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federico.massimi , 11-02-2021, 02:08 AM

Hi, for a PCB I have to go with an ethernet signal (4 differential pairs at 100 ohm) from one connector to another.
For the layout I have to go from the top layer to the bottom layer of a 4-layer PCB (Signal-GND-POWER-Signal).
Since I haven't found specific design guides for this topic, I have some doubts and a neophyte question:
1. How to size the Via to have a matched impedance?

2. The reference plane of the top layer is the ground plane below, while for the bottom layer it is the power plane, do I have to draw a continuous ground area also on the power plane under the traces I draw to have the same reference?

3. Since one connector is SMD and the other is Through hole, for how my layout is I have two choices, What's the best way?
a. With two differential pairs go directly to the SMD connector at the TH on the top layer and with the other two differential pairs go down to the bottom layer.
b. Go with all 4 differential pairs on the bottom layer by following the same path (Connector -> top layer -> Via -> bottom layer -> Connector).

Thanks in advance.

qdrives , 11-03-2021, 10:40 AM

1) There is a presentation titled "Anatomy, Modeling and Simulation of High Performance Vias: Why It Hasn't Mattered Until Now" on https://event.on24.com/eventRegistra...epage=register

2)There are plenty of video's on layer stack-up. Take those of Rick Hartley like this one https://www.youtube.com/watch?v=ySuUZEjARPY and this one https://www.youtube.com/watch?v=QG0Apol-oj0. Personally, I would ge for Gnd as reference.

3) So it is at least Gb ethernet? If the ethernet is just past through, then there is no reference other then the lines to each other (true differential pair, see Rick Hartley).
Make sure that the propagation delay (time delay) of the signals is the same. Via's add delay. If you route all on the bottom, chances are greater that all will have the same delay. SO, I would go for B. Do note that a through hole connector already has connections on both the top and bottom, so unless the SMD connector is on the top, you do not need via's.