[time-nuts] BNC question

Greg Broburg semiflex at comcast.net
Sun Apr 10 16:09:17 UTC 2011


>
>       Types
>
> BNC connectors exist in 50 and 75ohm 
> <http://en.wikipedia.org/wiki/Ohm>versions, matched for use with 
> cables of the samecharacteristic impedance 
> <http://en.wikipedia.org/wiki/Characteristic_impedance>. The 75 ohm 
> types can sometimes be recognized by the reduced or absentdielectric 
> <http://en.wikipedia.org/wiki/Dielectric>in the mating ends. The 50 
> and 75 ohm connectors are typically specified for use at frequencies 
> up to 4 and 2 GHz respectively.
>
> 75 ohm BNC Connectors are primarily used for video andDS3 
> <http://en.wikipedia.org/wiki/Digital_Signal_3>Telco central office 
> applications^[/clarification needed 
> <http://en.wikipedia.org/wiki/Wikipedia:Please_clarify>/] , whereas 50 
> ohm are used for data and RF. TheBBC 
> <http://en.wikipedia.org/wiki/British_Broadcasting_Corporation>had a 
> convention that BNC connectors used for video were always 50 ohm, 
> maybe because an accidentally connected 50 ohm plug would damage a 75 
> ohm socket.^[/dubious 
> <http://en.wikipedia.org/wiki/Wikipedia:Disputed_statement>--discuss 
> <http://en.wikipedia.org/wiki/Talk:BNC_connector#Video>/] Many VHF 
> receivers used 75 ohmantenna 
> <http://en.wikipedia.org/wiki/Antenna_%28radio%29>inputs, so they 
> often used 75 ohm BNC connectors.
>

I had remembered that it was a 75 ohm that would damage a
50 ohm socket. This article states that a 50 ohm would
damage a 75 ohm socket.

Greg

On 4/10/2011 9:25 AM, Greg Broburg wrote:
> From what I recall from our candle lit lab, the 75 ohm
> BNCs had a slightly larger pin and would open up the
> 50 ohm females just a skoosh so when you put a
> 50 ohm, into a 50 ohm that had been tweaked by a
> 75 ohm, they were noisy or intermittent.
>
> Greg
>
> On 4/10/2011 5:30 AM, Magnus Danielson wrote:
>> On 04/10/2011 01:51 AM, Mike S wrote:
>>> At 06:53 PM 4/9/2011, Joseph Gray wrote...
>>>> I have an old Arcnet hub that I want to salvage the isolated BNC
>>>> connectors from. Arcnet used 93 Ohm coax. I know that there are 50 Ohm
>>>> and 75 Ohm versions of BNC connectors, but the ones from the hub look
>>>> like a 50 Ohm BNC to me.
>>>
>>> The ARCNET spec states: "The MIC for use with coaxial cable is a
>>> conventional BNC per MIL-STD-348A." If you look at that spec, it's for
>>> 50 Ohm connectors. 75 Ohm ones don't have the insulator around the
>>> center socket. Since ARCNET only ran at 2.5 MHz, the mismatch 
>>> apparently
>>> didn't matter.
>>
>> Risetime is the key aspect. If the impedance missmatch is 
>> sufficiently small compared to the rise-time, it has no significant 
>> effect.
>>
>> As I recall it ARCNET wasn't running at very high speeds and hence no 
>> need for short rise-times.
>>
>> Cheers,
>> Magnus
>>
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>
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