cable assembly/connector choice - Page 1
Forum section: electronics / rf-microwave

author: kars
author_rank: Contributor
timestamp: January 16, 2019, 02:37:59 pm
content: hi all!for a project i need to carry a composite video signal across a distance of about 5m with some pesky bldc motors and power electronics between the two points.i could solve this with a normal video cable but we'd like something more propper.according to the interweb it needs to be 75 ohm impedance coax so if been browsing farnell and mouser for some cable assemblies.if got 2 questions about this:1) why is 75 ohm impedance coax needed, this isnt really explained.2) im having a hard time picking a suitable connector as we are trying to save as much weight as possible but that connector needs to be a lockable connector,if had a look at bnc connectors, 1.0/2.3 connectors and mcx connectors as the sites have these connectors on preassembled cables.do you guys/girls have any suggestions for a suitable connector?

author: sokoloff
author_rank: Super Contributor
timestamp: January 16, 2019, 05:29:51 pm
content: I'd use a standard F connector (as in CATV/antenna connections) by default, as that's overwhelmingly common, works well, and readily available.You could also use a BNC if there was a reason to do so, but most of those are 50 ohm.Composite video is pretty robust; I've run if over twisted pairs of Cat-3/Cat-5 before without visible degradation. (You could see/measure the impedance mismatch on a scope, but not on the screen.)

author: Gribo
author_rank: Frequent Contributor
timestamp: January 16, 2019, 06:31:39 pm
content: There are 75Ohm BNC and also mini BNC 75Ohm connectors.As for why 75 Ohm, someone in the distant past thought it was a good idea, and it was different enough from the RF guys. Usually your video output stages will be matched to 75 Ohm impedance.

author: vk6zgo
author_rank: Super Contributor
timestamp: January 17, 2019, 12:05:54 am
content: Quote from: Gribo on January 16, 2019, 06:31:39 pmFor analog video, the difference between 50 & 75 ohm connectors is small enough to be ignored.The TV Studio & transmitter sites I worked at used 50 ohm connectors as standard, as they are the most commonly available, & (a less often considered point), are more rugged than 75 ohm connectors, because the PTFE part extends further into the actual connecting part.They come in sizes to fit all the common 75 ohm coax.When the Studio started piping digital video signals around, they changed to "true 75 ohm BNCs".I'm a bit dubious whether it was necessary, as digital video is a pretty rugged format.Anyway, that Studio is now gone, replaced with a Housing development!By the way, as to worry about electrical noise getting into the video, using ordinary coax, I earlier worked at a site with a "high level modulated" TV transmitter.To achieve this end, the Tx produced several kW of video signal, which then grid modulated the RF final.You could hear the HF noise anywhere around the building on a HF radio, but we used (real) RG 59 to feed our (standard level) video around, with no problems with noise interference from this high level signal.And before you object "But it's the same signal, how would you know?",(1) We often tested equipment with different signals sourced elsewhere.(2) Phase difference/ time delays.

author: TheUnnamedNewbie
author_rank: Super Contributor
timestamp: January 17, 2019, 04:43:28 am
content: Quote from: Gribo on January 16, 2019, 06:31:39 pmThere is actually a pretty good reason for this, and it is quite well documented why.For distribution, you care about loss, and not power handeling (after all, the powers transmitted over 75 ohm coax are really nothing to write home about). When you look at air-filled or low-loss dielectric filled coaxial cables, you find that the point of least loss vs impedance occurs somewhere around the 75 ohm region (77 if my memory serves me right). As a result, this is chosen for transmission.When you look at power handling, again, for a given impedance and upper frequency limit, somewhere in the neighborhood of 30 ohm is 'optimal' (don't ask me about the math). I have been told (but cannot confirm this with actual evidence) that as a result, coax used for transmitting high power signals to antennas was 30-ish ohm.Test and measurement wanted a hybrid between the two - you want low loss, but still want to be able to carry acceptable power levels. As a result, 50 ohm was chosen as a middle ground.Another reason people say 75 ohm was chosen in the audio-video world, is that it matches very well a standard dipole antenna.There are some other theories, such as cheap cable having a solid core, and solid core being more flexible as it gets thinner, and a thinner core meant lower impedance. But who knows!As for the original question - often for video signals you can use 50 ohm connectors, the short impedance mismatch will not cause great issues. There are also some actual 75 ohm SMA if I remember correctly, but else SMA seems a pretty solid connector. It screws on so it can be quite a secure connection (there is a reason why SMA and its derivatives are the standard in connectors for anything higher frequency).

author: vk6zgo
author_rank: Super Contributor
timestamp: January 17, 2019, 06:11:16 am
content: Quote from: TheUnnamedNewbie on January 17, 2019, 04:43:28 amOh, I wouldn't sayanything------"N" connectors are still very common for mid range power levels at VHF & above, & "TNC" get a vote, too.