[SI-LIST] Re: diff pair questions
- From: Vinu Arumugham <vinu@xxxxxxxxx>
- To: si-list@xxxxxxxxxxxxx
- Date: Wed, 25 Jul 2001 09:43:10 -0700
The same driver can be used to drive differential traces whether they are
referenced to planes or to each other. You can cut out the reference planes
in a PCB and have the differential pairs spaced closer together to achieve
the same impedance. This may not always be the best option because, without
the planes, you could have more crosstalk from other signals and more skin
loss.
Thanks,
Vinu
signal hoss wrote:
> Chris,
>
> I think you made the assumption on the drivers that
> return current has to be on the planes.
> What about designing a driver to return current on
> opposite trace on pcb, so we don't need to care about
> reference to planes. A similar example is eithernet
> Cat-5 cable, which has no ground refernce.
>
> The reason I asked is the scenario:
>
> diff pair with planes refernce on pcb --> coaxial
> cable --> other pcb
>
> >From pcb diff pair to cable diff pair, there seems
> to have mismatch even though both have same
> impendence-
> one has reference to planes while the other don't.
>
> This really puzzled me. Please advise.
>
> Signal Hoss
>
> --- Chris Rokusek <crokusek@xxxxxxxxxxxx> wrote:
> >
> > Signal,
> >
> > Try thinking about it as two single ended traces.
> > Then in terms of return
> > current, a large proportion of the return current
> > for each diff trace
> > actually returns on the planes rather than the
> > opposite trace. So what you
> > really have in terms of currents is a separate
> > signal/return loop for each
> > diff half. These two loops are equal and opposite
> > but spaced very near each
> > other (in fact on the planes they overlap at all but
> > the highest frequencies
> > causing cancellation, less noise, good stuff).
> > Another way to visualize
> > this is that the E/H field lines are "contained" by
> > the planes. Thus
> > impedance will vary as the plane separation is
> > varied.
> >
> > Chris Rokusek
> > Innoveda
> >
>
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