[SI-LIST] Re: DDR2 2-slot design preference...

I have heard many times about cavity resonances between power and ground
planes being the source of problems.  However, I have yet to see any clear
proof that such things happen and I have never seen a product fail from
this in the design of 3000+ PCBs.  Could those who claim this happens
please supply some evidence of this?  Not simulations, but actual measured
results and not a statement that the last place it happened was at a client
who has an NDA in place that prevents revealing the data.

Lee Ritchey


> [Original Message]
> From: steve weir <weirsi@xxxxxxxxxx>
> To: <Monji.Jabori@xxxxxx>
> Cc: <si-list@xxxxxxxxxxxxx>
> Date: 10/27/2007 2:09:16 PM
> Subject: [SI-LIST] Re: DDR2 2-slot design preference...
>
> Jabori, the issue in each case will be the resonant frequency of the GND 
> - GND cavity.  Stitch the cavity with an adequate via density to get the 
> resonance well above your signal energy and either topology can be made 
> to work well.
>
> Steve.
> Jabori, Monji wrote:
> > Hi Experts,
> >  
> >
> > I am looking at two designs for a 2-slot DDR2-800 memory system. One is
> > a butterfly design and the other has the slot on opposite sides of the
> > motherboard (top and bottom).
> >
> > I am looking at the GND reference for both designs and have the
> > following question knowing that I have to use GND referenced layers for
> > each DDR2 channel.
> >
> >  
> >
> > For a butterfly design, my traces on an 8-layer design will flow from
> > the memory controller via 2 internal layers with 2 different GND layers,
> > i.e., one slot will have to change GND reference while the other will
> > not.
> >
> >  
> >
> > In an opposite 2-slot design, however, we can make the traces that go to
> > each internal layer have the same GND reference as the DIMM that is
> > closer to it. For example, the Top DIMM will have the same GND reference
> > (L2) as its channel traces coming on L3 while the Bottom DIMM will have
> > its GND reference (L7) as its channel traces coming on L6.
> >
> >  
> >
> > Having said the above, would you guys prefer one design over another
> > from an Signal Integrity point of view??
> >
> >  
> >
> > Thanks in advance.
> >
> >  
> >
> > Monji
> >
> >  
> >
> >  
> >
> >
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>
> -- 
> Steve Weir
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