FreeLists / si-list / [SI-LIST] Re: PCB layer stackup

["ChandraKanth Gajawada" <chandrakanth.gajawada@xxxxxxxxx>]

Hi,


I came to know that there are two types on Differential signal transmission
Common mode (Even) and Differential (Odd) Mode. How are they different from
each other?



Please explain?



Regards

Chandra Kanth


On 1/10/07, Zabinski, Patrick <zabinski.patrick@xxxxxxxx> wrote:
>
>
> > Do LVDS signals needs to refer GND plane (A plane in Specific!)
> > As differential pair has the other trace as return current,I
> > dont think
> > they need for a Continous plane underneath it for reference....
>
> Theoretically, a differential signal does not need to be referenced to
> a plane (ground or power plane).  For example, differential signals
> are frequently used in twisted-pairs of single-strand wires, and they
> work just fine.
>
> Practically, though, the printed wiring board (PWB) environment is
> comprised of metal and dielectric layers, and each layer has geometrical
> and electrical constraints that ultimately lead to the use of planes
> in one or more layers.  Once a sheet of metal is used, the geometries of
> the traces are often such that they have a higher coupling to the planes
> than to other traces.  As has been pointed out in the SI-List in the
> past, the coupling between two adjacent traces is often less than 15%
> of the coupling to the planes.  Most often, the differential coupling is
> closer to 5% of the plane coupling.
>
> Because the signal traces couple so strongly to the planes, the majority
> of local return currents are within the planes.  For example, in a
> differential
> pair with both true and complement signals, the return current for the
> "true"
> signal is mostly (90-95%) within the reference plane, and the return
> current
> for the "complement" signal is mostly (90-90%) within the reference
> plane.  Although
> the two return currents have the same magnitude and opposite polarities,
>
> they travel in distinctly different locations in the reference plane.
>
> Assuming the signal is truly differential (i.e., exactly balanced
> between
> true and complement) -and- there is minimal impedance between the true
> and complement return paths -and- the impedance can be maintained, then
> it is not necessary to have a continuous plane underneath a differential
> pair.
> However, in common packaging technologies (e.g., PWB, MCM, BGA, ...) it
> is quite difficult to maintain such constraints.  As such, it is
> easiest/cheapest
> to use reference planes to our advantage.
>
> Pat
>


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