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| How to tell the difference visually on BNC connectors |
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| dmills:
Modern video uses high speed digital, and I have had a cable fail because the installer has placed a zip tie every meter along a run over a suspended ceiling and had pulled them so tight they crushed the foam dielectric slightly. TDR revealed the problem, but the eye diagram looked really funky. Still rather deal with SDI then terminating Triax on Lemo. Regards, Dan. |
| TerraHertz:
Here's that pic I promised. The white cable is 75 ohm coax from a video installation - the old ABC studios at Gore Hill. You can be sure they were using the correct connectors. I have three big fruit boxes boxes full of these video cables, and I never noticed the connectors are not quite the same as standard 50 ohm BNCs (center in the pic.) I'm feeling pretty stupid now. Edit: I have to wonder though, if the connectors are actually significantly different in impedance. Or is the mechanical difference intended more to prevent accidentally using 50 ohm cables in 75 ohm systems? Since they won't fit. Too bad I don't have some 75 ohm ones new in pack (and their mating gender), or I could crimp a pair on some 50 ohm cable and have a look with my HP 54121T TDR. It can see really small impedance discontinuities, like holding a fingertip near a controlled impedance PCB stripline. |
| macboy:
50 ohm and 75 ohm BNC are 100% physically compatible, it is perfectly safe to mate then to each other. This is absolutely not the case for N type where different dimensions will result in damage to one or both. It is the presence or lack of dielectric between the pin and shield that results in different impedance. The BNC connectors on the white cable in Terrahertz's photo are definitely not 50 ohm, there is too little dielectric to bring the impedance down that far. |
| IdahoMan:
--- Quote from: dmills on September 06, 2016, 10:22:55 am ---At the sorts of frequencies and power levels most folks use BNC for it is not usually a huge deal (Outside of broadcast, where for mainly historical reasons we cart 3Gb/s serial digital video around on the 75 ohm versions). Sometimes someone gets an attack of the stupid and tries to do 6Gb/s or even 12Gb/s over the same cables, that is when the money for Pasternack or Huber & Schurner makes sense. CCTV of the composhite type is all so low frequency that you would have to really work at it to get a connector impedance discontinuity to cause a problem. I had to laugh at the notion of a specifically 75 (or 50) ohm BNC to RCA, them things tend to be nothing in particular impedance wise. The things that matter more on BNCs IME is using the right one for the cable and the right tooling when crimping the things, a poor crimp will cause far more pain then a 50 ohm BNC in a 75 ohm system. The good stuff for RF is always expensive, as it is in every field, have a look at VNA calibration kits or precision attenuators if you want to see really silly pricing. Regards, Dan. --- End quote --- Thanks Dan for posting that explanation. How effective is the shielding on coax cable? Shielding is very important for my use. It has to protect against malicious mischief: Someone using special equipment to detect it or disrupt/jam/destroy it. As for RCA*.. Yeah, laughable, but a lot of these little CCTV cams have either a BNC or a yellow RCA plug on the end. I want to assemble an adapter kit (BNC to RCA/F/N/, F-to-RCA, etc.) for testing. Just wanted everything to match for good organization. For a permanent setup, I'd use proper BNC properly crimped.. or solder the coax end straight to the two wires on the back of the cam in well-secured enclosure. *BTW, what is the alternative for video when coax is not an option (such as a wearable lapel cam)? s/pdif-rca? Sincerely, IM |
| helius:
Coaxial cables are rated for a specific shield coverage. 97% to 99% are pretty common. You can easily disrupt the signal if you are close to the cable, but it does a good job of screening typical interference. "Quad shield" type cables have more layers of screening and admit less interference. |
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