[SI-LIST] Re: Diff. Pairs

  • From: "Jeremy Plunkett" <jeremy@xxxxxxxxxxxxxxx>
  • To: weirsp@xxxxxxxxxx, Ravinder.Ajmani@xxxxxxxx,leeritchey@xxxxxxxxxxxxx
  • Date: Mon, 6 Oct 2003 18:40:18 -0700

Steve,
Ravinder's edges propagate faster in the 1st case because tighter spacing
between microstrip lines results in a greater portion of the field
propagating in air above/between the traces vs in the fiberglass under the
traces.  The exact amount of speedup depends on the details of his trace
geometry, soldermask geometry, and dielectric constants of the prepreg and
soldermask.

Outside of the special case of a mixed dielectric (as above), changes in
coupling will not affect velocity one way or another (I mean "pure"
propagation velocity here, not measured delay--see below).   However, if you
do not hold impedance constant, increasing coupling with make the impedance
will go down, which will result in "slower" edges throughout the system (in
V/ns) due to reduced signal amplitude (and vice versa, less coupling ->
higher Z -> "faster" edges due to larger signal swing).

Maybe not everyone agrees that there should be quotes around slower and
faster in the sentence above.  When I think about signals propagating on a
transmission line, I prefer to keep the "delay" effects separate from the
"amplitude" affects, even though they may both affect the measured delay.
For example there is one effect of changing coupling even in a uniform
dielectric while keeping Z constant; skin effect losses change due to the
proximity effect.  This will slightly affect the measured delay (if we
measure at 50% of the transition) because it changes the waveshape at the
receiving end of the line, but I don't consider it as changing the velocity
because if we measure at the earliest recognizable point on the transition,
it does not create any extra delay.

There are 2nd order effects (variation of Er with frequency) that can
complicate this mental separation of delay and amplitude, but I find it does
a nice job of clarifying the 1st order effects (impedance changes and
attenuation).  I'm interested to hear any comments people have on it's
usefulness or things I should watch out for.

best regards,
Jeremy



|>--/\/\/--((((((((()--|>

Jeremy Plunkett
Signal Integrity Engineer
Broadcom Corp
www.serverworks.com

|>--/\/\/--((((((((()--|>


-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx
[mailto:si-list-bounce@xxxxxxxxxxxxx]On Behalf Of steve weir
Sent: Friday, October 03, 2003 4:40 PM
To: Ravinder.Ajmani@xxxxxxxx; leeritchey@xxxxxxxxxxxxx
Cc: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Re: Diff. Pairs


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
> > >>
> > >>
> > >>
> > >>------------------------------------------------------------------
> > >>To unsubscribe from si-list:
> > >>si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field
> > >>
> > >>or to administer your membership from a web page, go to:
> > >>//www.freelists.org/webpage/si-list
> > >>
> > >>For help:
> > >>si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field
> > >>
> > >>List archives are viewable at:
> > >>                //www.freelists.org/archives/si-list
> > >>or at our remote archives:
> > >>                http://groups.yahoo.com/group/si-list/messages
> > >>Old (prior to June 6, 2001) list archives are viewable at:
> > >>                http://www.qsl.net/wb6tpu
> > >>
> > >
> > >
> > >
> > > ------------------------------------------------------------------
> > > To unsubscribe from si-list:
> > > si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field
> > >
> > > or to administer your membership from a web page, go to:
> > > //www.freelists.org/webpage/si-list
> > >
> > > For help:
> > > si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field
> > >
> > > List archives are viewable at:
> > >                              //www.freelists.org/archives/si-list
> > > or at our remote archives:
> > >
> http://groups.yahoo.com/group/si-list/messages
> > > Old (prior to June 6, 2001) list archives are viewable at:
> > >                              http://www.qsl.net/wb6tpu
> > >
> >
> > --
> > Duane Takahashi              phone: 408-720-4200
> > Greenfield Networks            fax: 408-720-4210
> > 255 Santa Ana Court          email: duanet@xxxxxxxxxxxxxxxxxxxxxx
> > Sunnyvale, CA 94085
> >
> > * MOVING!  Please note new numbers and address *
> >
> > ------------------------------------------------------------------
> > To unsubscribe from si-list:
> > si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field
> >
> > or to administer your membership from a web page, go to:
> > //www.freelists.org/webpage/si-list
> >
> > For help:
> > si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field
> >
> > List archives are viewable at:
> >                                //www.freelists.org/archives/si-list
> > or at our remote archives:
> >
> http://groups.yahoo.com/group/si-list/messages
> > Old (prior to June 6, 2001) list archives are viewable at:
> >                                http://www.qsl.net/wb6tpu
> >
>
>
>
>------------------------------------------------------------------
>To unsubscribe from si-list:
>si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field
>
>or to administer your membership from a web page, go to:
>//www.freelists.org/webpage/si-list
>
>For help:
>si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field
>
>List archives are viewable at:
>                                  //www.freelists.org/archives/si-list
>or at our remote archives:
>
>http://groups.yahoo.com/group/si-list/messages
>Old (prior to June 6, 2001) list archives are viewable at:
>                                  http://www.qsl.net/wb6tpu
>
>
>
>
>
>
>------------------------------------------------------------------
>To unsubscribe from si-list:
>si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field
>
>or to administer your membership from a web page, go to:
>//www.freelists.org/webpage/si-list
>
>For help:
>si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field
>
>List archives are viewable at:
>                 //www.freelists.org/archives/si-list
>or at our remote archives:
>                 http://groups.yahoo.com/group/si-list/messages
>Old (prior to June 6, 2001) list archives are viewable at:
>                 http://www.qsl.net/wb6tpu
>


------------------------------------------------------------------
To unsubscribe from si-list:
si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field

or to administer your membership from a web page, go to:
//www.freelists.org/webpage/si-list

For help:
si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field

List archives are viewable at:
                //www.freelists.org/archives/si-list
or at our remote archives:
                http://groups.yahoo.com/group/si-list/messages
Old (prior to June 6, 2001) list archives are viewable at:
                http://www.qsl.net/wb6tpu






------------------------------------------------------------------
To unsubscribe from si-list:
si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field

or to administer your membership from a web page, go to:
//www.freelists.org/webpage/si-list

For help:
si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field

List archives are viewable at:     
                //www.freelists.org/archives/si-list
or at our remote archives:
                http://groups.yahoo.com/group/si-list/messages 
Old (prior to June 6, 2001) list archives are viewable at:
                http://www.qsl.net/wb6tpu
  

Other related posts: