The_Dutchman
Member level 1
Dear All,
I'm planning on using a transistor to drive a 75 Ohm coaxial line (RG6) with just a digital signal. From collector to VDD there will be a 75 Ohm resistor. The signal at the collector is than AC-coupled onto a RG6 coaxial cable. At the end of the long cable (100m, neglect the attenuation for now) there is a 75 Ohm termination.
So if my transistor is off, the 75 Ohm pull-up resistor needs to pull the coaxial line to VDD, but there is also the 75Ohm termination where the current flows through. I was wondering how fast this would be as there is 5*R*C needed for proper settling. I was wondering what the capacitance of a coaxial cable is. In the datasheet I find 20.6pF/ft or 67.59 pF/m. Does this simply scale? So does my pull-up resistor see 6.759nF for a 100m cable? This would mean that 5*R*C= 2.5µs and that the fastest I can switch is around 400kHz?
Does this make any sense? Or am I missing some transmission line magic because these cables are typically used up to 1GHz?
Thanks
I'm planning on using a transistor to drive a 75 Ohm coaxial line (RG6) with just a digital signal. From collector to VDD there will be a 75 Ohm resistor. The signal at the collector is than AC-coupled onto a RG6 coaxial cable. At the end of the long cable (100m, neglect the attenuation for now) there is a 75 Ohm termination.
So if my transistor is off, the 75 Ohm pull-up resistor needs to pull the coaxial line to VDD, but there is also the 75Ohm termination where the current flows through. I was wondering how fast this would be as there is 5*R*C needed for proper settling. I was wondering what the capacitance of a coaxial cable is. In the datasheet I find 20.6pF/ft or 67.59 pF/m. Does this simply scale? So does my pull-up resistor see 6.759nF for a 100m cable? This would mean that 5*R*C= 2.5µs and that the fastest I can switch is around 400kHz?
Does this make any sense? Or am I missing some transmission line magic because these cables are typically used up to 1GHz?
Thanks