[SI-LIST] Re: Diff. Pairs

  • From: "Jeremy Plunkett" <jeremy@xxxxxxxxxxxxxxx>
  • To: weirsp@xxxxxxxxxx, Ravinder.Ajmani@xxxxxxxx,leeritchey@xxxxxxxxxxxxx
  • Date: Mon, 6 Oct 2003 21:46:40 -0700

Steve,
If you look back at Ravinder's post, his simulation result was that in case
1 (the tightly coupled case), "the propagation time was 63 ps less than the
second case" (the loosely coupled case).  You responded that this was
unexpected since coupling should "slow down" the transitions.

I originally jumped into the discussion to try to clarify that actually the
faster propagation times made sense if you think about which dielectric the
field is propagating in (and that also explains the lack of a difference for
the stripline case).  I also suggested that we should keep delay effects and
amplitude effects separate in our mental model of a signal propagating on a
transmission line.

Although I responded to your post, I didn't mean to start an argument with
you; I just wanted to point out that as long as we keep the total
differential impedance fixed while we vary the coupling, coupling does not
have any effect on rise time one way or another.

Regarding Lee's comment, I was never responding directly to it in my earlier
postings, since I really had entered the discussion based on Ravinder's
simulation results.  Now I that understand that you were focusing on Lee's
comment all along, I can see where you were coming from in your responses.

respectfully yours,
Jeremy



-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx
[mailto:si-list-bounce@xxxxxxxxxxxxx]On Behalf Of steve weir
Sent: Monday, October 06, 2003 8:59 PM
To: Jeremy Plunkett; Ravinder.Ajmani@xxxxxxxx; leeritchey@xxxxxxxxxxxxx
Cc: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Re: Diff. Pairs


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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