[time-nuts] BNC question

Magnus Danielson magnus at rubidium.dyndns.org
Sun Apr 10 12:41:11 UTC 2011


On 04/10/2011 01:44 PM, Bruce Griffiths wrote:
> 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
>>
> There's also the minor issue of 0.7mm and 0.9mm diameter centre pin BNC
> variants.
> They do exist, I've seen hundreds of them.

These days there exists a standard variant of the BNC for 75 Ohm which 
is mechanical compatible with 50 Ohm. So that is what I recommend using.

75 Ohm BNC dominate with margin over 50 Ohm BNC at work. Our rise-times 
makes connector-choices more relevant.

Cheers,
Magnus




More information about the Time-nuts_lists.febo.com mailing list