[SI-LIST] Re: Chassis and Signal ground

That's an excellent point, Steve. So many times engineers think that their 
single-point "ground" is applies from DC-to-daylight.
The coupling paths that we can't see will often negate the notion of 
single-point-"ground" at higher frequencies.

In addition to Henry Ott's book, Bruce Archambeault of IBM also gives an 
excellent treatment of this topic in his book "PCB Design for Real World 
EMI Control".

Regards,

Ron


Ron Matthews, NCE
Principal MTS
Signal Integrity/EMI
Charles Stark Draper Laboratory
555 Technology Square, MS 62
Cambridge, MA 02139
Voice: 617-258-2519
Fax:    617-258-1998
Email: rmatthews@xxxxxxxxxx



At 09:37 AM 12/15/2004 -0800, steve weir wrote:
>Lee, DC single pointing can work under the right conditions and may be
>required for safety in circumstances as I outlined to Jim.
>
>That said, DC single point connection does not eliminate AC paths, and can
>greatly reduce the SRF of the resulting structures.  For example, the
>proximity of a big signal ground plane near a chassis pan makes a nice big
>resonant cavity.  More often than not, a low cavity SRF is a bad thing that
>makes for efficient radiation / efficient reception across a broad range of
>frequencies.  This makes the folks who sell ferrite cable clamps happy, but
>not usually the poor compliance guy.
>
>Regards,
>
>Steve,
>At 08:46 AM 12/15/2004 -0800, Lee Ritchey wrote:
> >If I connected them at all, and I usually don't, it would be in only one
> >place, so that the opportunity for inceased EMI is held to a minimum.
> >
> >Lee W. Ritchey
> >Speeding Edge
> >P. O. Box 2194
> >Glen Ellen, CA 95442
> >Phone- 707-568-3983
> >FAX-    707-568-3504
> >
> >I just used the energy it took to be angry to write some blues.
> >Count Basie
> >
> >
> > > [Original Message]
> > > From: Peterson, James F (FL51) <james.f.peterson@xxxxxxxxxxxxx>
> > > To: <si-list@xxxxxxxxxxxxx>
> > > Date: 12/15/2004 8:26:04 AM
> > > Subject: [SI-LIST] Chassis and Signal ground
> > >
> > > Chassis and Signal ground, what happens to our noise susceptibility if we
> > > connect them together on our cards (some of them are analog cards)?
> > >
> > > We've always kept chassis ground and signal ground separate on our cards
> >and
> > > connected them together on our backplanes. The reason we even have a
> >chassis
> > > ground on a board is because we use them as thermal layers that are used
> >to
> > > remove heat from boards (that need to operate in the vacuum of space).
> > >
> > > We are currently evaluating connecting our signal ground on the boards to
> > > the "card guides" of the chassis. The benefit is we don't need thermal
> > > layers anymore. The signal ground layers can now conduct the heat out
> > > through the guides and onto the chassis....but what have we done to our
> > > radiated noise levels and to our susceptibility to incoming noise?
> > >
> > > Regards,
> > > Jim
> > >
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