FreeLists / si-list / [SI-LIST] Re: Ethernet magnetics question

[Joel Brown <joel@xxxxxxxxxx>]

Curt McNamara wrote:
> I think the app note writer and your EMI consultant have different views
> of the situation.
>
> Your Ethernet cable is a huge antenna. In most cases it is unshielded.
> Common mode noise present on the cable will radiate very effectively
> since its ground return path is uncontrolled and huge.
>
> So one function of the transformer and common mode choke is to reduce
> the amount of noise that gets on the cable.=20
>
> Transformers have different windings for each side. The common mode
> noise that couples across the windings is due to capacitive coupling
> between the two windings (due to transformer construction). The level of
> noise present on the windings is reduced by the common mode choke, a
> separate pair of windings. So there is always some coupling from the
> transformer itself.
>
> Back to the app note suggestions:
> If you put digital ground under the output side of the transformer you
> form a small capacitor between the output winding//Ethernet cable and
> your digital ground. This will couple noise from the digital side to the
> line side. Eliminating digital ground in this area reduces the coupling.
>   
I am wondering if you could get the same benefit by eliminating ground 
on the line side on all layers except keep a continuous ground on the 
bottom layer.
Now for a board that .093 thick you have reduced the coupling 
capacitance from the ground plane to the magnetics and there is no plane 
split.
I am not sure how close the magnetics in a Ethernet jack are to the 
surface of board and how much capacitance exists between the board and 
the magnetics.
> Since ground is usually a good thing :-) they recommend using chassis
> ground (sheet metal) under the output side. The gap is to eliminate an
> even tinier capacitor ...
>
> As to the caps between the plane splits ... these are "hacks" which
> 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.
>   
So now you have a plane split which is a slot antenna that is being 
driven by the RF potential difference between the digital ground and 
chassis ground.
I will admit the length of the plane split around a Ethernet jack is 
somewhat short and would only radiate at very high frequencies, hence 
the need for the bridging caps.

Thanks - Joel

>                                               Curt
>
> Curt McNamara. P.E.=20
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> -----Original Message-----
> From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx]
> On Behalf Of Joel Brown
> Sent: Wednesday, April 05, 2006 4:00 PM
> To: si-list@xxxxxxxxxxxxx
> Subject: [SI-LIST] Ethernet magnetics question
>
>
> The following is from an Intel Ethernet  controller app note: *The
> magnetics module chassis or output ground (secondary side of=20
> transformer) should be
> separated from the digital or input ground (primary side) by a physical=20
> separation of 100 mils
> minimum. Splitting the ground planes beneath the transformer minimizes=20
> noise coupling between
> the primary and secondary sides of the transformer and between the=20
> adjacent coils in the magnetics.
> This arrangement also improves the common mode choke functionality of=20
> magnetics module.
>
>
> Figure 9 illustrates the split plane layout for a discrete magnetics=20
> module. Capacitors are used to
> interconnect chassis ground and signal ground.
> Figure 10 below shows the preferred method for implementing a ground=20
> split under an integrated
> magnetics module/RJ-45 connector. The capacitor stuffing options (C1=20
> through C6) are used to
> reduce and filter high frequency emissions. The values of the capacitor=20
> stuffing options may be
> different for each board. Experiments will need to be performed to=20
> determine which values provide
> best EMI performance.
> The placement of C1 through C6 may also be different for each board=20
> design (in other words, not
> all of the capacitors may need to be populated). Also, the capacitors=20
> may not be needed on both
> sides of the magnetic module.
>
> *I sent this app note to our EMI consultant he said it does not work.=20
> Plane splits cause EMI problems, they not solve them.
> Adding capacitors across the plane split effectively creates an RF short
>
> across the plane split which defeats the isolation that is trying to be=20
> achieved in the first place.
> Our EMI consultant recommends using a solid continuous  plane underneath
>
> the Ethernet magnetics. I have also seen this plane split recommendation
>
> for other types of I/O connectors. Does anybody know the origin of this=20
> idea and if it has any practical validity?
>
> Thanks - Joel
>
> *
> *=20
>
>
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