Subject: Re: Bird 28.2 - Comments and Suggestions

From: [email-masked] (Dileep Divekar)
Date: Tue, 22 Aug 1995 08:54:03 +0800

Editorial Comment:
The character used to separate the section information
is a slash (/) and not a backslash (\) as mentioned.

Comment:
It would be helpful to clarify if ALL of L/R/C have
to be specified. That is, is any of the following allowed?
/Len=1.3 L=1.3n/
/Len=2.5 L=3n C=0 R=0/

Suggestion:
I think the off diagonal entries in the C matrix
should be specified as negative numbers, if they
are indeed negative, as they are in this case,
instead of making them positive.

Suggestion:
For the sake of completeness and generality,
the specification should allow an optional G
specification which is a conductance in parallel
with C, similar to R in series with L.

------------------------------------------------------------
Dileep Divekar
Contec CAE, Ltd.
2188 Bering Drive
San Jose, CA 95131

Phone - (408)-434-6767 x 126
Fax   - (408)-434-6884
Email - [email-masked]
------------------------------------------------------------

---

From: Stephen Peters <[email-masked]>
Date: Thu, 24 Aug 1995 09:13:37 -0700

Hello Dileep:
     Thanks for taking the time to review the Bird and
offor your comments and suggestions.  My answers are below, 
prefixed by >>>.

             Best regards,
             Stephen Peters
             Intel Corp.

-----------------
On Tue, 22 Aug, Dileep Divekar at Contec wrote

Editorial Comment:

   >>> Will fix

Comment:

    >>> Paragraph three of the usage rules states 
    >>> the following: "Specifying a length or L/R/C value 
    >>> of zero is allowed."  Paragraph C) under Legal
    >>> Subparameter Combinations does say the 'any combination'
    >>> of L, R and C are allowed, but I will clarify the
    >>> statement to indicate that all three are not required
    >>> to be specified.

Suggestion:

    >>>  When I looked at the acadimic and technical 
    >>>  papers on the subject (not to mention the software
    >>>  we use in house) the off-diagonal elements are
    >>>  always positive quanities.  For what ever reason 
    >>>  this seems to be the convention and I don't see a need
    >>>  to do it differently.

Suggestion:

    >>>  Good point. I did not include a 'G' parameter
    >>>  because the current IBIS 2.1 Matrix descriptions
    >>>  do not include one and I was trying to limit the
    >>>  scope of the bird.  If the committe did want
    >>>  to add the a conductance parameter it could be
    >>>  in a seperate proposal.
    >>>

---

From: [email-masked] (Dileep Divekar)
Date: Thu, 24 Aug 1995 11:46:59 +0800

>>>  this seems to be the convention and I don't see a need

There is no standard convention. I can give many references
published in technical journals, in which off-diagonal elements
of the C matrix are given as negative numbers. Following are
few examples:
Colin Gordon, Thomas Blazeck and Raj Mittra, "Time-Domain
Simulation of Multiconductor Transmission Lines with
Frequency-Dependent Losses", pp. 1372-1387, IEEE Trans.
on Computer-Aided Design, Vol. 11, No. 11, November 1992.
Tak Tang, Michel Nakhla and Richard Griffith, "Analysis
of Lossy Multiconductor Lines using the Asymptotic Waveform
Evaluation Technique", IEEE Trans. of Microwave Theory and
Techniques, pp. 2107-2116, Vol. 39, No. 12, December 1991.
A.J.Gruodis and C.S.Chang, "Coupled Lossy Transmission Line
Characterization and Simulation", IBM Journal of Res. Develop.,
pp. 25-41, Vol. 25, No. 1, January 1981.

In my opinion, why hide the fact that they are negative ?
Why not just specify them as they are ?

    >>>  to add the a conductance parameter it could be
 
Why add more delay and more paperwork ?
------------------------------------------------------------
Dileep Divekar
Contec CAE, Ltd.
2188 Bering Drive
San Jose, CA 95131

---

From: J Eric Bracken <[email-masked]>
Date: Mon, 28 Aug 1995 10:46:09 -0400

Howdy, EIA/IBIS folks:

  I have a couple of comments on some earlier posts:

Point 1:  [Regarding the signs of the off-diagonal C matrix entries]
----------------------------------------------------------------

  One interpretation of the capacitance matrix is that it relates
the charges Q1, Q2, ... Qn on a set of n conductors to the potentials
V1, V2, ... Vn on those same conductors.  For two conductors, we'd 
have

            Q1 = C11 * V1  +  C12 * V2
            Q2 = C12 * V1  +  C22 * V2

(This is just writing out the "matrix" multiplication [Q] = [C][V].)

  The simplest two-conductor situation is a parallel plate capacitor C.
In this case, we know that the charge on the top plate (conductor 1)
is Q1 = C*(V1 - V2); and so expanding the equation out, C11 = C and
C12 = -C.  The charge on the bottom plate is Q2=-C(V1-V2), so everything
is consistent if the off-diagonal terms are assigned a negative sign.

  To think of it another way, suppose that we connect conductor 1 to a
constant voltage source, and then we raise the potential on a second
nearby conductor.  Additional NEGATIVE charge will appear on
conductor 1 in order to terminate the extra field lines coming from
the second conductor.

  Many field solvers actually compute "charges", and so it is natural
for them to report the off-diagonal entries as negative.

Footnote: The situation is rather different for INDUCTANCE matrices.
In this case, we can arbitrarily assign a certain direction as being
"positive" for current flow.  Depending on the orientations of the
currents in two neighboring wires, the mutual inductances between them
could have either positive or negative sign.

Point 2: [Regarding the need for G matrices]
----------------------------------------------------------------

  Here's the problem that I have with specifying the G matrix:  suppose
we conduct the measurement of G at DC.  Chances are that the dielectrics
used in the package are so good that G is immeasurably low; all the 
entries of the G matrix are then zero.

  You _can_ run into nonzero G values at high frequencies.  This occurs
because the poor little atoms in the dielectrics can't spin fast enough
(they have some mass) to reorient themselves to the rapidly changing 
electric fields.  The lag causes some very frequency-DEPENDENT losses.

  So, at what frequency do you specify G?  And is data from just one
frequency point good for a transient analysis?

Another footnote: certainly the R and L matrices can change due to
skin effect as we increase the frequency.  But the changes in L are
very minor (usu. less than 10%); changes in R are much more
substantial, but at least there is a well-defined non-zero DC value
for R that's easy to measure.

--Eric