Hi Ben,

you are absolutely wright concerning the intend of designing 
differential lines on pcb.

In the workshop one of my results for good noise reduction and EMC 
reason is, that the emission and susceptablity to external EM fields as 
well as for noise immunity requires a parallel routing of the 2 traces 
over the same ground area - but not necessarily very tight. Loose 
coupling as you have been summarized (and Lee Richtey as well) is even 
better for constancy of the impedance (due to lower mutual capacitance).
Thinking in illustrative (correct) modells of the signal flow as a wave 
resp. field _inbetween_ trace and ground rather than in electrons (which 
are moving very slowly!) supports understanding and deriving logic 
consequences in practice by the designer him-/herself.

Hope we will meet on the Designers Day at APEX.

Cheers
Rainer


Am 28.02.2014 00:08, schrieb Barry Olney:
> Hi Ben,
>
> I tend to agree with you on this.
> But the return current does flow in the low inductance plane rather than the
> opposite signal trace.
>
> Routing differential signals together will give a higher level of noise
> immunity provided there is sufficient separation from other adjacent traces.
> Tightly coupled diff pairs are good for short distances where the traces
> remain coupled for the entire length but loose coupling, say 4/8
> (trace/clearance), is better if the pair have to separate around an obstacle
> at some point as the impedance variation is smaller.
>
> Cheers,
> Barry
>
>
> -----Original Message-----
> From: DesignerCouncil [mailto:[log in to unmask]] On Behalf Of Ben
> Jordan
> Sent: Friday, 28 February 2014 5:02 AM
> To: [log in to unmask]
> Subject: Re: [DC] The real signal flow in differential pairs - referring to
> Lee Ritchey's statement
>
> Thanks Rainer,
>
> This is really well summarized piece of research confirmation you have
> done.
>
> I think that the reason there's confusion is that the original and only
> purpose for a differential transmission line was to create a balanced line
> for noise immunity. In analog systems (such as public address audio, or
> POTS) using balanced lines is critical for moving sensitive low-voltage
> signals through harsh environments without losing information or
> significant quality.
>
> Differential signals on a PCB also are differential for noise immunity. The
> odd part about this is that the transmitters and receiver circuits do not
> terminate the signals the same way as an audio microphone input on a mixing
> console (ie. with a transformer), but rather by high-impedance inputs to
> logic comparators. Therefore it's not so much a true balanced line but
> rather two single-ended lines which are routed side-by-side so that common
> mode noise can be easily ignored.
>
> Would you agree also that for EMC purposes differential signals should be
> routed as pairs coupled together? Intuitively to me it seems that two equal
> and opposite signals traversing the board in far-apart locations will emit
> more noise than if they are tightly coupled. Along the same lines - ground
> return paths should also be uninterrupted.
>
> Regards,
> Ben.
>
>
> On Thu, Feb 27, 2014 at 10:29 AM, 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)
>>>
>>>
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-- 
------------------------------------------
Prof. Dr. Rainer Thüringer
FB Elektro- und Informationstechnik
Elektronik-Design & -Technologien

THM Technische Hochschule Mittelhessen
University of Applied Sciences
Wiesenstr. 14, 35398 Giessen
Tel. 0641/309-1917, -1919 (Labor)



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