impedance calculation
I have 6-layers stackup:
-----SIGNAL
-----GND
-----SIGNAL
-----SIGNAL
-----VCC
-----SIGNAL
the board is for PCI express cards, with differential lines, that must be impedance controlled to 100 Ohm differential.
For top layer and layer 3, the reference layer is the GND layer and I can use the microstrip model for calculationg the impedance.But how to calculate impedance for layer 4 and bottom layer? They don't have adjacent reference GND layer.