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
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