[SI-LIST] Re: diff pair questions
- From: signal hoss <simails@xxxxxxxxx>
- To: si-list@xxxxxxxxxxxxx
- Date: Wed, 25 Jul 2001 11:42:25 -0700 (PDT)
Vinu,
I don't quite understand this-
say we use pecl, lvds or cml driver- they all
have diff amplfier, which implies current needs
to be returned on power/ground. For the way you talked
about- cut out the reference plane, it will make
return
current path unusually long and creates side-effects
like coupling to other signals and more EMI.
One example(may not be good) is ethernet driver use a
transformer to converta diff pair with reference to
gnd into a diff pair with reference to each other in
order to put on cat 5 cable. I dont know what happen
if just use ethernet driver to drive cable without
going thru
transformer.
--- Vinu Arumugham <vinu@xxxxxxxxx> wrote:
>
> 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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