09-21-2009 04:48 AM
09-21-2009 10:09 AM
09-24-2009 03:30 AM - edited 09-24-2009 03:53 AM
I don’t know exactly why Xilinx suggest using only inner layers (I suppose they don’t want to bother with questions that might arise from various users…).
Using outer layers is quite OK as long as you are aware of signal speed at the outer PCB layer. As we all know voltage signal reach the speed of light. But speed of light isn’t the constant in all medium. Speed of light in inner layers is smaller compared to outer layers, that is because in inner layer the conductor is surrounded by dielectric from both sides. At the outer layer from one side it is the same dielectric as in inner layer but at the other side the dielectric is different.