[SI-LIST] Re: High speed signal-return path

I meant in 1st para:
But, if driver local PDN is proved adequate, p/s/g could be done.

On 9/24/07, agathon <hreidmarkailen@xxxxxxxxx> wrote:
>
> Uh... it matters I believe...     g/s/g  vs.  p/s/g
>
> The g/s/g is preferred since that will have minimal pull-down return
> current going through drivers decaps.  But, if local PDN is proved adequate,
> g/s/g could be done.  It's all in the proof for the individual case.
> Ref:  Hall, Hall, McCall; sec. 6.1.4.
>
> It's a GTL example, which has the same structure as CML, typical of diff.
> drivers. The example is for just 1 ref plane but it's useful...
> Fact: The pulldown current is shared between driver and load.  Now, if gnd
> is referenced then the same return current magnitude occurs in the source
> decaps for pull up and down.  For pwr referencing, pulldown decap current is
> 2I, pullup is 0.  Not good if driver PDN decaps are poor or too distant.
>
> Diff pair case...
> T+/- would have 2 ref planes, p & g, so pulldown current = 'I' through
> decaps and pulldown = 0 still.  In general, the g and p planes would be
> different paths, having different inductance through the PDN, thus in
> general unbalancing the return current for p vs. g.  Ie: a differential '1'
> would travel differently than a '0' since T+ return differs from T- return.
> This would end up being an impedance discontinuity of the data kind, ergo DJ
> and or Vcm noise.  This is my general view, not a prescription for any given
> case.  Symmetry is almost always best.
>
> -Agathon
>
> On 9/23/07, olaney@xxxxxxxx <olaney@xxxxxxxx> wrote:
> >
> > Keeping an eye on the original question, which was the choice between:
> > >  Option1:
> > > L4 - Power
> > > L5 - Signal
> > > L6 ? GND
> > >
> > >  Option2:
> > > L4 - GND
> > > L5 - Signal
> > > L6 ? GND,
> >
> > ...are we agreed that these options are equivalent from a high frequency
> > standpoint?  And that the potential problems you mention regarding
> > layout
> > apply equally to power and ground plane implementations, and therefore
> > are a separable problem from the choices shown above?
> >
> > Current always needs a return path.  For a high frequency signal
> > sandwiched between the planes of either option, the important concept is
> > that the return current flows on both planes, in a ratio determined by
> > the proximity to each.  For a trace centered between them, the return
> > current in each plane will be half that on the signal trace.  Of course,
> > any DC component is restricted to returning via ground, but that is not
> > the same as the AC case.  There is a classic EMC demo that shows the
> > transition region between the two regimes, but basically, once the
> > frequency is high enough for the bypass caps to make a difference, then
> > both planes in the microstrip sandwich participate as return paths, and
> > it doesn't matter whether both are ground planes, or one is power and
> > one
> > is ground, or both are power, all else being equal.  In other words, the
> >
> > assignment of a plane to power or ground is not, in and of itself, an SI
> > issue.
> >
> > BTW, it is not entirely true that differential pairs have no ground
> > return current.  If a plane is close enough to affect the impedance of
> > the pair, it will carry ground currents in both directions at opposite
> > polarities, one for each trace.  There will be a current null at the
> > point midway between the traces and a peak underneath each trace.  The
> > sum will be zero, but the actual currents are not each zero because the
> > simple fact that the signals are balanced does not mean that the
> > distance
> > between the traces has no effect.  For sufficiently distant planes or
> > small enough separations (or both), the paired return currents might be
> > trivially small.  For typical board stacks and trace pitches, Murphy
> > says
> > they're not.
> >
> > Orin
> >
> > On Sun, 23 Sep 2007 10:11:03 -0700 (PDT) Jory McKinley
> > <jory_mckinley@xxxxxxxxx> writes:
> > This is interesting and hopefully not too long winded and off the
> > track...........
> >
> > Correct me if I have this wrong but current always attempts to return to
> >
> > its source no matter what frequency we are talking about, if it does not
> > then we have a problem such as radiation in the path.  For AC signals
> > the
> > return or image current is instantaneous to the traveling signal
> > attempting to get back to the source through any low inductance path.
