[SI-LIST] Re: High speed signal-return path
- From: olaney@xxxxxxxx
- To: jory_mckinley@xxxxxxxxx
- Date: Sun, 23 Sep 2007 17:40:24 -0700
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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