[SI-LIST] Re: PCB layer stackup
- From: <Cortex.Chen@xxxxxxxxxxxx>
- To: <chandrakanth.gajawada@xxxxxxxxx>
- Date: Wed, 10 Jan 2007 15:48:20 +0800
Chandra Kanth,
Simply, it's based on your viewpoint.
Generally, Even and Odd mode is defined from signal to GND (reference =
plane) and differential and common mode is between signals. For example, =
there are two transmission lines (A and B). If there are both waveforms =
(amplitudes are the same, in phase, 0 degree) are on both lines, then =
the (single-ended) impedance is called Even mode impedance, Zoe. On the =
other hand, if waveforms are out of phase, 180 degree, then the =
(single-ended) impedance is Odd mode impedance, Zoo. If coupling effect =
arises, tight coupling, Zoe is more than Zo and Zo is more than Zoo (Zoe =
> Zo > Zoo). For loose coupling, Zoe is almost similar to Zoo.
A ------
B-------
In addition, if you like to know the impedance between A and B, it's =
called common mode impedance for in phase case and the other is =
differential mode impedance for out of phase. Based on the current =
direction and equalized circuit. You could observe that common mode =
impedance is Zoe/2 and differential mode impedance is 2 Zoo.
In details, refer to the chapter 6 of high-speed signal propagation: =
advanced black magic.
Regards,
Cortex
-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] =
On Behalf Of ChandraKanth Gajawada
Sent: Wednesday, January 10, 2007 2:51 PM
To: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Re: PCB layer stackup
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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