Re: [SI-LIST] : Comments from your SI seminar (SendII)`

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From: Jim Freeman (freeman@broadcom.com)
Date: Mon Nov 01 1999 - 11:03:13 PST


> The otherwise in your email leaves a big gulf and I hope that thewhole
> response was sarcastic.

Jim Freeman

> two 50 ohm lines work just as well as one differential 100 ohm line.
>
> HOWEVER, There are some rather nice things with differential lines:
>
> 1. EMI radiation and susceptibility is reduced as the lines become
> closer coupled, just as you would expect from a twisted pair of wires.
>
> 2. PCB space is reduced.
>
> 3. When crossing a ground discontinuity it is possible to operate
> differentially without a ground. That is, with no change in coupling
> the impedance goes up crossing the ground break but not as much
> as with seperate traces. If it is possible to get enough coupling the
>
> ground plane is not required.
>
> Otherwise, who cares, two 50s or diff 100.
>
> Ron Miller
>
> Larry Miller wrote:
>
>> Follow the UltraCad URL below and have a look at Doug Brooks' papers
>>
>> diff_z.pdf and terminations.pdf.
>>
>> These papers are
>>
>> 1) Clear
>>
>> 2) Mathematically rigorous
>>
>> 3) Correct.
>>
>> >">By the way, has anyone who advocates differential impedance shown
>> why it is
>> >necessary and what happens if isn't maintained? I've never seen
>> anyone
>> >demonstrate this. I'd welcome some analysis that supports this
>> point of
>> >view, if it exists."
>>
>> What happens is that you get impedance mismatches. This causes
>> ringing and
>> EMI. The analysis is presented in Dr Johnson's book and may be
>> easily
>> demonstrated on a PCB by doing it wrong, especially at frequencies
>> above
>> 200 MHz. We had to tweak all of our Gigabit Ethernet designs to get
>> the
>> impedance trimmed sufficiently that we could pass EMI (the symptom
>> being a
>> big 2500 MHz component).
>>
>> What you have below is just poop.
>>
>> Is there some specific reason you continue to bring these things up?
>> We are
>> talking physics here, not opinion.
>>
>> The fact that something is published does not necessarily make it
>> correct.
>> There are whole journals devoted to the veriest nonsense.
>>
>> Larry Miller
>>
>> At 12:26 PM 10/29/99 -0400, you wrote:
>> >Hi Steve I just got an e-mail back from Lee about his article:
>> (Doesn't
>> >touch the length difference though)
>> >Let me know what you think.
>> >
>> >Lee worte:
>> >"The intention of my article is to show that proper operation of a
>> digital
>> >differential signalling circuit does not depend on a particular
>> differential
>> >impedance existing between the two lines. If you look at the
>> termination
>> >specifications for LVDS you will see that this is true. The
>> termination
>> >value is
>> >2X the impedance of either line and the termination is placed
>> across the
>> >ends of
>> >the two lines. What matters is that each of the two signal paths
>> is
>> >properly
>> >terminated with enough accuracy that the erosion of each signal
>> from
>> >reflections
>> >is within system noie margin limits. (By the way, both LVDS and
>> ECL
>> >differential signalling can tolerate surprisingly large amounts of
>> >reflections
>> >and work properly.)
>> >
>> >. Often, by the geometry we pick when routing, there will exist a
>> >differential
>> >impedance between the two lines. In this case, we might need to
>> allow for
>> >the
>> >differential impedance as well as the "common mode" impedance
>> between each
>> >line
>> >and system ground. Whether this is necessary depends on the
>> tolerance the
>> >signalling protocol has for impedance mismatches. In LVDS and ECL,
>> there is
>> >enough tolerance that this can be ignored. In level sensitive
>> analog like
>> >video, this may not be true. But, remember the article covers
>> logic
>> >signalling
>> >only.
>> >
>> >By the way, has anyone who advocates differential impedance shown
>> why it is
>> >necessary and what happens if isn't maintained? I've never seen
>> anyone
>> >demonstrate this. I'd welcome some analysis that supports this
>> point of
>> >view, if
>> >it exists."
