[SI-LIST] Re: High speed signal-return path
- From: Jory McKinley <jory_mckinley@xxxxxxxxx>
- To: olaney@xxxxxxxx
- Date: Sun, 23 Sep 2007 10:11:03 -0700 (PDT)
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
DIV {
MARGIN:0px;}
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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