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

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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