I just stumbled into this thread while I was looking for a different email.
funny that I never saw it before, so I apologize for the delayed response!

Anyway, I don't think Figure 2 in Lee's article (PCBDesign007 Aug2013
page11)
was intended to show electrons like marbles rolling through a plumbing
pipe.

http://www.magazines007.com/emag/pub/PCBD-Aug2013/

The energy field will be building up between the signal path and the return
path as the wave propagates down the line. As a board designer, I should be
able to trace that path (even if I have to use my finger), keeping in mind
that I should also create an unbroken copper path along-side it for the
return. Otherwise the field will distort and spray out to who-knows-where
looking for another conductor to glom onto.

In Rick Hartley's presentation "Grounding to Control Noise and EMI", one of
his slides asks,
"Where is the Energy in a circuit? In the Voltage? In the Current?
Neither... Energy is mostly in the Fields!!!"

and in the next slide he asks,
"Where are the Fields in a circuit? In the Traces? In the Planes?
Neither... in the Space between the Traces and the Planes... in the
Dielectric!"

Kind of a mind-blower the more you think about it.
If I ever write a book about designing circuit boards, I think I would call
it
"The Field Guide to Guiding Fields"

onward thru the fog,
Jack


On Thu, Feb 27, 2014 at 12:29 PM, Rainer Thüringer <
[log in to unmask]> wrote:

> *Is it true, that**parallelism is really not required for the two
>> differential traces on PCB routed over a common ground plane?*
>>
>> Lee Ritchey's article "Differential Signal Design" in the PCB Design
>> Magazine from August 2013 has been discussed controversially. Especially
>> his statement "/The property that these two signals have in common is that
>> they are equal and opposite and they are tightly timed to each other.
>> _Beyond these two characteristics there are no other properties that matter
>> when a design uses differential pairs_"/has raised the question, if
>> parallelism is really not required for the two differential traces routed
>> over a common ground plane.
>>
>> In my master class on Electronic Design we examined this question by
>> modeling a differential pair of traces over ground with a 3D-Field solver
>> from CST. If Lee's statement is true, the signal flow in each of the two
>> lines should behave independently like two single ended traces. This
>> behavior has been described very vividly by Howard Johnson and Martin
>> Graham in the "Advanced Black Magic" book (chapter 2.3 Transmission line)
>> concerning the return current, building up simultaneously with the signal
>> current in the ground plane underneath as the rising edge propagates
>> through a transmission line.
>>
>> For differential traces over ground there are two options for the return
>> current flow: (1) the return current of each trace flows underneath each
>> trace or (2) the return current will switch over to the inversed trace
>> returning to the driver. In case (1) an orthogonal slot in the ground plane
>> underneath the trace would disturb each of the 2 signals in case (2) a
>> parallel slot between the 2 parallel traces would do so.
>>
>> Making a long story short, even a wide ground slot of 1mm running
>> parallel between two differential traces with 1mm spacinghas no effect on
>> the return current of each trace. Both return currents are running
>> separately underneath each signal trace, being distorted if a transversal
>> (orthogonal) slot is inserted in ground.The reason therefore is simple: the
>> return current always takes the path of least impedance which is the path
>> of the smallest loop i.e. underneath each trace. Lee's statement is
>> correct. Nevertheless, for cancelling out ground noise generated by other
>> circuits the two differential traces should be routed over the same ground
>> area -- but not necessarily _very_ tight together.
>>
>> Having this model in mind, it is also obvious that the current does not
>> go down the signal conductor , reach the end and then begin to make its way
>> back. Unfortunately Lee's figure 2 (current flow is electron flow) in his
>> article could be misunderstood in that way. If the two lines are
>> substantially different in length or of different impedance both traces
>> must be terminated separately to ground as Lee did explain for the 2.4GB/s
>> case (with a small cap).
>>
>> But independent of the switching problem for the receiver, different
>> trace lengths or impedances (trace width) will generate reflections i.e.
>> EMC problems if terminated by one resistor only rather than two separately
>> to ground. Using the correct return current model this becomes obvious even
>> without any receiver. Will say, using the vividly return current model from
>> Howard Johnson helps understanding the signal propagation in between signal
>> trace and reference plane, so that design rules i.e. for placing correct
>> return vias can be derived by yourself.
>>
>> Therefore I am using vividly models in my classes at university. On
>> Monday afternoon at APEX you could "*Becoming an EMC Competent Board
>> Designer --- Understanding What Happens rather than Learning Rules (PD
>> 26)". **I am following Einstein's recommendation: Make things as simple as
>> possible -- but not simpler!***
>>
>> Rainer Thüringer, CID Master Instructor; Member of the IPC-DC Steering
>> Committee
>>
>> Professor for Electronic Design, THM - University of Applied Sciences,
>> Giessen (Germany)
>>
>>
>
> ______________________________________________________________________
> This email has been scanned by the Symantec Email Security.cloud service.
> For more information please contact helpdesk at x2960 or [log in to unmask]
> ______________________________________________________________________
>
> ------------------------------------------------------------
> ---------------------
> DesignerCouncil Mail List provided as a free service by IPC using LISTSERV
> 16.0.
> To unsubscribe, send a message to [log in to unmask] with following text in
> the BODY (NOT the subject field): SIGNOFF DesignerCouncil.
> To temporarily stop/(restart) delivery of DesignerCouncil send: SET
> DesignerCouncil NOMAIL/(MAIL)
> For additional information, or contact Keach Sasamori at [log in to unmask]
> or 847-615-7100 ext.2815
> ------------------------------------------------------------
> ---------------------
>

______________________________________________________________________
This email has been scanned by the Symantec Email Security.cloud service.
For more information please contact helpdesk at x2960 or [log in to unmask] 
______________________________________________________________________

---------------------------------------------------------------------------------
DesignerCouncil Mail List provided as a free service by IPC using LISTSERV 16.0.
To unsubscribe, send a message to [log in to unmask] with following text in
the BODY (NOT the subject field): SIGNOFF DesignerCouncil.
To temporarily stop/(restart) delivery of DesignerCouncil send: SET DesignerCouncil NOMAIL/(MAIL)
For additional information, or contact Keach Sasamori at [log in to unmask] or 847-615-7100 ext.2815
---------------------------------------------------------------------------------