[Elecraft] 50 and 75 ohm BNC Connectors

[Jack Smith]

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