[SI-LIST] Re: Reference plane resonance
- From: Steve Weir <weirsi@xxxxxxxxxx>
- To: joepaulm@xxxxxxxxxxxxx, si-list@xxxxxxxxxxxxx
- Date: Sun, 06 Feb 2005 14:25:42 -0800
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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