BNC - now I've seen it all ...

"Reputed"? I don't think so.

MrT.

Indeed. I had a trainee radio tech try to tell me that a BNC connector was both 50 and 75 ohm compatible.

Jason

There are 50 Ohm and 75 ohm versions. IBM used 93 ohm RG/62 coax with BNC connectors for terminals, as well. The pin diameter is different, to maintain constant impedance.

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Well, for some applications it my not make a difference....

geoff

Yeah,,..he thought a 50 ohm BNC was good for 75 as well.

IBM used 93 ohm RG/62 coax

93 ohms! I wonder why? Guess you could use 50 ohm test equip etc and dial in the mismatch loss.

Jason

A their site

Amphenol say about their BNC connectors: '50 ? and 75 ? connectors are intermateable to ensure non-destructive mating'.

Chris

... further proof at

wherein: 'Center contact .053" diameter, 30µ" Au. True 75 ohm'. Compare with: where the mating diameter of the pin of a 50 ohm BNC plug (or TNC) is given as 0.052 to 0.054 inches.

It's the PTFE components that are different between the two impedances - the

75 ohm components have more air between the mated inner and outer parts.

Chris

Not until after you try mating a 50ohm male with a 75 ohm female :-)

MrT.

err, it was IBM. If you understand, then you'll know. If you don't, think lock in with proprietary HW. All the dinosaurs did it.

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Apple have been making a fortune that way for years, and have yet to become extinct. They even change their own products to prevent accessories from some models working with others. The lambs keep lining up to buy however.

MrT.

Apple is a mere amateur.

but you are correct.

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They used 93 ohm coax to minimize line loss. That way they could run hundreds of feet between the concentrator and the terminals. The alternative was RS-232, at lower data rates.

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I was referring to the ratio of OD of the pair of center contacts to the ID of the shell. That is what determines the impedance.

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At low frequencies, it won't. Try a 100 foot of 50 ohm cable for video and see how much detail you lose compared to RG/59 or RG/6. We had both 50 and 75 Ohm output options in our telemetry products. A six foot cable would made a good module look bad when you used a 50 Ohm cable on a 75 Ohm port.

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The diameters of the mating parts are the same for 50 ohm and 75 ohm BNCs according to the Amphenol BNC catalogue at

Compare the drawings on pages 107 and 124. The difference is the PTFE 'filler' between the mating parts in the 50 ohm case.

The pin diameter behind the mating part is different for the two cases, but that has no bearing on whether the parts can be mated without damage.

Chris

Sigh. What a waste of time. I have bought & used both types for over 30 years. I don't need the damn data sheets. I have paper copies from the thousands I've used.

I didn't say the mating parts were different in the Amphenol parts. I explained what determines the impedance.

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