[SI-LIST] Re: Ethernet magnetics question
- From: "Curt McNamara" <CurtM@xxxxxxxxxxx>
- To: "Joel Brown" <joel@xxxxxxxxxx>
- Date: Wed, 5 Apr 2006 16:58:33 -0500
From: Joel Brown [mailto:joel@xxxxxxxxxx]=20
>
>> Back to the app note suggestions:
>> If you put digital ground under the output side of the transformer
you=20
>> form a small capacitor between the output winding//Ethernet cable and
>> your digital ground. This will couple noise from the digital side to=20
>> the line side. Eliminating digital ground in this area reduces the=20
>> coupling.
=20
>I am wondering if you could get the same benefit by eliminating ground=20
>on the line side on all layers except keep a continuous ground on the=20
>bottom layer.
>Now for a board that .093 thick you have reduced the coupling=20
>capacitance from the ground plane to the magnetics and there is no
plane=20
>split.
>I am not sure how close the magnetics in a Ethernet jack are to the=20
>surface of board and how much capacitance exists between the board and=20
>the magnetics.
Yes, the capacitor is defined by surface area, dielectric, and
separation. You could calculate or measure it. I am puzzled by why you
would want digital ground in this area, since it will have common mode
noise on it. If you have some concern about chassis ground in this area
you could eliminate all planes under the output side.=20
I didn't mention but for some CE marked products you also need
separation from Ethernet line side to SELV for safety reasons (Ethernet
can get connected to the AC line accidentally via a wiring error).=20
>> As to the caps between the plane splits ... these are "hacks" which=20
>> allow you to dump digital noise onto your case via the chassis ground
>> plane. If your system is well designed you should not need them.
> =20
>So now you have a plane split which is a slot antenna that is being=20
>driven by the RF potential difference between the digital ground and=20
>chassis ground.
>I will admit the length of the plane split around a Ethernet jack is=20
>somewhat short and would only radiate at very high frequencies, hence=20
>the need for the bridging caps.
This could happen, however the practice is often simpler. You have an
emissions peak at 333MHz. By coupling digital ground to chassis you
spread this energy across a wider bandwidth, hopefully decreasing the
peak so you can pass. Obviously there are lots of problems that can
happen with this approach.
Curt
Curt McNamara. P.E.=20
Senior Electrical Engineer=20
Logic Product Development=20
411 Washington Ave. N Suite 101=20
Minneapolis, MN 55401=20
Tel 612-436-5178=20
Fax 612-672-0443=20
curtm@xxxxxxxxxxx=20
www.logicpd.com=20
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