Bob:
No... at least according to Amphenol, who makes the connectors, the
mating end of the center pins are identical diameter in both the 50 and
75 ohm BNC versions and are mechanically interchangeable.
Not so with Type N connectors, where the pins are different diameters
and can cause the problems you mention.
Jack K8ZOA
Bob Nielsen wrote:
The reason it *might* stop the signal is that a 75 ohm center pin
would not necessarily make contact with the 50 ohm center contact of
the barrel. The diameter is quite a bit smaller in order to achieve
75 ohm impedance. Likewise, a 50 ohm center pin could damage a 75 ohm
receptacle.
Bob, N7XY
On Aug 6, 2007, at 10:46 AM, Jack Smith wrote:
Richard:
I would be amazed if a single 50 ohm barrel would "stop" a digital
video signal.
A high bit rate digital waveform, such as digital video, is subject
to destructive interference due to reflected signals--if the bit rate
is sufficiently high and the rise/fall time sufficiently short, and
if the noise margin of the signal is sufficiently low for reflected
signals to cause bit errors. I really doubt that a single 50 ohm
connector, however, would cause a major problem. There's a much
larger problem with high frequency attenuation in sending data over
coaxial cable, which is normally fixed via equalizers and
regenerators in the telecom world. That's one reason fiber has proven
to be desirable even for relatively low bit rates.
There's a lot of "ifs" in that statement, but still if I were
building a video plant with 75 ohm coaxial cable and 75 ohm
equipment, I would certainly use 75 ohm BNC connectors to reduce
reflections and because they fit 75 ohm cable mechanically.
When distributing a 10 MHz reference sine wave over 50 feet, however,
the impedance bumps from mixing 50 and 75 ohm impedances do not prove
to be a significant issue. In fact, I could probably use 50 feet of
zip cord connected via alligator clips to distribute timing signals
if I had to. (My measurements on zip cord show it to have an
impedance around 100-125 ohms but to be on the lossy side.)
Jack K8ZOA
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