I am designing a circuit that will receive a 20 MHz square wave (LVDS), convert the LVDS to single ended, and drive a 75 Ohm line. I've found some 75 Ohm drivers from TI, but the applications engineers are telling me they are not sure if they can handle those speeds at that load. I've decided to use a Darlington Array type of driver with open collector outputs based on some feedback from the applications engineers.
Some transmission line calcs (not my specialty, correct if needed):
λ = 300 / fMHz = 300 / 20 = 15 meters
Transmission line if:
≥ λ / 8 = 1.875 meters ~= 6 feet
So, if I use greater than a 6' BNC cable, I need to worry about impedance matching, correct? Assuming the BNC cable is 75 Ohm, the receiver is 75 Ohms, I need to make the source (open collector output) 75 Ohms as well? I get confused about the ability to drive a 75 Ohm load (i.e. current drive capability) with the need for impedance matching. If this is the case, how do you impedance match an open collector output?