1 GND Vias beside high-speed signal via. If there are more than one gnd layer, the GND via can should be used to provide a continuous signal returning current path, right? But if there is only one gnd layer, are the gnd vias still usefull?If useless, should the gnd vias be removed?2 how to understand signal returning current on a Plane underneath that is not a GND layer?ex: a VCC plane layer stacked closely under a signal layer, how to understand the returning current on VCC layer?
Yes, you can use GND vias for a signal return path. If you have only one ground layer, hopefully you can route the signal from one side of that layer to the other. Return current will also travel on VCC planes. If your signal transistions from using a GND layer to a VCC layer it helps if the two power layers are tightly coupled with as thin a dielectric as possible. Nearby decoupling capacitors also help shunt high-speed return currents between two planes, as long as their mounting inductance is minimized.
Donal Telian, Thank you very much for your help!