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[SI-LIST] Re: Diff. Pairs
- From: steve weir <weirsp@xxxxxxxxxx>
- To: Ravinder.Ajmani@xxxxxxxx, leeritchey@xxxxxxxxxxxxx
- Date: Fri, 03 Oct 2003 16:39:32 -0700
Ravinder, something seems very wrong with the physics here. I don't know
if it is round-off error in your simulation, but Lenz' law agrees with
Lee. Differential coupling resists any change. The more tightly you
couple the two nets in a diff pair, the more it slows down the
transitions. To cause a speed-up, they would have to switch in the same
direction. If we could manufacture a machine that accelerated transitions
in opposite directions we could solve the world's energy needs.
Regards,
Steve.
At 01:12 PM 10/3/2003 -0700, Ravinder.Ajmani@xxxxxxxx wrote:
>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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> > >
> > >
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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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>
>
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