[SI-LIST] Re: Current Flow
- From: <Aubrey_Sparkman@xxxxxxxx>
- To: <HreidmarKailen@xxxxxxxxxxx>, <scott@xxxxxxxxxxxxx>, <doug@xxxxxxxxxx>
- Date: Wed, 10 Aug 2005 08:57:55 -0500
I love it!
Beer's to you!=20
Aubrey Sparkman=20
Enterprise Engineering Signal Integrity Team
Dell, Inc.=20
Aubrey_Sparkman@xxxxxxxx=20
(512) 723-3592
-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx]
On Behalf Of HreidmarKailen
Sent: Wednesday, August 10, 2005 3:47 AM
To: scott@xxxxxxxxxxxxx; doug@xxxxxxxxxx
Cc: steve weir; si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Re: Current Flow
I have a good analogy...
You know "the wave" that's performed ad hoc at football, and other,
arenas by the fans?
Well, to simplify, each rank of humans, perdendicular to the direction
of wave travel, decide in unison to stand up, raise their arms, and
think beer-influenced thoughts. They must use some energy to move and
wave.
Some strange interaction among these humans causes the next rank to do
the same, and so forth. Something very human.
The result is something that travels around the arena, sure as the sky
is blue, well controlled by the gravity block of the floor which serves
to make constant the wavefront size, as long as each rank is a clone of
the last and has consumed the same requisite
portion of beer. Occasionally, a frankfurter outgassing or the tipsy
human scattering off a chair can disturb perfection. Anyway,
the result is a constant beer to wavefront ratio -- let's call that the
BeerWavefront Impedance.
Back to the wave... Said wave proceeds, as seen from the Blimp, quite
nicely around the bend. All electrons, er ... humans, are happy. It's
a community.
Well, that's all folks. The wave proceeds --- a flow of energy. It's
really cool.
The electrons move little, some up/down some towards the ones they're
inspiring to imitate them. At the terminus of the arena
seats the last rank typically "matches" the wave by terminating it
perfectly. At some later time, the same - or other - instigators will
try the same thing, to varying degrees of success. The driver must be a
low impedance source of beer, or the result is likely to be an under or
over-driven Wave.
How can it be that electrons and humans are so alike??
Yours,
Agathon
----- Original Message -----
From: "Scott McMorrow" <scott@xxxxxxxxxxxxx>
To: <doug@xxxxxxxxxx>
Cc: "steve weir" <weirsi@xxxxxxxxxx>; <si-list@xxxxxxxxxxxxx>
Sent: Friday, August 05, 2005 3:51 PM
Subject: [SI-LIST] Re: Current Flow
> Enough.
> Electrons do not flow down a a PCB trace, electromagnetic fields,
which
> propagate as waves, do. Electrons do not flow across the plates of
> capacitors, but an electromagnetic field does. We can model traces,
> plates, transmission lines, cables ... etc as resistors, capacitors
and
> inductors, but do not forget that they are just that, models.
>
> If we were talking about light traveling down a piece of fiber,
between
> mirrors, through windows ... etc, no one would have a problem with a
> wave or particle scattering model. Nor would anyone invoke capacitor
> bucket brigade models to explain the signal propagation and "return
> path." It would be intuitively obvious to anyone that the light will
> scatter off of, and be constrained by, reflecting boundaries (return
> path.) With light, does anyone have a problem understanding how a
> signal can be launched and become totally disconnected from the
> launching ground reference? Yet, so many people have a problem
> understanding that an electronic signal is an electromagnetic wave
> (light), traveling through a medium (dielectric), and is constrained
by
> boundaries (metal), which either guide the signal, or reflect it.
Once
> the signal is launched as an EM wave, the original ground, and
voltage,
> that was used by the transistor, does not matter. The signal (wave)
has
> a life of it's own, and is constrained only by the metal around it.
As
> long as all the metal used to guide the wave is continuous, you
usually
> have a very good transmission medium. As soon as there is a break or
> disruption in the guiding metal, serious problems arise. Usually this
> break or disruption occurs in what we call the "return path."
However,
> make no mistake. There's no such thing as far as the EM wave is
> concerned. All it knows is that it is following a bunch of metal
around
> in what we call the path of least impedance. Yank the metal out from
> under it along the signal conductor or the "return path" conductor,
and
> it will find a better path, instantaneously.
>
> One you look at electronic signals as propagating waves, understanding
> them becomes much easier than other oversimplified models. As the
wave
> travels down a trace on a board, it is very easy to envision the EM
> field "touching" the plane and trace, which guide it, and enveloping
> itself around the trace and extending off into the infinite universe.
> One can imagine what happens when a signal passes down a via and past
> planes, and how some of the "light" will leak into each of the planar
> cavities, and ripple back and forth, like waves on a pond.
