[SI-LIST] Re: Diff line

  • From: "zanella, fabrizio" <zanella_fabrizio@xxxxxxx>
  • To: "'pikeda@xxxxxxxxxxx'" <pikeda@xxxxxxxxxxx>,si-list@xxxxxxxxxxxxx
  • Date: Thu, 14 Feb 2002 21:39:44 -0500

Paul, a large percentage of nets on pc boards are now differential.  
By using 20 mil spacing for your loosely coupled differential pairs, I don't
see how you can route high density boards without using 20-30 layers, which
increases cost, creates manufacturability problems, increases via
capacitance, etc.  The trend is to pack more and more routes in a given
space.  5 mil lines with 5 mil spacing, edge coupled on stripline layers is
quite common for differential pairs and works very well for 2.5Gbs signals. 
For the differential skew value, make sure you look at the entire path,
through the BGA, connectors, vias, etc, the total lenghts may be closer than
300 mils.

Regards,
Fabrizio Zanella
Signal Integrity
EMC Corporation
fzanella@xxxxxxx


-----Original Message-----
From: pikeda@xxxxxxxxxxx [mailto:pikeda@xxxxxxxxxxx]
Sent: Thursday, February 14, 2002 9:08 PM
To: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Re: Diff line





Pravin,

This is just one more argument to loosely couple your differential lines. If
they have around 20 mils seperation, you can treat them as single ended and
serpentining has no effect on impedance. Here is an article by Dr. Howard
Johnson on why you don't need to tightly couple your diff pairs which I
agree
with wholeheartedly. By the way I commonly do 3.125GHz loosely coupled and
they
work very well.
http://www.sigcon.com/news/2_30.htm


Paul




" pravin patel" <fairfax100@xxxxxxxxxxxxx> on 02/14/2002 06:25:04 AM

Please respond to fairfax100@xxxxxxxxxxxxx

To:   si-list@xxxxxxxxxxxxx
cc:    (bcc: Paul Ikeda/Marvell)

Subject:  [SI-LIST] Diff line




I am doing layout of 2.5Ghz diff. line and having hard time to match length
of a
pair by 300 mil. I was going to make one line serpentine to make both line
matched. By doing serpentine, I am creating impedance discontinuty or I will
create skew.
Any Ideas. Thanks

--


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