>> >
>> >Regards,
>> >
>> >Paul Denomme
>> >
>> >> -----Original Message-----
>> >> From: sweir [SMTP:weirsp@a.crl.com]
>> >> Sent: Friday, October 29, 1999 8:28 AM
>> >> To: si-list@silab.eng.sun.com
>> >> Subject: RE: [SI-LIST] : Comments from your SI seminar
>> (SendII)`
>> >>
>> >> Paul,
>> >>
>> >> Where is this article? I think the claim on matched lengths is
>> going to
>> >> cause quite a bit of controversy.
>> >>
>> >> For just the signal integrity, never mind the EMI:
>> >>
>> >> If the risetime is known, then any mismatch in length can be
>> equated to a
>> >> differential voltage offset during transitions. The offset
>> appears in
>> >> opposite polarity for opposite edges, so a duty-cycle distortion
>> results.
>> >> This shows up in both simulation and real lab measurements. If
>> the
>> >> risetime is slow compared to the mismatch, then the effects will
>> be
>> >> minimal. For the examples given this is definitely not the case.
>>
>> >>
>> >> 180pS/inch @ 1.6 in = 298pS, none of the signal transition is
>> >> differential. The receiver will switch when the voltage on the
>> shorter
>> >> trace satisfies the receiver hysteresis. This is hardly a good
>> situation
>> >> for skew and jitter control.
>> >>
>> >> 180pS/inch @ 0.5in = 90pS for LVDS @ 350mV/300pS per lead,
>> introduces an
>> >> effective offset of about -95mV in the longer lead for - to +
>> transitions,
>> >> and +95mV in the longer lead for + to - transitions. This
>> approximates
>> >> the maximum voltage required at the receiver to switch, 100mV.
>> So, again,
>> >> the switching suffers skew and jitter because we have lost the
>> benefits of
>> >> differential signaling for switching. Not a problem if a 200pS
>> out of the
>> >> timing budget doesn't matter. If we look at eye patterns at
>> 50MHz no one
>> >> will notice. However, at common transmission rates of 1-2.5Gbps,
>> 200pS is
>> >> a lot to give up.
>> >>
>> >> These are just the static effects. If we then look at the
>> impairment
>> >> caused by the physical antenna paths such differences cause
>> relative to
>> >> aggressors, this situation may deteriorate quite a bit more.
>> >>
>> >> If you would like a more intuitive approach, consider that the
>> advice(
>> >> routing around holes, etc ) is tantamount to claiming that you
>> can make
>> >> equally accurate high-speed oscilloscope measurements with an
>> extra 0.5
>> >> inches in the ground lead rather than maintaining coax all the
>> way to the
>> >> signal source. Most of us know all too well such a notion is
>> hogwash.
>> >> Why does the author claim our digital circuits are different?
>> >>
>> >> Regards,
>> >>
>> >>
>> >> Steve.
>> >>
>> >> At 08:04 AM 10/29/99 -0400, you wrote:
>> >>
>> >>
>> >> It is also stated in this article that critically
>> matched
>> >> lengths is
>> >> unnecessary, however you MUSt take into account your noise
>> >> margins(parallel
>> >> aggressors). It states that differential transmissions only
>> need
>> >> about 15mV
>> >> difference to switch.
>> >> Assuming no noise/ crosstalk and a 300pSec rise time
>> in
>> >> FR-4(180
>> >> pSec/In), the pairs length could be mismatched by as much as
>> 1.6
>> >> inches(Tr/Td) and still function. So a 500 mil difference
>> in line
>> >> length
>> >> would be acceptable. What he is really stating is that if
>> you need
>> >> to route
>> >> one of the traces around a through hole, just do it.
>> >>
>> >> It only shows an example for ECL and LVDS Logic in
>> the
>> >> acrticle.
