Re: Ethernet design guidelines

Mac wrote:
> 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
>

Thanks ! Exactly what I was looking for.


Sylvain
.

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