[SI-LIST] Re: diff pair questions

Dan,

questions as follow marked by ****

--- "Daniel, Erik S." <Daniel.Erik@xxxxxxxx> wrote:
> Signal-
> 
> For a diff pair in a PCB, some amount of current
> does have to flow in the
> reference ground planes, because each signal line is
> coupled to the planes
> to some degree, even though they are also coupled to
> each other to some
> degree.  This degree of coupling is reflected in the
> relative magnitude of
> the even and odd impedances of the diff pair.  As
> coupling between the
> signals increases and/or coupling between signals
> and planes decreases, the
> even mode impedance rises relative to the odd mode
> impedance.  Consider two
> extreme cases:
> 
> 1.  Two separate coax cables.  There will be
> extremely little coupling 
>     (call it zero) between the two signals, as they
> are shielded from 
>     one another by the coax ground traces. 
> Therefore, the even mode 
>     impedance will be very nearly (call it exactly)
> the same as the
>     odd mode impedance, equal to the impedance of
> the cables (assuming
>     they are the same).
> 
> 2.  Twisted pair cable.  In this case, consider
> ground to be infinitely
>     far away.  Then, the even mode impedance will be
> infinite, but the 
>     odd mode impedance will be finite, dictated by
> the coupling
>     between the signal traces.
> 
> The diff pair in PCB case will be somewhere in
> between.  One can tune this
> to some degree by changing the relative spacing of
> the signal lines to the
> planes and to each other (within practical limits).
> 
> When stringing together a system as you describe
> (PCB - cable - PCB),
> because the even and odd impedances will typically
> differ in the different
> parts of the string, you can only match either the
> odd or even mode
> impedances at the interface.  In your case, you'd
> almost certainly design to
> match the odd mode impedances, but you'd want to be
> aware of the even mode
> impedance mismatch, as this will lead to common mode
> reflections which may
> or may not impact your system performance.

***** I do care common mode mismatch because I want
***** diff signal to transfer to the other pcb with
***** best fidelity, so coax cable
***** is better suited for my case since it has 
***** reference to ground as compared to twisted pair
***** cable, which has infinite even mode impedence.
***** Correct me if I were wrong.
***** Also thanks for your clear answer. 
 There
> are a lot of past SI
> postings on this issue.
> 
>                                       - Erik
>     
> 
>
==================================================================
> Erik Daniel, Ph.D.                   Voice:  (507)
> 538-5461      
> Mayo Foundation                      Fax:    (507)
> 284-9171
> 200 First Street SW                  E-mail:
> daniel.erik@xxxxxxxx
> Rochester, MN  55905                 Web:   
> www.mayo.edu/sppdg/
>
==================================================================


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