>> >>
>> >> Regards,
>> >>
>> >> Paul Denomme
>> >>
>> >>
>> >> > -----Original Message-----
>> >> > From: S. Weir [SMTP:weirsp@a.crl.com]
>> >> > Sent: Thursday, October 28, 1999 6:04 PM
>> >> > To: si-list@silab.eng.sun.com
>> >> > Subject: RE: [SI-LIST] : Comments from your SI
>> seminar
>> >> (SendII)`
>> >> >
>> >> > Paul,
>> >> >
>> >> > Are you assuming:
>> >> >
>> >> > 1. The traces have been specified to remain at a constant
>> >> separation?
>> >> > 2. The trace pair will have a minimum separation from
>> parallel
>> >> aggressors?
>> >> > 3. The traces will have matched lengths?
>> >> >
>> >> > I am not sure how such things can be assumed and make it
>> to the
>> >> physical
>> >> > design. If the trace lengths don't match, the signal will
>> have
>> >> lots of
>> >> > common mode for fast enough edges. If there is not enough
>>
>> >> separation from
>> >> >
>> >> > parallel aggressors, then aggressors can inject
>> differential mode
>> >> > noise. If the traces do not maintain a constant
>> separation, the
>> >> impedance
>> >> >
>> >> > will vary by about half the variation in the coupling
>> coefficient.
>> >> >
>> >> > Regards,
>> >> >
>> >> >
>> >> > Steve.
>> >> >
>> >> > At 03:30 PM 10/28/1999 -0400, you wrote:
>> >> > > I have read an article recently that states that
>> the use
>> >> of
>> >> > >specifying the differential impedance of two traces on a
>> circuit
>> >> board is
>> >> > >unnecessary. The only thing you need to worry about is
>> the
>> >> individual
>> >> > trace
>> >> > >impedance. If you need a differential impedance for two
>> lines to
>> >> be 100
>> >> > >ohms, just use two 50 ohm lines rather than using two
>> signals
>> >> whose
>> >> > >differential impedance is 100 ohms. Also when connecting
>> a 110
>> >> ohm
>> >> > twisted
>> >> > >pair to PCB you should just connect it to two 55 ohm
>> traces to
>> >> achieve
>> >> > the
>> >> > >110 ohm differential impedance. I have done enough
>> research to
>> >> draw my
>> >> > own
>> >> > >conclusions, but I would like to get the reaction from
>> people in
>> >> this
>> >> > forum
>> >> > >regarding this issue.
>> >> > >
>> >> > >Thank you,
>> >> > >
>> >> > >Paul Denomme
>> >> > >Viasystems Inc.
>> >> > >
>> >> > >
>> >> > >
>> >> > > > -----Original Message-----
>> >> > > > From: Doug Brooks [SMTP:doug@eskimo.com]
>> >> > > > Sent: Thursday, October 28, 1999 12:59 PM
>> >> > > > To: si-list@silab.eng.sun.com
>> >> > > > Subject: RE: [SI-LIST] : Comments from your SI
>> seminar
>> >> (SendII)`
>> >> > > >
>> >> > > > >But a comment on our industry in general,
>> >> > > > >
>> >> > > > > I went to several courses at the PCB Design
>> East, and
>> >> each
>> >> > course
>> >> > > > >instructor had their own opinion on what they believe
>> is the
>> >> correct
>> >> > way
>> >> > > > of
>> >> > > > >doing things.
>> >> > > > >It is sad that our industry cannot take a concensus
>> and come
>> >> up with
>> >> > the
>> >> > > > >CORRECT way of doing things. Instead of using testing
>> and
>> >> empirical
>> >> > data
>> >> > > > to
>> >> > > > >determine what is accurate, they bicker about why
>> ones
>> >> methods will
>> >> > or
>> >> > > > won't
>> >> > > > >work.
>> >> > > > >
>> >> > > >
>> >> > > >
>> >> > > > As a seminar presenter at PCB East, and one who is
>> also
>> >> concerned
>> >> > about
>> >> > > > the
>> >> > > > fact that students hear different things in different
>> courses,
>> >> I'd
>> >> > like to
>> >> > > > offer a few random comments here.
