FreeLists / si-list / [SI-LIST] Re: diff signaling

[steve weir <weirsp@xxxxxxxxxx>]

Jim, another way to visualize the behavior is to model the pair running 
over a longitudal split in the ground plane along the center line 
projection of the pair, and bridging that split only at the driver and the 
receiver.  Clearly, if we use a large lateral separation between each of 
the complementary signals, it is easy to see that we just have two 
transmission lines with very little interaction along the path.  The return 
currents in each ground strip look like almost identical to the complement 
signal currents.  As we push the two signals closer together we get more, 
but never a lot of coupling, the behavior doesn't change much.  If we then 
bridge the gap, the complementary currents of the two transmission lines 
cancel, and only the common current remains.
Steve.

At 01:37 PM 4/2/2004 -0500, Knighten, Jim L wrote:
>Jon,
>
>Actually, return current is in the other conductor, not the ground plane.
>While the ground plane may have induced currents they net to zero in the
>longitudinal direction (perfectly balanced case).  Return current in the
>ground plane is common-mode, not differential-mode.  This can be generated
>by imbalances of many sorts.  This common-mode in the ground plane does not
>react well to gaps in the ground plane.
>
>Jim
>
>_____________________
>James L. Knighten, Ph.D.
>Teradata, a division of NCR                 http://www.ncr.com
>17095 Via del Campo
>San Diego, CA 92127
>tel: 858-485-2537
>fax: 858-485-3788
>
>
>-----Original Message-----
>From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] On
>Behalf Of Jon Powell
>Sent: Friday, April 02, 2004 10:13 AM
>To: mohammadt@xxxxxxxxxxxx; zabinski.patrick@xxxxxxxx;
>chris_landrum@xxxxxxxxxx; si-list@xxxxxxxxxxxxx
>Subject: [SI-LIST] Re: diff signaling
>
>I wouldn't count much on the EMI help. The vast majority of the return
>current is still in the return plane and thinking that enough jumps into the
>diff signals at a gap crossing is generally not a good analysis.
>
>jon
>
>
>-----Original Message-----
>From: si-list-bounce@xxxxxxxxxxxxx
>[mailto:si-list-bounce@xxxxxxxxxxxxx]On Behalf Of Mohammad Tabatabai
>Sent: Friday, April 02, 2004 9:55 AM
>To: jonpowell@xxxxxxxxxxxx; zabinski.patrick@xxxxxxxx;
>chris_landrum@xxxxxxxxxx; si-list@xxxxxxxxxxxxx
>Subject: [SI-LIST] Re: diff signaling
>
>
>Also Routing diff-pairs closely coupled preserves edge rates
>which will help greatly in reducing ISI jitter (for high-speed
>data). Another added benefit is as the diff pairs couple more
>strongly together the impedance matching also becomes a function
>of their field coupling and not entirely to the return ground
>below hence providing less susceptibility to impedance mismatch
>when a diff pair crosses over boundaries which do not have ground
>plane for return currents not to mention an added benefit for
>reduction in EMI as well.
>
>-Mohammad
>
>-----Original Message-----
>From: si-list-bounce@xxxxxxxxxxxxx
>[mailto:si-list-bounce@xxxxxxxxxxxxx]On Behalf Of Jon Powell
>Sent: Friday, April 02, 2004 9:04 AM
>To: zabinski.patrick@xxxxxxxx; chris_landrum@xxxxxxxxxx;
>si-list@xxxxxxxxxxxxx
>Subject: [SI-LIST] Re: diff signaling
>
>
>In addition, by routing the next to each other they are more likely to
>receive similar amounts of crosstalk noise (from the same sources) than if
>you routed them, say, across different parts of the board, even if you did
>length matching. Also you can get some timing benefits and additional
>dynamic impedance control from the mutual crosstalk between the adjacent
>differential signals.
>
>
>-----Original Message-----
>From: si-list-bounce@xxxxxxxxxxxxx
>[mailto:si-list-bounce@xxxxxxxxxxxxx]On Behalf Of Zabinski, Patrick J.
>Sent: Friday, April 02, 2004 6:32 AM
>To: chris_landrum@xxxxxxxxxx; si-list@xxxxxxxxxxxxx
>Subject: [SI-LIST] Re: diff signaling
>
>
>Chris,
>
>I  believe there's a bit a confusion...
>
>I agree that the traces/interconnect do not necessarily
>reject common mode noise any better than single ended
>traces.  If radiated energy is applied to a copper
>trace (or a pair of traces in the case of differential),
>there will be energy transferred to the traces.  So,
>a pair of differential traces do not have any special
>properties that reject common mode noise.
>
>However, what's more important is how the receiver electronics
>react to this noise.  In a good differential receiver,
>the circuit only considers the difference in voltage
>between the two input nodes and does not care what the
>common mode voltage is (within the limits of the power
>rails).  Thus, if there is common-mode noise on
>both traces, then a the receiver will effectively
>ignore (i.e., reject) the common-mode noise and
>only consider the difference in voltage
>between the two traces.
>
>Accordingly, I don't believe it's the interconnect
>that provides good common-mode rejection, but it's
>the receiver where you get the most benefit.
>
>Taking this a step further, the line driver also benefits
>from differential signals by providing a better,
>more controlled path for the return signal, where
>a single ended driver must often rely on decoupling
>elements to provide the return path.
>
>With respect to line length matching, this becomes
>important from the perspective of common-to-differential
>mode conversion.  If an external source applies
>common-mode energy to a differential pair (say,
>through crosstalk), a difference in interconnect
>lengths between the true and complement signals will
>effectively convert the common-mode noise to diff-mode
>noise, and the receiver will not be able to reject
>this.
>
>Accordingly, it's important to consider common-to-diff
>conversion and line-length matching to maintain the
>benefits of differential signaling.
>
>Hope this helps,
>Pat
>
> >
> > I have been searching through the archive for information
> > regarding this but
> > I have not come up with anything concrete.
> > Many people mention using differential signaling in an effort
> > to reduce
> > common mode noise. Looking through Lee Ritchey's book "Right the First
> > Time", his discussion implies that this is not the case. For
> > there to be
> > common mode noise rejection between two traces, the fields
> > would have to be
> > equal and opposite. In a PCB structure with a diff pair
> > referencing GND
> > planes (say in a stripline topology) this seems impossible!
> > Routing signals
> > next to eachother is convenient for length matching (skew
> > management) but I
> > don't see much else.... Any thoughts.
> >
> > And I do apologize if this has been discussed in detail before.
> > Thanks
> > Chris
> >
> >
> >
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Steve Weir
Teraspeed Consulting Group LLC
2926 SE Yamhill St.
Portland, OR 97214
(503) 239-5536
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