I'm just starting to read up on RF circuits, and I'm a little confused about what the characteristic impedance of a system is (not the characteristic impedance of a transmission line, I understand that pretty well). For example, in Pozar's book he talks about a 3 dB attenuator with a 50 Ω characteristic impedance (represented by a T configuration of resistors with a port on either side). Does that just mean that all the transmission lines connected to the ports have a 50 Ω characteristic impedance? Is that 50 Ω value determined by the lumped elements in between the ports in any way, or am I just deciding what all the connecting transmission lines will be? Basically, what is required for me to say "my system has a 50 Ω characteristic impedance?" Hopefully what I'm asking is clear... I feel like this should be a fairly simple thing.