[SI-LIST] Re: Diff. Pairs

  • From: steve weir <weirsp@xxxxxxxxxx>
  • To: "Jeremy Plunkett" <jeremy@xxxxxxxxxxxxxxx>,Ravinder.Ajmani@xxxxxxxx, leeritchey@xxxxxxxxxxxxx
  • Date: Mon, 06 Oct 2003 20:58:35 -0700

Jeremy,

I think if you follow the thread Ravinder was responding to Lee's comment 
about flattened rise times.  Respectfully, this evening when you brought it 
up is the first that I have seen on this particular thread about 
propagation time.  The assumption of an unadulterated wave shape relies on 
perfect impedance matching at the load, which is a bit rare.

Regards,


Steve.

At 08:14 PM 10/6/2003 -0700, Jeremy Plunkett wrote:
>Hi Steve,
>That was my point, that in these discussions we should be careful to
>distinguish between amplitude effects (things that affect measured prop
>delay by changing rise time or waveform shape) and delay effects (which are
>only based on the effective dielectric constant).
>
>In Ravinder's simulation, he got the result that the tightly coupled traces
>have a faster propagation delay than the loosely coupled, which is what we
>would expect based on the different effective dielectric constants for the
>two cases.  That's the thing he was missing.
>
>Your response that tight coupling should slow down the transitions is true,
>if you don't keep the total differential impedance constant while you vary
>the coupling.  But it was a non-sequitur, since Ravinder was measuring
>shorter propagation delay, not transition time.
>
>And actually, the rise time will not change due to changes in impedance,
>only the slew rate.  That's another situation where keeping the amplitude
>and timing effects separate in your head can be very important.  The only
>things that will change rise time are frequency-dependent effects that
>create phase shifts, amplitude differences, or both between different
>frequency components in your signal (examples include reactive loads,
>dielectric constant that varies with frequency, skin effect, and dielectric
>loss).  Impedance discontinuities by themselves do not have any frequency
>dependence and will not change the rise time, only the slew rate (by
>changing the amplitude of the entire signal).
>
>Best regards,
>Jeremy
>
>-----Original Message-----
>From: steve weir [mailto:weirsp@xxxxxxxxxx]
>Sent: Monday, October 06, 2003 7:05 PM
>To: Jeremy Plunkett; Ravinder.Ajmani@xxxxxxxx; leeritchey@xxxxxxxxxxxxx
>Cc: si-list@xxxxxxxxxxxxx
>Subject: RE: [SI-LIST] Re: Diff. Pairs
>
>
>Jeremy,  I was referring to percentage rise-time, not propagation
>velocity.  To the extent that each line in a pair couples to the other, the
>opposing transition on each requires more energy for any given excursion
>than without coupling, and that flattens the transitions at any real load.
>
>Regards,
>
>
>Steve.
>
>At 06:40 PM 10/6/2003 -0700, Jeremy Plunkett wrote:
> >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
> > > > >>
> > > > >>
> > > > >>
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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
> > > >
> > > > * MOVING!  Please note new numbers and address *
> > > >
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