Is this a 50 or 75 ohm BNC connector? - diyAudio
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Old 28th January 2011, 03:59 AM   #1
wwenze is offline wwenze  Singapore
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Default Is this a 50 or 75 ohm BNC connector?

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Click the image to open in full size.

Correct me if I'm wrong, but isn't this a 50 ohm connector?
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Old 28th January 2011, 03:39 PM   #2
Pano is offline Pano  United States
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It could be. Normally on the male 75 ohm connectors you don't see the insulator. That's true on some female, but not all. It can be hard to tell.
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Old 28th January 2011, 04:56 PM   #3
LAJ is offline LAJ  United States
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Default Is it an input or an output?

If its an input, you should be able to tell be measuring the resistance of the center pin and outer shell.
If its an output, you'll have to look at the buildout resistor in the circuit.
For most applications, it doesn't matter. In broadcast TV both are used interchangeably.
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Old 29th January 2011, 08:21 PM   #4
DF96 is offline DF96  England
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The important thing is not the connector, unless you are working at high RF frequencies. The cable needs to match the presented impedance unless the cable is quite short.
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Old 29th January 2011, 11:56 PM   #5
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It's not about the cable, it's about the connectors.

The center pin of the 50 Ohm and the 75 Ohm connectors are of a different diameter.
So if you connect a small male to a large female you have a poor connection.
But if you connect a large male to a small female then you damage the female!
That all sounds so dirty.

For the short cable lengths and low frequencies that many of us work at, characteristic impedance is no big deal.
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Old 30th January 2011, 04:30 PM   #6
DF96 is offline DF96  England
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There have been huge debates about that, which were inconclusive. Anecdotal evidence points both ways. Official drawings suggest the pins are all the same, with the impedance difference coming from including or omitting the plastic dielectric.
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Old 2nd February 2011, 01:22 AM   #7
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Wow "DF96" I just learned something from you. I had always though that it was a mechanical size difference issue and we would warn the equipment installers about the damage that it would cause.

But Amphenol Connectors has a page:

Amphenol Connex- BNC Connector Series

Two distinct types of 75 Ω BNCs are available, and both mate with each other and with 50 Ω BNCs.
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Old 2nd February 2011, 10:53 AM   #8
DF96 is offline DF96  England
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Some people are sure there is a difference. Others are equally sure there is not. It may depend on who made them - there is now a lot of Chinese junk around.

See here and here.
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Old 2nd February 2011, 01:09 PM   #9
oshifis is offline oshifis  Hungary
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50 ohm pin (and female part) has smaller diameter, 75 ohm pin/female has larger diameter, as Speedskater wrote. There might be some difference in the insulator part, too.
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Old 2nd February 2011, 04:11 PM   #10
LAJ is offline LAJ  United States
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Default The difference in diameter...

is for the hole for the center conductor to go into. Fifty ohm cable center conductors are smaller than 75 ohm cable center conductors. Obviously the larger center conductor will not fit into the smaller pin.
BUT, I have done this. You can solder the pin on to the center conductor. Many years ago I attended an ADC seminar where they recommended soldering the pins on to the center conductor for digital applications. It seems that it reduces reflections at high frequencies.
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