[SI-LIST] Re: Diff. Pairs

  • From: Ravinder.Ajmani@xxxxxxxx
  • To: leeritchey@xxxxxxxxxxxxx
  • Date: Fri, 3 Oct 2003 13:12:32 -0700

One obvious benefit is the reduction in EMI because of reduced loop area. 
I have verified this through simulation.  However, I am not able to 
understand the phenomenon of edge degradation due to close coupling.  I 
ran some quick simulations on six inch long differential Microstrip nets 
under the following conditions:
Case 1: Trace width 9 mils, separation 4 mils, differential impedance 99.3 
ohms
Case 2: Trace width 12.5 mils, separation 40 mils, differential impedance 
98.8 ohms 

The driver had a rise time of 250 ps.  The only difference I observed 
between the two waveforms was that in the first case the propagation time 
was 63 ps less than the second case.  This is understandable since the 
signals in the two branches of differential net have opposite polarity, 
the coupling effect will speed them up.

Next I tried the same simulation for Stripline nets.  In this case, there 
was practically no difference between two waveforms (less than 5 ps 
difference in propagation delay).

Am I missing something here. 

Regards, Ravinder
Server PCB and Flex Development
Hitachi Global Storage Technologies

Email: Ravinder.Ajmani@xxxxxxxx






"Lee Ritchey" <leeritchey@xxxxxxxxxxxxx>
Sent by: si-list-bounce@xxxxxxxxxxxxx
10/03/2003 11:41 AM
Please respond to leeritchey

 
        To:     "Duane Takahashi" <duanet@xxxxxxxxxxxxxxxxxxxxxx>, 
si-list@xxxxxxxxxxxxx
        cc: 
        From:   si-list-bounce@xxxxxxxxxxxxx
        Subject:        [SI-LIST] Re: Diff.Pairs




More than that, it does not have any benefit.  Tight coupling of
differential pairs forces the traces to be narrower increasing the skin
effect losses.  Also, this tight coupling is going to result in good old
cross talk that actually degrades the edges.

How the notion of tight coupling of differential pairs as beneficial got
started is a mystery to me.  There are several references that show that
tight coupling is not beneficial, one of them is Howard Johnson's latest
book, at least one column he has written and my recently released book.

Lee Ritchey


> [Original Message]
> From: Duane Takahashi <duanet@xxxxxxxxxxxxxxxxxxxxxx>
> To: <si-list@xxxxxxxxxxxxx>
> Date: 10/2/2003 3:58:59 PM
> Subject: [SI-LIST] Re: Diff.Pairs
>
> Hi Juergen:
>
> Aligning the stack up for the broadside coupled diff lines is expensive. 

>    You can do this, but it drives up the cost of the board.
>
> Duane
>
> > Hi Juergen,
> > You can find lots of  application notes
> > especially with respect to process variation
> > on differential pairs here:
> > 
> > 
> > www.polarinstruments.com/support/cits/cits_index.html
> > 
> > In particular this one may be of interest:
> > 
> > 
> > How measured impedance may vary from field solver calculations when
> > using woven glass reinforced 
> > <http://www.polarinstruments.com/support/cits/AP139.html>laminates
> > 
> > www.polarinstruments.com/support/cits/AP139.html
> > 
> > 
> > And this note:
> > 
> > Copper thickness, edge coupled lines and
> > characteristic 
> > <http://www.polarinstruments.com/support/cits/AP151.html>impedance
> > 
> > 
> > www.polarinstruments.com/support/cits/AP151.html
> > 
> > 
> > 
> > Hope this helps....
> > 
> > 
> > Kind regards
> > Martyn Gaudion
> > www.polarinstruments.com
> > T: +44 1481 253081
> > F: +44 1481 252476
> > M: +44 7710 522748
> > E: martyn@xxxxxxxxxxxxxxxxxxxx
> > 
> > ============================================
> >   Controlled Impedance & Signal integrity tools
> >   for the Printed circuit fabrication industry
> > ============================================
> > 
> > 
> > 
> > 
> > 
> > 
> > At 19:00 02/10/2003, you wrote:
> > 
> >>I am seeking help in finding enlightenment regarding electrical 
> >>performance pros and cons and how manufacturing tolerances play a role 

> >>when comparing side by side and tandem differential pairs. I'd
appreciate 
> >>your opinion, experience, analysis, pointers to papers and articels,
etc.
> >>
> >>In return, I would offer to share a summary of the finding/discoveries 

> >>with interested parties.
> >>
> >>Thanks
> >>
> >>Juergen
> >>
> >>
> >>
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>
> -- 
> Duane Takahashi              phone: 408-720-4200
> Greenfield Networks            fax: 408-720-4210
> 255 Santa Ana Court          email: duanet@xxxxxxxxxxxxxxxxxxxxxx
> Sunnyvale, CA 94085
>
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