[SI-LIST] Re: UltraCAD ESR and Bypass Capacitor Caculator

  • From: Kim Flint <si@xxxxxxxxxxxx>
  • To: si-list@xxxxxxxxxxxxx
  • Date: Fri, 15 Aug 2003 15:25:46 -0700

Hi Charles-

At 02:39 PM 8/14/2003, Grasso, Charles wrote:
>Hi Kim,
>First - thanks for putting the slides up on the bweb for
>all to see. I think that you may have missed the point
>a little. In your scenario ( a signal trace switching
>planes )the location of the caps is vital.

In fact, the simulations I did seemed to show otherwise. Sometimes cap 
location makes a difference, and sometimes it makes no difference at all. 
For example, in the large size board with the signal switching across 3 mil 
spaced planes, there was effectively no difference between the cap being 
right next to the via or removed from the board totally. The interplane 
capacitance was far more effective. On the other hand, with a 50 mil plane 
spacing the cap location had a more obvious effect. The lesson I get from 
that is to make sure and consider the application and the actual conditions 
at hand when making such design decisions.


>The discussion was centered on the location of caps
>wrt power distribution. The location of the capacitors
>(within reason) will not affect a S11/S21 measurement
>that much.

yes, I was aware of that aspect of the discussion. I was also aware that 
the discussion had become a bit mixed up with the return current aspect, 
and that people were becoming confused by that. I was hoping to help 
clarify the distinction between the two in terms of capacitor usage. I 
think I explained that in the second paragraph of my post, but maybe I 
wasn't very clear. Larry just did a much better job of explaining it! In 
any case, it seemed to me the s-parameter aspect had been discussed enough 
already so I thought it would be more interesting to look into the layer 
switching cases. After I did a bunch of simulations for myself, I figured, 
hey why not share!

>Fancy tackling that little problem?

Does it need more tackling? I don't mind playing around with it but it 
seems to me others have already done a lot of work in this area. I might 
play around with it anyway for my own edification, and if something 
interesting comes of that I'll let you know!

kim


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