit scary over BNC terminated copper. I see sometimes dual link is used?I also note that attenuations of 20 to 30dB are acceptable, so you might get 50' out of an RG6 cable at 12Gbps.

author: vk6zgo
author_rank: Super Contributor
timestamp: March 14, 2018, 12:24:37 pm
content: Quote from: Howardlong on March 13, 2018, 09:52:14 pmNope! At around 5 to 6MHz highest video frequency, the mismatch is not discernible.( no ghosting).Analog TV stations used 50 ohm BNCs for many years.Strangely enough, they were designed to physically fit on 75 ohm cable !When they went to digital, all the new cabling used true 75 ohm connectors.

author: Howardlong
author_rank: Super Contributor
timestamp: March 14, 2018, 01:49:53 pm
content: Quote from: vk6zgo on March 14, 2018, 12:24:37 pmYou're absolutely right!Now had I said, "you will risk seeing ghosting effects with mis-matchedcables", it would've been more accurate.

author: NiHaoMike
author_rank: Super Contributor
timestamp: March 14, 2018, 03:03:58 pm
content: Ask Micah Elizabeth Scott? She's one of the few hobbyists to run SDI at home. I think she even sent SDI through a few inches of 50 ohm micro coax because that was all that she was able to find that would fit in the cat tracking robot.

author: Howardlong
author_rank: Super Contributor
timestamp: March 14, 2018, 03:12:42 pm
content: Over a few inches, there wouldn't be much attenuation, and as it's digital, no ghosting.

author: cvanc
author_rank: Frequent Contributor
timestamp: March 14, 2018, 03:30:02 pm
content: Agreed; I've not seen one like that before either. Not sure what the intended application would be?Quote from: edpalmer42 on March 13, 2018, 05:07:44 pm

author: capt bullshot
author_rank: Super Contributor
timestamp: March 14, 2018, 05:26:18 pm
content: Quote from: cvanc on March 14, 2018, 03:30:02 pmFound one look alike in my junk box. It came attached to an el cheapo chinese "kelvin" set of tweezers. First thing I did was to remove these obvious "not a BNC" connectors ...

author: NiHaoMike
author_rank: Super Contributor
timestamp: March 15, 2018, 12:43:55 am
content: Quote from: Howardlong on March 14, 2018, 03:12:42 pmThat's not the full story. After the few inches of 50 ohm micro coax to get the signal from the camera PCB to a BNC connector on the chassis, it then goes through a few tens of feet of thin coax (to keep weight down) before going to a repeater and then to the control PC in the server rack.

author: David Hess
author_rank: Super Contributor
timestamp: March 15, 2018, 01:16:27 pm
content: Quote from: vk6zgo on March 14, 2018, 12:24:37 pmI still have my old high definition BNC input VGA monitors and cables and the BNCs are 50 ohms and the cables are 75 ohms. Of course the VGA 15 pin D-connectors on the other end of the cables causes an impedance mismatch as well but neither the effect from it nor the 50 ohm BNC connectors was ever visible at the 100s of MHz of bandwidth which high definition required which makes sense; even for high definition component video, the 300+ MHz of bandwidth just not high enough for the connector impedance to matter.I have a few 75 ohm BNC adapters and they look like the examples shown above with the metal fingers present but the insulator missing. They have no trouble interfacing with 50 ohm BNC connectors.

author: fmunson
author_rank: Newbie
timestamp: March 15, 2018, 03:43:38 pm
content: If one thing, I now know I still have soooo much to learn about cables and connectors.Thanks everyone for the replies (so far - but please keep 'em coming!:)If it can be of any use, here are the pics of the connectors on the Decklink card and on the HDLink monitoring converter. Cable attached is the one with the connectors shown in the last three pictures in my first post. Everything works fine, all types of video up to and including 2K are shown without glitches on the monitor. The other cable also works fine. That's not to say there isn't a 50/75 mismatch on one or both cables...Cheers

author: Quarlo Klobrigney
author_rank: Super Contributor
timestamp: March 15, 2018, 04:34:27 pm
content: In my too long of experience, video or digital video is always 75. The connectors make a difference in HDSDI or SDI, as well does the coax.

author: edpalmer42
author_rank: Super Contributor
timestamp: March 15, 2018, 04:53:49 pm
content: Oh, for fudge sake! Those female connectors aren't standard either!In the picture that fmunson posted above look at the thickness of the center insulator on the female connector. Now look at the following picture from Wikipedia. Fmunson's connectors don't match either one.Maybe they're 62 1/2 ohm connectors.My head hurts.

author: fmunson
author_rank: Newbie
timestamp: March 15, 2018, 05:16:35 pm
content: Now this is interesting...https://www.eevblog.com/forum/chat/how-to-tell-the-difference-visually-on-bnc-connectors/msg1020633/#msg1020633This even more (copied fromhttp://www.satcure.co.uk/reviews/satcure_blog_170.htm)In this month's newsletter (169) you say:"What always puzzles me about CCTV is that it uses 75 Ohm cable but BNC connectors are 50 Ohm."- 'fraid not! :-)BNC connectors are made in 50, 75, and rarely 93 ohm impedances.I have 50 and 75 ohm examples, and I was going to send a photo showing the difference, but as this is a form rather than an actual email I can't, but you can see it here:http://www.hibernaculum.org.uk/Secret/Radio/bnc_50-75ohm.jpgThe 50 ohm plug and socket are on the left, the 75 ohm on the right.The differences are subtle - the 75 ohm version has a slightly wider centre pin and a slightly narrower centre insulator, by about a quarter of a millimetre as far as I can measure them.Some 75 ohm sockets I have omit the teflon insulator surrounding the "socket pin" and it stands up from the back of the mating chamber on its own - I have no idea why they'd do that.Just thought you'd like to know - (...)

author: edpalmer42
author_rank: Super Contributor
timestamp: March 15, 2018, 05:56:47 pm
content: This just shows how much confusion there is on this topic. The Wikipedia pictures agree with most of the connectors I've seen, both on cables and equipment, so, until somebody shows me proof to the contrary, that's what I use.Until someone puts these things on a VNA, we're all just throwing up more confusion. I suspect that the VNA tests will show that most of the differences are trivial, but a few are significant if you're running very high frequencies and/or very sensitive measurements. Remember that BNC connectors aren't really suited to either of those situations. SMA, N, or maybe even TNC are better.

author: Martin.M
author_rank: Frequent Contributor
timestamp: March 15, 2018, 06:14:59 pm
content: to be extendet:BNC plugs for RG62 = 93 OhmsgreetingsMartin