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

Some design-guides about  power integrity require important signals =
should
set ground planes as their reference plane, for power planes have more
noises than gound planes.
If  the power plane is integral,  there is no difference to set ground =
plane
or power plane as reference plane, as you said.  Is this design-guide =
about
reference plane right or not?
And the question Raja asked is the problem of  stripeline,  is there any
difference for the situation of microline?

Best Regards,
Zhengrong



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=D6=F7=CC=E2: [SI-LIST] Re: High speed signal-return path

Keeping an eye on the original question, which was the choice between:
>  Option1:
> L4 - Power
> L5 - Signal
> L6 ?GND
>=20
>  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@yahoo.
com> 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.=20
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. =20
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.=20
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@yahoo.
com> 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@yahoo.
com> writes:
Yes, however every plane is a potential return current plane, not =
ground,
less important for true differential more important for single ended.=20
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=20
> required to take these signals between Gnd Planes.  Which option=20
> should I go for from below if my stack up is symmetrical?
>=20
>  Option1:
> L4 - Power
> L5 - Signal
> L6 ?GND
>=20
>  Option2:
> L4 - GND
> L5 - Signal
> L6 ?GND
>=20
> Regards,
> Raja
>=20
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