[SI-LIST] Re: diff pair questions
- From: Vinu Arumugham <vinu@xxxxxxxxx>
- To: simails@xxxxxxxxx
- Date: Wed, 25 Jul 2001 13:44:31 -0700
Signal,
The path is:
Driver power lead->pull up structure ->true output->true trace->complement
trace(return current) ->complement output ->pull down structure->driver
ground lead.
Switch true and complement for the other transition.
An ethernet driver can drive the cable directly. The transformer is used
(among other reasons) to provide DC isolation between the boxes the cable
connects.
Thanks,
Vinu
signal hoss wrote:
> 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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