FreeLists / si-list / [SI-LIST] Re: Reference plane resonance

[Steve Weir <weirsi@xxxxxxxxxx>]

Joe, no it is not a myth.  Istvan has provided a good summary.  Most planes 
have very little to damp the cavity resonances.  A square board will 
line-up the X and Y axis modes in the center, all other things considered 
equal.   But as Istvan said, alter the shape of the board and the modes 
will just line up somewhere else.  You can see this in any number of tools 
like Ansoft.

Whether or not plane resonance modes cause concern depends on the 
approaches that you take to:  signal integrity, EMI control, and power 
distribution.  Probably entire books could be devoted to the various 
approaches and the trade-offs of each.  From a signal integrity standpoint 
though, I think you would be wise to follow the beat of Chris Cheng's drum 
and make certain you define and manage your signal returns 
properly.  "Death by autrorouter", can be a slow and painful way to 
go.  For power distribution, be sensible and keep your bypass capacitors 
relatively close to the ICs they support, and unless you are using an 
exotic dielectric with very short wavelength your power distribution should 
not suffer for modes.  EMI is the trickier issue.  The plane design and 
altering it, or just coping with it is something your EMC consultant can 
help you with.

Steve.


At 10:43 PM 2/6/2005 +0550, Joe Paul wrote:

>I have a query on resonance of reference planes, (referring to
>http://www.sigcon.com/Pubs/edn/ppresonance.htm)
>
>I under stand that resonance freq corresponds to the time, a disturbance 
>in reference plane ,take
>to return to its origin after reflecting from edges of the board. I have 
>also read in a different
>article that , symmetric board shapes like Square should be avoided as far 
>as possible for reducing
>problems with resonance of reference planes .
>
>My doubt is
>
>Is the shape concern due to chance of noise originating at center of board 
>after reflecting from
>all four sides , reaching the center at same phase and adding up  ? or is 
>there any other reason ?
>or is the shape concern a myth ?
>
>Regards
>Joe
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