Re: gEDA-user: Eliminate separate Vcc planes?

[Larry Doolittle <ldoolitt@xxxxxxxxxxxxxxx>]

Neil -

On Mon, Oct 19, 2009 at 01:20:23PM -0700, Neil Hendin wrote:
> If you look at the RF S-Parameters of the capacitor at frequencies
> above the self resonance, they look inductive, not capacitive.

I'm not sure what the precise definition is for the S-Parameters
of a two-terminal device, but the conclusion is accurate.

> 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?

OK, I suppose if you're building cell phones and selling them by
the hundreds of thousands, the nickel you could save by using
a lower value would add up.  But for me, the cost of assembly and
documentation probably exceeds the cost of the component itself.

   - Larry


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