RE: [SI-LIST] : Upper limit of interplane capacitance

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From: Istvan Novak - Board Design Technology (inovak@pompom.east.sun.com)
Date: Thu Jun 15 2000 - 08:11:18 PDT


Greg, Erik,

The dielectric losses do help at higher frequencies to dampen out resonances. With FR4
material, resonance impedance ripple is minimal above 1.5GHz. However, to suppress the
resonances just with the dielectric loss on a large PCB, the loss tangent should be
fairly high. On many printed circuit boards through vias are still common, so the
signal via should pass through the lossy material, which affects the signals unless we
surround the immediate vicinity of the via with lower-loss dielectrics.

Intentionally increasing the conductive loss by using lower conductivity metalization is
also a possibility, but it will add to the total board thickness, and as it was pointed
out, it will adversely impact the DC capability of the planes.

Since low spreading inductance of the plane structure is the main goal in high-speed
designs, we can achieve both low inductance AND suppressing resonances by putting the
planes sufficiently close.

Regards

Istvan Novak
SUN Microsystems

        From: gedlund@us.ibm.com
        X-Lotus-FromDomain: IBMUS
        To: si-list@silab.eng.sun.com
        Date: Thu, 15 Jun 2000 09:23:26 -0500
        Subject: RE: [SI-LIST] : Upper limit of interplane capacitance
        Mime-Version: 1.0
        Content-Disposition: inline
        
        Larry and Erik,
        
        [Erik Daniel wrote]
> I agree with most of your comments on power plane capacitance, but I have
        to
> disagree with one point -- dielectric loss does NOT decrease with
        decreased
> thickness of the dielectric -- dielectric loss is independent of the
> dielectric thickness in particular, and all geometrical concerns in
        general
> (unlike skin-effect loss).
        
        Someone once explained dielectric loss to me as absorption of the energy
        in the E/M field by quantum resonances in the molecules that make up the
        dielectric material. In my mind, I thought this sounded analogous to the
        photoelectric effect, where you shine light of a particular frequency on
        an atom and it gets absorbed if the frequency corresponds to one of its
        quantum states. Is this truly the physical origin of dielectric loss?
        If so, that would explain why it's strictly a material property and not
        geometry-dependent...
        
        Greg Edlund
        Advisory Engineer, Critical Net Analysis
        IBM
        3605 Hwy. 52 N, Dept. HDC
        Rochester, MN 55901
        gedlund@us.ibm.com
        
        
        
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Istvan Novak Sun Microsystems, Inc.
Istvan.Novak@Sun.Com Workgroup Servers, BDT Group,
                        One Network Drive, Burlington, MA 01803
                        Phone: (781) 442 0340

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