[SI-LIST] Re: Bypass vs Decoupling capacitors

Steve,

Actually the opposite is true.  As it was reported and published,
for the RC termination to be effective, the cumulative inductance
of RC termination parts for a wide
range of board geometries has to be approximately 20% of the
plane inductance or less.  If you do a Vcc / Sig / Sig / Gnd sandwich,
the plane inductance becomes higher, and it will be proportionally
easier to satisfy the inductance requirement from the RC termination
components.

Regards,
Istvan

----- Original Message -----
From: "steve weir" <weirsp@xxxxxxxxxx>
To: "Istvan NOVAK" <istvan.novak@xxxxxxxxxxxxxxxx>; <cpad@xxxxxxxxx>;
<si-list@xxxxxxxxxxxxx>
Sent: Friday, August 27, 2004 9:01 AM
Subject: [SI-LIST] Re: Bypass vs Decoupling capacitors


> Istvan, the assumption, is that mounted inductance will make discrete
> devices ineffective at HF. That assumption then leads to dedicated Vcc /
> Gnd pairs as necessary with lossy material to damp the cavity, as opposed
> to Vcc / Sig / Sig / Gnd sandwiches.  So we lose the second surface of the
> GND layer as an image return.
>
> Regards,
>
>
> Steve.
>
> At 08:55 AM 8/27/2004 -0400, Istvan NOVAK wrote:
> >Steve,
> >
> >I am curious, what do you mean by "it is plane
> >  termination that requires not only the extra
> >parts, but extra layers"?
> >
> >Regards,
> >Isatvan
> >
> >
> >----- Original Message -----
> >From: "steve weir" <weirsp@xxxxxxxxxx>
> >To: "Istvan NOVAK" <istvan.novak@xxxxxxxxxxxxxxxx>; <cpad@xxxxxxxxx>;
> ><si-list@xxxxxxxxxxxxx>
> >Sent: Friday, August 27, 2004 2:03 AM
> >Subject: Re: [SI-LIST] Re: Bypass vs Decoupling capacitors
> >
> >
> > > Istvan, well we do agree on both the points that it requires judgment
and
> > > cost is a big issue, especially if it means adding layers.  I can see
a
> > > number of scenarios where it works the other way, and it is plane
> > > termination that requires not only the extra parts, but extra layers.
The
> > > cost of placements needs to be accounted as well.  But then, we get
paid
> >to
> > > figure out the appropriate compromises.
> > >
> > > Regards,
> > >
> > >
> > > Steve.
> > > At 08:48 PM 8/26/2004 -0400, Istvan NOVAK wrote:
> > > >Steve,
> > > >
> > > >Eventually it is a judgement call of the designer.  If you have to
> > > >increase the number of layers by one (and to keep the stackup
> > > >symmetry, it is never just one), the board cost may go up by
> > > >$5-15, depending on the size and complexity.  For that money
> > > >one can buy many hundred resistors and capacitors to quiet
> > > >the planes.
> > > >
> > > >Regards,
> > > >Istvan
> > > >
> > > >
> > > >
> > > > > Istvan, yes on most of the points we agree.  There are always
> >exceptions
> > > >to
> > > > > any hard rule.  A large Vcc plane that does not have thin / lossy
> > > > > dielectric is subject to resonance at pretty low frequencies.  So
in
> >the
> > > > > cost equation, we have to figure out which costs more:
> > > > >
> > > > > 1) Dealing with the increased route complexity of an islanded
scheme
> > > > > 2) Buying lossy dielectric, or special parts / materials to edge
> >terminate
> > > > > the planes.
> > > > >
> > > > > I am very biased towards 1)
> > > > >
> > > > > Regards,
> > > > >
> > > > >
> > > > > Steve.
> > >
> > >
>
>
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