I am going to route Ethernet differential par in a four layer PCB which contains Top, PWR planes, GND plane and Bottom stackup. These diff pair are going to be routed in the top layer.
In order to maintain the impedance, I would like to use a power plane as reference beneath the signals, instead of the GND plane, as it provides better dimensions for routing (width and spacing) in the PCB toolkit calculator. But I am afraid that, even an continuous power plane will not provide a proper reference for those Eth differential pairs.
This is because, the supply voltages of the Ethernet Phy is not the same as the board I am designing, they are two different PCBs. But they share the GND net
So my question is: is there any chance that a 3v3/1v8/5v power plane could provide a valid reference for those Ethernet MDI signals?