On what does the signal velocity depend for a signal traveling through a PCB trace?
According to Wikipedia, it depends exclusively on the relative permittivity (ε_r) of the medium (well, it also depends on c, but that one is a constant), under the approximation that for PCBs, the relative permeability of the medium is 1.
My question is: Shouldn't the signal velocity be a function of the characteristic impedance of the transmission line?
My thought process is: If I have a trace that has a width change, then there will be a characteristic impedance change, and that would cause partial reflection of the incoming wave. But there is no energy absorption anywhere, so the only cause for a partial reflection can be a change in the wave's propagation speed, right?
According to the equation in Wikipedia's signal velocity page, there would be no change in velocity, since the medium is the same, so the relative permittivity remains the same.
Can someone shed some light on this?