> > This path may be the opposite line for differential it may be as you
> > pointed out power or ground or both.  However, there could be many
> > hurdles for this return current, signal via's, reference plane splits,
> > and as you mentioned improper decoupling to name a few.
> > One of the issues on a board with a gnd/sig/gnd configuration is the
> > potential long return loop for single ended signals routed on this layer
> > and potential issues with any unbalanced common mode differential noise.
> >
> > I will say that for "ideal" differential with well balanced decoupling,
> > and return vias the gnd/sig/gnd works but ideal is not that easy.
> > -Jory
> >
> >
> >
> >
> > ----- Original Message ----
> > From: " olaney@xxxxxxxx" <olaney@xxxxxxxx>
> > To: jory_mckinley@xxxxxxxxx
> > Cc: raja.anand@xxxxxxxxx; si-list@xxxxxxxxxxxxx
> > Sent: Sunday, September 23, 2007 11:17:49 AM
> > Subject: Re: [SI-LIST] Re: High speed signal-return path
> >
> >
> > That makes sense for DC current, but the point is that the power and
> > ground planes are shorted together in the RF sense by a properly
> > implemented bypass network.  Therefore, one is as good as the other for
> > RF.  RF energy never reaches the power supply -- if it does, something
> > is
> > very wrong.
> >
> > Orin
> >
> > On Sat, 22 Sep 2007 22:11:06 -0700 (PDT) Jory McKinley
> > <jory_mckinley@xxxxxxxxx> writes:
> > Hello Orin,
> > There is in that the return current has to do just that, return.  So for
> >
> > single ended or common mode differential sourcing or sinking from power
> > or ground could make a significant difference, somewhat regardless of
> > edge rate.  All mute for ideal differential of coarse.
> > -Jory
> >
> >
> >
> >
> > ----- Original Message ----
> > From: "olaney@xxxxxxxx" <olaney@xxxxxxxx>
> > To: jory_mckinley@xxxxxxxxx
> > Cc: raja.anand@xxxxxxxxx; si-list@xxxxxxxxxxxxx
> > Sent: Saturday, September 22, 2007 12:49:27 PM
> > Subject: [SI-LIST] Re: High speed signal-return path
> >
> >
> > We all know differential versus single ended, but "return current plane,
> > not ground" needs explanation if you feel that there is some nontrivial
> > difference at high frequencies between power and ground planes.
> > Orin
> >
> > On Sat, 22 Sep 2007 10:36:14 -0700 (PDT) Jory McKinley
> > <jory_mckinley@xxxxxxxxx> writes:
> > Yes, however every plane is a potential return current plane, not
> > ground,
> > less important for true differential more important for single ended.
> > How about any common mode noise on the differential signals, I would
> > think Option 1 would allow for better potential isolation.
> > -Jory
> >
> >
> >
> >
> > ----- Original Message ----
> > From: "olaney@xxxxxxxx" <olaney@xxxxxxxx>
> > To: raja.anand@xxxxxxxxx
> > Cc: si-list@xxxxxxxxxxxxx
> > Sent: Saturday, September 22, 2007 6:51:30 AM
> > Subject: [SI-LIST] Re: High speed signal-return path
> >
> >
> > Every power plane is a ground plane for high frequency purposes.  There
> > should be no material difference between the two implementations.  If
> > there is, something is really wrong with the power supply bypassing
> > strategy.  The choice between stripline and microstrip is generally
> > driven by the particulars of the board layout.  There is no definitive
> > choice of one over the other based on general principles, only as
> > circumstances dictate.
> >
> > Orin Laney
> >
> > On Sat, 22 Sep 2007 11:12:19 +0530 "Raja Anand" < raja.anand@xxxxxxxxx>
> > writes:
> > > Hi All,
> > >  For high speed differential signals (3.125Gbps ), is it really
> > > required to
> > > take these signals between Gnd Planes.  Which option should I go for
> > > from
> > > below if my stack up is symmetrical?
> > >
> > >  Option1:
> > > L4 - Power
> > > L5 - Signal
> > > L6 ? GND
> > >
> > >  Option2:
> > > L4 - GND
> > > L5 - Signal
> > > L6 ? GND
> > >
> > > Regards,
> > > Raja
> > >
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