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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