>
> Scott McMorrow
> Teraspeed Consulting Group LLC
> 121 North River Drive
> Narragansett, RI 02882
> (401) 284-1827 Business
> (401) 284-1840 Fax
>
> http://www.teraspeed.com
>
> Teraspeed(r) is the registered service mark of
> Teraspeed Consulting Group LLC
>
>
>
> Doug Brooks wrote:
>
> >At 02:18 PM 8/5/2005, you wrote:
> >
> >
> >>Doug,
> >>
> >>In the fluid model, we would see current propagate down the signal
> >>conductor and then later back in the return lead would we not?
> >>
> >>
> >
> >That's why I said I didn't think "fluid" was the best descriptor. As
I
> >tried to point out the first time, electrons start flowing down the
line,
> >onto the "plates" of the distributed capacitance, repelling electrons
(like
> >charges repel) from the other "plates" of the distributed
capacitance, and
> >back, completing the loop. As the first "plates" charge up, the
current
> >flows past them and charges the next "plates". By the time the
current gets
> >to the end if the line, all the "plates" are charged up, and the flow
looks
> >like a DC flow would look. This is exactly what Figure 7-19 in
Bogatin's
> >book is describing. You describe this from the standpoint of
"waves". I
> >can equally well (no better, no worse) describe it as electron flow.
I
> >don't see a difference and I don't see a problem.
> >
> >The "fluid flow" model breaks down because we can't envision fluid
crossing
> >between the plates of a capacitor. But electron flow CAN cross the
plates
> >of a capacitor because of the property that "like charges repel each
> >other." Electrons don't physically cross the space between the
plates, but
> >they build up on one side and repel those on the other, so that the
same
> >number of electrons return to the source as left it.
> >
> >Doug
> >
> >
> >
> >
> >
> >>But in real life, we observe that current propagates in one polarity
from
> >>the signal conductor portion of the wave guide, and simultaneously
in the
> >>opposite polarity from the return conductor side of the wave guide.
The
> >>fluid flow model has problems both with time, and with the fact that
the
> >>wave propagates down an infinitely long open transmission line just
as
> >>well as it does an end terminated line. In the open, or infinite
length
> >>line electrons never passed from one conductor to the other.
> >>
> >>How does a circular fluid flow analogy model this behavior? At the
far
> >>end of an open transmission line the conduction path is broken, the
fluid
> >>has no contiguous path.
> >>
> >>We can agree that electrons in the conductors move in response to
the
> >>propagating fields, sic wave. But I have to reiterate that back at
our
> >>switch it is the fields interacting with the conductors that push on
those
> >>electrons you observe moving in the conductors. When the dv/dt
switches
> >>direction later in time, the charge will go the other way in each
> >>conductor, but as far as charge between the two conductors: never
the
> >>twain shall meet.
> >>
> >>Regards,
> >>
> >>
> >>Steve.
> >>
> >>At 02:12 PM 8/5/2005 -0700, Doug Brooks wrote:
> >>
> >>
> >>>(I have changed the subject line to better represent what I think
we are
> >>>talking about.)
> >>>
> >>>You raise an excellent example. Let me deal with the two points.
> >>>
> >>>1. I'm not sure I understand what you are getting at here. The
focus
> >>>should be at the point of the switch.
> >>>
> >>>2. I have introduced the problem in some of my transmission line
classes
> >>>that deal with point 2. Assume that (a) there is a propagation time
for a
> >>>signal, (b) current (i.e. electrons) flows in a closed loop, (c)
current
> >>>is constant everywhere in that loop ---- aren't these mutually
exclusive
> >>>conditions? The answer is no! The current flows down the
transmission
> >>>line from one side to the other through the distributed capacitance
(as
> >>>suggested in Bogatin's Figure 7-19). This is a current flow (i.e.
> >>>electron flow) picture. If you want to call it a wave flow, well
that's
> >>>fine. But you can also describe it as current (electrons) flowing
to the
> >>>point of the distributed capacitance, repelling charge away from
the
> >>>other side of the capacitance back to the beginning of the line,
charging
> >>>the capacitance up (with electrons) along the way. At the steady
state,
> >>>current (electrons) is flowing in the DC loop we would expect. If
we
> >>>don't have a transmission line ---- well, we always have a
transmission
> >>>line of sorts. The question is whether it's ideal or REAL crummy.
There
> >>>is always a characteristic impedance, even if it is only that of
air.
> >>>
> >>>So the "fluid" analogy (I don't think that's the best descriptor)
can
> >>>deal with this issue perfectly fine. Likewise, it can deal with the
> >>>crosstalk coupling issue equally as well. (I don't have a figure
like
> >>>Eric's in my book, but there is a very detailed illustration of how
> >>>crosstalk coupling works in my book that doesn't need Maxwell and
wave
> >>>theory to understand.)
