[SI-LIST] Re: diff pair questions
- From: "Daniel, Erik S." <Daniel.Erik@xxxxxxxx>
- To: si-list@xxxxxxxxxxxxx
- Date: Thu, 26 Jul 2001 06:59:41 -0500
Signal-
> ***** 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.
This logic doesn't sound quite right to me. If you were to care *only*
about differential signal transfer from a diff pair on one PCB through two
coax cables to another diff pair on another PCB, then you would only have to
match the odd mode impedances. In this case, a twisted pair would work as
well as two coax cables (provided the odd mode impedances were the same).
It sounds like this should be your primary concern -- matching the odd mode
impedances.
There is a secondary concern, though, regarding the even mode impedance
mismatch, and related design tradeoffs. In principle, you could match the
even mode impedances as well, if, say, you use distinct (i.e., uncoupled)
coax cables for each signal in the diff pair, and if you were to make the
spacing between the traces on the board large so that there was little
coupling between them. But this throws away one of the niceties of coupled
differential lines on a PCB -- the more closely coupled the lines are, the
stronger the rejection of "common mode" noise from other sources on the PCB.
It seems the appropriate balance point to pick between "no coupling" and
"maximum coupling", and hence "no even mode mismatch" and "large even mode
mismatch" at the PCB-coax interface depends on a lot of other system issues.
Again, there has been a fair amount of past SI list traffic related to this,
and I'm probably not the best one to try to summarize it. In my limited
practical experience with this in a few candidate systems, the benefits of
fairly strongly coupling the diff pair outweigh the negatives associated
with the larger common mode impedance discontinuity, although if you were to
go to the extreme of using a twisted pair between boards (i.e., effectively
infinite common mode impedance mismatch), this can lead to problems.
- Erik
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