[SI-LIST] Re: Common Mode vs. Even Mode


 Hi All:

 I would like to put one more question on this subject. My interest is
interconnection in rigid disk drive. Two-trace  with ground in 2D
cross-section. If possible, I would like to hear  in terms of inductances
and resistances instead of impedance since impedance is derived term
from L,R and C. (Is there any reason people talk with Z?)

 Fact: Regardless of the definitions posted here, we say we excite the
interconnection differentially and common mode appears as conseqeunce of
impedance mismatch at both source and receiving ends. The transmission
line parameters L_DM, L_CM etc from field solver with differential set-up
and common mode set-up agree well with VNS measurement in frequency domain
and TDR measuremetn in time-domain. Here all the parameters are ONE SINGLE
VALUE.

 Problem: I simulated same layout(s) with another  field solver that spits
out line parameters as form of MATRICES [R],[L], and [C]. There are lots of
books (like S.H Hall/G.W Hall/Macall) that show L_DM = (L11-L12)*2 and
L_CM = (L11+L12) etc.  I cannot see these relationships. In some cases,
L_DM = (L11-L12)*2 looks Ok, but some cases are not. I would say this
equations do not work. (Is this, possibly, something to do with
definition of matrix size.  When there are (N+1) conductor, size of
matrixis N X N.)

 Thanks in advances.
 Sincerely,
 se-ho You


On Thu, 16 May 2002, Doug Brooks wrote:

>
> I think the issue is a simple misunderstanding and confusion in terms.
>
> In a coupled pair of traces, the individual trace termination value is
> differential          Z11 - Z12
> Common Mode  Z11 + Z12
>
> In a practical sense, the differential resistors are placed in series, so
> the net "differential" impedance is 2X(Z11-Z12)
>
> Others say the common mode resistors can be visualized as being in
> parallel, so the net parallel combination result is .5X(Z11+Z12)
>
> So, differential impedance is "approximately" 2 times or 4 times common
> mode impedance, depending on your definitions.
>
> Doug Brooks
>
>
>
> At 08:50 AM 5/16/2002 -0700, you wrote:
>
>
> >In the current literature, the equations used to analyze a differential
> >system's traces are fairly consistent: differential impedance is equal to
> >twice the odd mode impedance, where the odd mode is the difference between
> >the single-ended and coupling impedances, etc.
> >
> >One point of difference is the definition of common mode impedance.  Several
> >sources, including Douglas Brooks' 1998 Printed Circuit Design article,
> >state that the common mode impedance is equal to one-half the even mode
> >impedance.  Similar statements can be found in industry specifications, such
> >as those for Serial ATA.
> >
> >On the other hand, Eric Bogatin has explicitly stated in his differential
> >training materials ("Differential Impedance Finally Made Simple", 2000) that
> >common mode impedance is equal to the even mode impedance; yet, on this
> >forum, he has recently stated (in an SI-LIST e-mail, Feb. 25, 2002) that
> >common mode impedance is one-half of even mode impedance.  The "common mode
> >Z = even mode Z" definition is also given in a 1999 Printed Circuit Design
> >article by Kaufer and Crisafulli.
> >
> >What is the "correct" definition of common mode impedance?  I suspect that
> >there is a referencing or perspective discrepancy here, but I would like to
> >hear a formal explanation, preferably from the above experts or others on
> >the list.
> >
> >Thanks!
> >
> >- Michael Mirmak
> >   Intel Corp.
> >
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