[SI-LIST] Re: Capacitors arrays. worth it or not?
- From: steve weir <weirsi@xxxxxxxxxx>
- To: Mark.Randol@xxxxxxxxxx,"si-list" <si-list@xxxxxxxxxxxxx>
- Date: Mon, 22 May 2006 08:08:41 -0700
Mark, eddy currents limit the field into the lower mil or so at
100MHz. With a vertical plate arrangement, this effect isolates the
upper regions of the capacitor very similarly to the spreading
inductance effect of a PCB on a much smaller scale, magnified by the
square root of eR. When the plates are horizontal, the physical
extents of the package set-up a cavity resonator, the same way we get
one with multple PCB plane cavities if we don't stitch.
Steve.
At 07:33 AM 5/22/2006, Mark Randol wrote:
> > -----Original Message-----
> > From: si-list-bounce@xxxxxxxxxxxxx=20
> > [mailto:si-list-bounce@xxxxxxxxxxxxx] On Behalf Of Jerry Martinson
> >=20
> > I've always wondered how discrete cap performance is affected=20
> > if the caps plates are parallel with the underlying plane or=20
> > if they are perpendicular. I'd think that having them rolled=20
> > 90 degrees
> > (perpendicular) might make them perform better in some=20
> > regions. I wonder how this would extend to arrays and=20
> > whether arrays are configured rolled or not. Does anyone=20
> > know? =20
>
>American Technical Ceramics (ATC) used to recommend 'vertical' placement
>of their porcelain caps for just this reason. I've seen it make several
>100MHz's of difference in the measured resonance frequency. That was on
>a relatively thick 2 layer PCB, so on a board with a thinner component
>to ground layer spacing this could be more significant <guess>. I
>didn't find it on their web site, but here is their link.
>
>http://www.atceramics.com/
>
>Now how much of this was due to plate coupling to the substrate, or
>reduced effective capacitance and inductance because of current crowding
>towards the new 'bottom' of all the plates, beats me. It seems to me in
>the horizontal orientation, the upper plates would have slightly more
>inductance due to the greater loop area. Which effect dominates,
>capacitance or inductance? =20
>
>The problem at the time was fixed, so we didn't investigate further.
>
>--
>Mark Randol, RF Evaluation & Application Engineer
>Not speaking for my company, etc
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