I'm currently re-designing a 12-layer pcb. The only thing I am still struggling with is the routing of the ethernet.
The layer stackup is as follows:
- Top Layer
- Bottom Layer
I have read a lot of ethernet design guidelines which mostly stated that preferably the RJ45, Ethernet magnetics and Tx and Rx lines should have a ground keepout beneath them. Now here is my confusion I just read this paper: Ethernet Ground and it states
When designing 4 layer boards, the ground plane should exist on layer 4, assuming the differential pair is routed on layer 1. On 2 layer boards, the ground plane can be located on layer 2, the adjacent layer to the TX and RX signal pairs. Under no circumstances should a ground plane exist under the magnetics, the RJ45 connector or in between the magnetics and RJ45 connector.
I find the part of "when designing 4 layer boards, the ground plane should exist on layer 4 (layers further away?)" and the "under no circumstances should a ground plane exist under the magnetics, the RJ45" very conflicting and would love some clarification.
When following the rule of having a ground keepout around the ethernet, does this keepout have to be on all 12 layers. Or only the nearest adjacing layers?
PS: there are some related ethernet questions about layout, Stackexchange question but it doesn't completely satisfy the "why" part I am interested in and does not address the multilayer design.