[SI-LIST] Re: Ethernet emissions.
- From: Joel Brown <joel@xxxxxxxxxx>
- To: bryan@xxxxxxxxxxxxxx
- Date: Tue, 9 Apr 2013 09:56:12 -0700
Almost every company that makes Ethernet controllers and Phys (Including
Intel) recommends to put a cut in the ground plane around the RJ45
connector with built in magnetics and to ground the connector to chassis
ground and then strap some capacitors across the plane split. Is it better
to just use a solid plane all the way to the connector or does the plane
split sometimes solve a problem? In general I avoid plane splits. One case
where we did use a ground plane split was with an SDI phy and the
manufacturer (Genum) said it wouldn't work without it.
On Sun, Apr 7, 2013 at 7:15 PM, Bryan Ackerly <bryan@xxxxxxxxxxxxxx> wrote:
> Lee, I try to never cut up ground planes. This is the one "golden rule" i
> took away from one your own training courses I attended many years ago....
>
> Regards,
> Bryan Ackerly
>
>
> On 07/04/2013, at 4:19 AM, "Lee " <leeritchey@xxxxxxxxxxxxx> wrote:
>
> > Todd,
> >
> > Thanks for reiterating the need to avoid cutting up ground planes. I am
> > puzzled as to why anyone would recommend such a tactic. But, then, there
> are
> > a host of such "off the cuff" things that get circulated year after year
> > without any proof they are valid. Even more puzzling is why engineers
> > accept such claims without supporting evidence that they are valid.
> >
> > They keep me very busy most of the time fixing problems that they cause!
> >
> > -----Original Message-----
> > From: Todd Hubing
> > Sent: Saturday, April 06, 2013 10:54 AM
> > To: <si-list@xxxxxxxxxxxxx>
> > Subject: [SI-LIST] Re: Ethernet emissions.
> >
> > Bryan,
> >
> >
> > Please don't cut up your ground. I wouldn't want anyone on this list to
> get
> > the idea that isolating grounds is going to solve a radiated emissions
> > problem above 100 MHz. From the symptoms and the measurements you
> describe,
> > it appears that you are driving the cable relative to your circuit board
> > ground with a CM voltage produced directly by the PHY and insufficiently
> > attenuated by the connector magnetics. If you've maintained balance in
> your
> > layout between the PHY and the connector, you may need to consider using
> a
> > different connector with better CM rejection.
> >
> >
> >
> > At these frequencies, nothing you do with your ground is going to improve
> > upon the solid plane that you currently have. Trying to establish
> separate
> > grounds that are at different potentials is likely to make things worse.
> >
> >
> >
> > Todd
> >
> > -----
> >
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