[SI-LIST] Re: diff pair questions
- From: signal hoss <simails@xxxxxxxxx>
- To: "Daniel, Erik S." <Daniel.Erik@xxxxxxxx>, si-list@xxxxxxxxxxxxx
- Date: Wed, 25 Jul 2001 11:28:35 -0700 (PDT)
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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