[SI-LIST] Re: diff pair questions
- From: "Chris Rokusek" <crokusek@xxxxxxxxxxxx>
- To: <si-list@xxxxxxxxxxxxx>
- Date: Tue, 24 Jul 2001 18:12:03 -0700
Signal,
The driver design will only control where the very low frequency components
of the signal travel (DC). At the higher frequencies each lengthwise
section of T-Line configuration can be considered separate from those
further away.
In each of the cross sections of the configurations, the easiest way for a
current to return is through the plane (it has large surface area, and is
realtively near). If you have access to a field solver, you will see a
larger proportion of E field lines terminating on the plane(s) rather than
the adjacent trace. Similar effect for tangential H lines near plane.
Another example of it not mattering about the driver configuration is that
non-DC return currents don't care whether they travel on GND or PWR to get
back to their source because the high frequency components can jump between
due to coupling between planes.
If there is no GND or PWR plane nor coaxial shield then the field lines will
be uncontained but still overlap (couple) and cancel out (at lower
frequencies).
Chris
> -----Original Message-----
> From: si-list-bounce@xxxxxxxxxxxxx
> [mailto:si-list-bounce@xxxxxxxxxxxxx]On Behalf Of signal hoss
> Sent: Tuesday, July 24, 2001 5:35 PM
> To: si-list@xxxxxxxxxxxxx
> Subject: [SI-LIST] Re: diff pair questions
>
>
>
> 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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