Re: gEDA-user: Eliminate separate Vcc planes?

[Dan McMahill <dan@xxxxxxxxxxxx>]

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


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