Re: gEDA-user: Eliminate separate Vcc planes?

[Darrell Harmon <dlharmon@xxxxxxxxxxxx>]

On Mon, Oct 19, 2009 at 5:27 PM, Dan McMahill <dan@xxxxxxxxxxxx> wrote:
> 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

I have done some testing of various passives (mostly 0402) and came to
the conclusion. I tested both shunt and series capacitors on a 50 ohm
transmission line with a VNA. What was most interesting to me was that
the large value capacitors performed better as series coupling caps
than the small ones. The single layer caps in the 1 to 100 pF range
frequently had parallel resonances in the 10 to 30 GHz range. Most of
the multilayer caps (10 nF to 1 uF) performed well to at least 30 GHz.
I like to use 3 terminal feedthrough capacitors when I am concerned
about frequencies > 1 GHz getting on the power supply.

Darrell Harmon


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