Re: How to determine per length RLC in a board

All,

Huh?

"Return path" as it is applied to AC Impedance, is the
instantaneous path that the fields take in traveling between multiple
conductors.

For balanced stripline, PWR-SIG-GND, the fields most absolutely
do propagate between the signal and both the power and ground
planes. The field equally divides between the two. As a result,
the high speed currents will use both planes. Even a floating
conductor will "capture" fields and carry a "return current".
The fields don't know to stop until they reach a metalic object.
This follows from Maxwell's first Law.

The RLC characteristic of a stripline trace (with the same dimensions)
between two ground planes, two power planes, one power and one ground
plane are identical. They are also identical for floating metal once
the fields are established.

The only difference in all of these configurations is in the "transition
region" to get onto the structure. Here things get interesting. However,
IBIS ebd models, and most of the Intel literature do not concern
themselves with modeling these effects, anyway.

regards,

scott

--
Scott McMorrow
Principal Engineer
SiQual, Signal Quality Engineering
18735 SW Boones Ferry Road
Tualatin, OR  97062-3090
(503) 885-1231
http://www.siqual.com
Alan Hilton-Nickel wrote:
> Yaping,
>
> In the SIG-GND-PWR, the PWR plane is not a return path for the signal
> layer. I suggest you model it as SIG-GND (microstrip). The high-speed
> currents will always use the GND plane, assuming good decoupling between
> PWR and GND.
>
> Alan
>
> Yaping Zhou wrote:
> >
> > Hi, All:
> >
> > If the stackup in a board is a symmetric stripline structure (GND-SIG-PWR),
> > the characteristic impedance is the same for switching high and low, and the
> > per length RLC are also the same for both switching cases.
> >
> > How to determine the per length RLC in a board with a stackup like
> > SIG-GND-PWR? The characteristic impedance and per length RLC are
> > switching-dependent, but there is no way to put two values for each trace in
> > an ebd file.
> >
> > Intel uses ebd to describe packages used for Pentium processors,I have the
> > same question there on the way to determine RLC values.
> >
> > Your help is appreciated.
> >
> > Thanks,
> >
> > --
> > **************************************
> > Yaping Zhou (r3aadv)
> > (512) 933-5803
> > Motorola Semiconductor Products Sector
> > Final Manufacturing Technology Center
> > Ed Bluestein, Austin, Texas
> > **************************************