Ethernet PCB routing

[AndrewHodgson]

Hi,

If I am using external magnetics, do people think it is a good idea to have a separate ground plane under the signal lines post transformer? I've had a look at some TI guidelines and they said don't put a ground plane under the signal lines between the transformer and the RJ45, but the Toradex routing guidelines seems to suggest it's a good idea.

https://docs.toradex.com/102492-layout-design-guide.pdf
page 34.

If i include the ground plane, which net should it connect to? At the moment I've got it connected to the termination plane, that is the point where all the bob smith termination resistors are connected. Then there is a capacitor connecting the plane to the shield ground.

Ethernet is 100MB

Thanks

[Daixiwen]

I don't put any plane under those lines and it works fine. If you use a plane then it must be the one that you are using right now ("chassis") and not your logic ground.
At only 100 MB I don't think you'll see a huge difference anyway, especially with such a short distance between the magnetics and the connector.

[AndrewHodgson]

Thanks! I'll probably just remove it then.

In the Toradex guidelines the reference plane does look like it has been separated. That makes sense to me because presumably the signal lines need to be isolated from the shield/enclosure ground. If you rely on the PCB for the isolation, then i would have thought the distance between the reference plane and the signal lines would be quite large, meaning you need very large tracks to get the correct characteristic impedance. 

[T3sl4co1l]

Use differential pairs, 100Ω, no ground plane.  May not actually be able to achieve that within acceptable PCB limits (trace spacing), so just get it as close as practicable and keep the routed lengths short so the discontinuity doesn't affect it too much.

Note that you can do edge-coupled differential on a single layer, buried edge-coupled (on an inner layer), or broadside coupled stripline (on top of each other, different layers).

Tim

[AndrewHodgson]

Interesting, sounds like plan.

Thanks guys! Much appreciated