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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