Hmm, this seems to be just another question on line impedances.
I understand that when we say "transmission line" effects we talk about things like cross talk, reflections and ringing (I guess that is just about it). These effects are not present at low frequencies where the PCB trace behaves like an "ideal" transmission medium, more like we expect a wire to behave in our early school days.
I also understand that the 50 ohm value comes not from the line resistance which is going to be very small and less than 1 ohm. This value comes from the ratio of L and C on the line. Changing C by changing the trace height above ground plane or changing L by changing the trace width shall change the impedance of the line.
We all know that the reactance of L and C is dependant on the signal frequency as well. Now my questions:
Why should we not call this as line reactance only rather than line impedance?
How can it be just 50 ohm? It has to be signal frequency dependent right? E.g 50 ohm at 1 MHz
Will the world end if I chose to do a 100 ohm or 25 ohm trace instead? I know that while we like to say 50 ohm as a magic number, it will be within some range around 50 ohm and not 50.0000 ohm exactly.
Is there any time when the actual resistance of a PCB trace may matter?