Re: Ethernet design guidelines

On Sat, 02 Jul 2005 18:57:58 +0200, Sylvain Munaut wrote:

> Hello,
>
> I'm working on a PCB where there is an ethern 10/100 RJ45 port.
> But I'm having a hard time finding guidelines to route thos signal.
> I can put the magnetics close to the RJ45 (traces ~0.5 inch) but I need
> to carry the data signals further (about 3 inches). They are
> differential signals so routing them as a differential pair sounds
> obvious but any other recommandation ? Particular differential impedance
> to match ?
>
>
> Thanks for any hint.
>
>
> Sylvain

I used to have a differential pair impedance calculator. I don't have
it anymore, but for the most part, this rule will work: route the two
signals which make up the pair immediately adjacent to each other on the
same layer and make them the same length. Then, most importantly, use
a trace width which makes the individual tracks have a characteristic
impedance of around 55 Ohms to ground (or the nearest power plane). Then
the differential impedance will be about right.

Oh, I guess I should explain my methodology. You are trying for 100 Ohms
differential impedance. This is from the Ethernet spec. The 55 Ohm figure
comes from this highly technical formula:

Zo = Zdiff / 2 + epsilon.

Zdiff is the desired differential impedance, epsilon is uh, or rather has
been experimentally determined to be around 5 Ohms. ;-)

--Mac

.

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