>> >> > > >
>> >> > > > First, people in our industry need a better
>> understanding
>> >> about
>> >> > > > fundamental
>> >> > > > electrical engineering!! And I am not just talking
>> about those
>> >> without
>> >> > an
>> >> > > > engineering degree, but also those with an engineering
>> degree
>> >> who (1)
>> >> > > > didn't take certain kinds of classes related to such
>> high
>> >> speed issues
>> >> > as
>> >> > > > crosstalk, transmission lines, and stray trace/lead
>> >> inductance, etc.
>> >> > (2)
>> >> > > > took them and didn't understand them, or (3) took them
>> and
>> >> forgot
>> >> > them!!
>> >> > > > And I am not criticizing them --- in my second job out
>> of
>> >> college my
>> >> > > > company was designing state-of-the-art components for
>> the
>> >> > state-of-the-art
>> >> > > > Illiac IV computer that were water cooled ECL devices
>> running
>> >> at the
>> >> > > > remarkable speed of 3 MHZ! Things DO change.
>> >> > > >
>> >> > > > Second, it's nice to have rules of thumb, but it is
>> better to
>> >> > understand
>> >> > > > where those rules of thumb came from and when they
>> might (and
>> >> might
>> >> > not)
>> >> > > > apply. I often get comments like "In so-and-so's class
>> HE said
>> >> ...".
>> >> > My
>> >> > > > response is to try to make the issue UNDERSTANDABLE
>> for the
>> >> student so
>> >> > > > he/she can make up his/her OWN mind about what
>> position seems
>> >> more
>> >> > > > reasonable. But that can be a challenge when the
>> student has
>> >> very
>> >> > little
>> >> > > > technical understanding.
>> >> > > >
>> >> > > > Thirdly, as has been pointed out, there aren't a lot
>> of
>> >> absolutes in
>> >> > our
>> >> > > > industry. If there were, we'd all understand and be
>> teaching
>> >> the same
>> >> > > > (absolute) rules of thumb. While I am a strong
>> supporter of
>> >> studies
>> >> > (and
>> >> > > > have contributed to two of them --- the effects of
>> vias on
>> >> traces and
>> >> > the
>> >> > > > effects of 90 degree corners) this is not always the
>> answer.
>> >> Because
>> >> > ...
>> >> > > > each design has a unique environment. So, what works
>> in one
>> >> > environment
>> >> > > > might not apply to another. Once again, my approach is
>> usually
>> >> to try
>> >> > to
>> >> > > > present to the student the ISSUES and the alternative
>> >> opinions, so
>> >> > they
>> >> > > > can
>> >> > > > recognize problems and (hopefully) potential solutions
>> when
>> >> they
>> >> > arise. As
>> >> > > > before, it is improved understanding that helps the
>> designer
>> >> (and the
>> >> > > > engineer) solve problems, not rules of thumb or
>> others'
>> >> studies.
>> >> > > >
>> >> > > > Finally one last observation about studies. We lead a
>> study on
>> >> right
>> >> > angle
>> >> > > > corners where the measurements were taken by the
>> respected
>> >> people at
>> >> > the
>> >> > > > University of Missouri (Rolla). The results of that
>> study were
>> >> > > > independently confirmed by Mark Montrose with (a) a
>> board of
>> >> his own
>> >> > > > design
>> >> > > > and (b) another board from our study. These results
>> have
>> >> appeared in
>> >> > at
>> >> > > > least two publications. Nevertheless, take a position
>> on right
>> >> angle
>> >> > > > corners in one of these e-mail forums and see how much
>>
>> >> discussion it
>> >> > > > generates!! Some people's minds are made up, facts be
>> darned!
>> >> > > >
>> >> > > > Doug Brooks
>> >> > > >
>> >> > > >
>> >> > > > .
>> >> > > > ****************************************************
>> >> > > > Doug Brooks, President doug@eskimo.com
>> >> > > > UltraCAD Design, Inc.
>> <http://www.ultracad.com/>
>> >> > > >
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>
> --
> Ronald B. Miller _\\|//_ Signal Integrity Engineer
> (408)487-8017 (' 0-0 ') fax(408)487-8017
> ==========0000-(_)0000===========
> Brocade Communications Systems, 1901 Guadalupe Parkway, San Jose, CA 95131
> rmiller@brocade.com, rbmiller@sjm.infi.net
>
>

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