Re: gEDA-user: Eliminate separate Vcc planes?

[asomers@xxxxxxxxx]

I second this request.  All of the quantitative data that I have seen
basically says that for a given dielectric, the inductance is a
function of the package size and shape regardless of the capacitance.
And yet the rule of thumb continues to be mixing capacitor values to
handle a range of frequencies.  Is that advice wrong (outdated,
perhaps?) or is there more to it than the datasheets show?
-Alan

On Mon, Oct 19, 2009 at 3:40 PM, Larry Doolittle
<ldoolitt@xxxxxxxxxxxxxxx> wrote:
> 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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