Re: gEDA-user: Eliminate separate Vcc planes?

[Gene Heskett <gene.heskett@xxxxxxxxxxx>]

On Monday 19 October 2009, Dan McMahill wrote:
>Larry Doolittle wrote:
>>> Good RF decoupling standard practice is to use a smaller cap
>>> (e.g. 20pF in parallel with some larger ones such as 1000pF
>>> _and_ 0.1uF or larger as needed) to get a good broad band
>>> capacitive reactance across frequency).
>>
>> I have yet to see a 20pF or 1000pF cap with less parasitic inductance
>> than a decent (e.g., X5R) 0402 cap up in the uF range.  Say, in
>> particular,
>>  TaiyoYuden  JMK105BJ225MV-F  2.2uF  0402  6.3V  X5R  0.1560 in 100's
>> If you're going to occupy board area with a cap and its connection
>> to the power nets, can anyone explain why I should choose anything
>> other than the largest value available in that size and voltage?
>
>my recent experiences are more in line with Larry's.  Most C for a given
>package and voltage seems to be the best meaning that above resonance it
>is no worse than smaller capacitance value devices and below resonance
>it is better.  And yes, this seems to fly in the face of what has been
>recommended in the past.  I've not done any careful measurements of
>older technology bypass caps but I wonder if this is one of those
>"rules" which became obsolete 15-20 years ago and no one noticed...
>
>-Dan

It may have, having been replaced with surface mounted, there leadless 
packages.  Unforch, the traces leading to the pads then become the leads, so 
I have doubts one is completely home free with that approach.
>
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-- 
Cheers, Gene
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