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75 ohm or 50 ohm help

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rmn_tech

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Hi I've been to the boot sale today and picked up a bag of BNC connectors (about 50 chassis sockets) for £1. Can't go wrong.

However some are 75 ohm and some are 50 ohm I have some ideas in mind for the 50 ohm sockets but just by sight how do I know one from the other.

I have researched on the web but sellers just use the same photo for both types. anybody know how to tell them apart ?.
 
should be able to get out your DMM and measure the resistance of them mannualy,post a picture up of the connectors so we can have a look though
 
Now they are connectors and not terminations right? Usually the 75ohm coax has a slightly larger diameter than 50ohm (In the same loss/grade category). You may be able to determine the type with a pair of vernier calipers or just try and insert the core of a 75ohm coax into one. If it doesn't fit, it is a 50 ohm connector.
And yes, I know that that RG-8 is larger than RG-59 coax. Hence the "loss/grade category" disclaimer. ;)
should be able to get out your DMM and measure the resistance of them mannualy
That'll only work if they are terminations (mini dummy loads) and not connectors designed to go on the end of the coax.
 
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Chassis Sockets

They are chassis sockets.

A DMM will tell me nothing they are sockets.
 
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kchristie's got this one covered, 75ohm is slightly larger than 50ohm. Both cable and connector.
 
So what your'e saying a standard BNC socket is in 2 sizes and not standard at all.

A BNC connector at any impedence is the same size.

My origional question still stands.

Is there a way to tell the difference visually ?

Ps 50 ohm and 75 ohm is their impedence not their resistance.

socket.jpg
 
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Oppps. I was thinking of BNCs that would go on a coax cable end. The one in the PIC is not designed for high frequency use and no matter how you wired it would show up as a impedance anomaly on a TDR (Time Domain reflectometer) because there is no way to maintain the characteristic impedance of the transmission line when going from coax to connector.
So what your'e saying a standard BNC socket is in 2 sizes and not standard at all.
No, but that the internal structure and materials are different. The parts that plug together are the same sizes and this is why BNCs aren't really that good of connectors for RF, but are great for test equipment connections due to their ease of removal/attachment.
However some are 75 ohm and some are 50 ohm I have some ideas in mind for the 50 ohm sockets but just by sight how do I know one from the other.
Are all your connectors this style? Any without the solder tab for the shield? How do you know that you have some 50 and some 75 ohm BNCs in the first place?
 
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I intend to use these for video signals conecting to long cables.

The Box is labled as BNC Chassis sockets 50ohm / 75ohm

They are all as in the picture

The solder tab is connected to a washer so is removable
 
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Don't worry about!, it will make no difference for your video signals which ones you use - incidently, for video signals it should be 75 ohms - and normally 50 ohms for RF use.
 
rmn_tech said:
Hi I've been to the boot sale today and picked up a bag of BNC connectors (about 50 chassis sockets) for £1. Can't go wrong.

However some are 75 ohm and some are 50 ohm I have some ideas in mind for the 50 ohm sockets but just by sight how do I know one from the other.

I have researched on the web but sellers just use the same photo for both types. anybody know how to tell them apart ?.

hi,
**broken link removed**

As pointed out in a previous post, for your video application it will make no visible difference to the signal quality.

Eric
 
I ran into a problem with this issue many years ago.
The male pin in the 50 ohm BNC coax connector is slightly larger in diameter than that of the 75 ohm BNC (this is how they get the different impedances). The female receptacles in the two connectors are correspondingly sized. The difference is subtle to the eye, but if you plug a 50 ohm coax into a 75 ohm receptacle, it will spring (distort) the receptacle. If you then switch back to 75 ohm coax, the connection may be unreliable.
I don't know if 75 ohm coax plugged into a 50 ohm connector is reliable or not.

EDIT: I just now read the Amphenol page that Eric referenced. They have obviously solved this problem, but the fact that they mentioned that the two connectors can be mated nondestructively suggests that it was a problem in the past. As I said, I encountered the problem many years ago - like 30. Surplus connectors could have this problem.
 
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Roff said:
I ran into a problem with this issue many years ago.
The male pin in the 50 ohm BNC coax connector is slightly larger in diameter than that of the 75 ohm BNC (this is how they get the different impedances). The female receptacles in the two connectors are correspondingly sized. The difference is subtle to the eye, but if you plug a 50 ohm coax into a 75 ohm receptacle, it will spring (distort) the receptacle. If you then switch back to 75 ohm coax, the connection may be unreliable.
I don't know if 75 ohm coax plugged into a 50 ohm connector is reliable or not.

EDIT: I just now read the Amphenol page that Eric referenced. They have obviously solved this problem, but the fact that they mentioned that the two connectors can be mated nondestructively suggests that it was a problem in the past. As I said, I encountered the problem many years ago - like 30. Surplus connectors could have this problem.

hi Roff,
For ref only, link to:
https://www.electro-tech-online.com/custompdfs/2007/08/bnc.pdf

Eric
 
ericgibbs said:
hi,

I think I have found a link that describes some of the differences 50/75ohm bnc.

**broken link removed**

and some pins dimensions

http://www.conec.com/section24/s3/bnc.dimension.php3
**broken link removed**

Eric
Good find, Eric.
Both pins of 50 Ohm and 75 Ohm plugs have the same diameter in the mating area. Therefore, both 50 Ohm and 75 Ohm connectors mate mechanically without any physical damage.
I am certain this was not the case in the early '70's, at least not with the brand we were buying.
 
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Thanks Guys for your responses.

Most enlightening

Rick
 
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