From: Bill Dempsey ([email protected])
Date: Mon Nov 01 1999 - 11:02:48 PST
Mark Randol wrote:
> 2nd, most ceramic 0603's and 0402's can have SRFs up around
> 3-4GHz. Mounted parasitic inductances are another matter left
> for the user :) The values top out at about 33pF for 1.5GHz, but
> they are usable. Of course, I'm using them in a different
> application than most on the list. I'm predominately concerned
> with analog circuits, i.e. "discrete" frequency and relatively
> low amplitude swings), so small capacitances are generally
> sufficient. With digital circuits, I can understand where a
> single small capacitance may not supply sufficient
> "instantaneous" current. I don't know how much it costs to buy
> and mount one of these, but from what I've been told by people
> that should, 20 cents would be quite high unless you are using
> porcelains or another special device.
When you say "most 0603/402's can have SRF around 3-4GHz" you can NOT be referring to the typical ceramic X7R type decoupling caps used by "most" on this list. 0.1uF ceramics (0805) SRF around 30 MHz (depends...) and .001uFs
~150-200 MHz~. Add a long squirrley trace to this and watch the capacitor's effectiveness drop.... Todd Hubing published a good paper in the IEEE a few years back on the effectiveness of decoupling caps vs. board capacitance.
It would behoove all to review this before making leaps of faith on decoupling strategies.
> Mark Randol, RF Measurements Engineer
> Motorola SPS, Inc.
> M/S EL381
> 2100 E. Elliot Road
> Tempe, AZ 85284
> (480)413-8052 Voice
> (480)413-3455 FAX
> [email protected]
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