> >>>
> >>>So I don't see the difficulty here.
> >>>
> >>>Doug
> >>>
> >>>
> >>>
> >>>
> >>>
> >>>At 01:18 PM 8/5/2005, steve weir wrote:
> >>>
> >>>
> >>>>Doug, in the fluid model, there are two misleading elements:
> >>>>
> >>>>1. The focus is on the source of EMF, sic the battery,
> >>>>2. It implies a time lag between the foward current starting from
some
> >>>>point and the matching return current closing that path.
> >>>>
> >>>>If we take the switch example you offered, one might imagine a
couple of
> >>>>different cases:
> >>>>
> >>>>a. The switch is located very close to one terminal of the battery
and a
> >>>>say 300m wire connects it to the other through some load resistor.
> >>>>b. The switch is located at the end of two 300m wires back to the
> >>>>battery through some load resistor.
> >>>>
> >>>>Now, what will each the fluid analogy, and wave propagation tell
us
> >>>>about each case? Where does each model show the propagation
beginning
> >>>>and ending? How accurate is each? Which model can explain
behavior
> >>>>
> >>>>
> >>>>from virtual DC to any frequency we like? I don't think it's the
fluid
> >>>
> >>>
> >>>>analogy.
> >>>>
> >>>>On a PCB with switching I/Os the time and distance scales have
changed
> >>>>but not the behavior. What we have is in essence case b from
> >>>>above. The wave emanates from the switches in our ICs, not from
the
> >>>>power supply. The wave model makes this clear, as it does the
> >>>>propagation path. The wave model makes clear the critical point
that
> >>>>the return and forward currents propagate together. The fluid
analogy
> >>>>with its unidirectional emphasis fails us badly.
> >>>>
> >>>>Where has the fluid analogy brought us? How many times have you
seen
> >>>>people talk about bone-headed ideas like the PCB planes or bypass
> >>>>capacitors supplying current to high speed edges, when the entire
edge
> >>>>has completed long before the wave front through power pins can
reach
> >>>>significant charge in the planes, much less even reach the PWB
bypass
> >>>>caps? Yet this kind of junk mythology sadly makes its way into
books
> >>>>and other publications on a regular basis. I don't like it one
bit.
> >>>>
> >>>>For my money, I find the fluid analogy terribly misleading, and a
source
> >>>>of much misunderstanding. One doesn't need to be able to derive
Maxwell
> >>>>to understand wave propagation. I think that as Eric's book
> >>>>demonstrates, most SI concepts are not that difficult to
> >>>>understand. Even a dummy like me gets them from time to time.
> >>>>
> >>>>Regards,
> >>>>
> >>>>
> >>>>Steve.
> >>>>At 12:51 PM 8/5/2005 -0700, Doug Brooks wrote:
> >>>>
> >>>>
> >>>>>A couple of people have interpreted my statement re "flow" of
electrons
> >>>>>as meaning electron drift. Let's kill that right now.
> >>>>>
> >>>>>One electron in =3D one electron out is the flow of electrons. =
One
> >>>>>electron in =3D SAME electron out is electron drift --- not at =
all
the
> >>>>>same thing.
> >>>>>
> >>>>>Certainly I don't argue against Maxwell's equations. But I don't
argue
> >>>>>against the fundamental definition of one amp of current either
--- the
> >>>>>flow of one coulomb of charge (6.25 x 10^18 electrons) across a
> >>>>>surface in one second. I spend a lot of time with engineers (and
> >>>>>technicians) who never took Maxwell's equations and didn't
understand
> >>>>>them if they did. My goal has been to take our difficult SI
concepts
> >>>>>and explain them in terms that these "poor" people can
understand. To
> >>>>>suggest that you can't explain what happens during planar
transitions
> >>>>>without Maxwell's equations (I believe) is simply wrong. To say
that
> >>>>>the classical description of current can't explain the difference
> >>>>>between DC and high frequency is also (I believe) flat wrong. To
say
> >>>>>that one description is "more accurate" than the other --- well I
> >>>>>suggest that depends a lot on whose working with them! And while
people
> >>>>>have been misled by seminar leaders teaching without the benefit
of
> >>>>>Maxwell's equations, we all know seminar leaders whose ability to
> >>>>>mislead wasn't one bit hampered by a thorough knowledge of
Maxwell's equations!
> >>>>>
> >>>>>Don't sell these more basic principles short when it comes to
> >>>>>understanding what is happening on circuit boards. They can very
> >>>>>effectively explain what is happening, and why one design
approach may
> >>>>>be more effective than another depending on the important design
> >>>>>considerations. Especially for all those board designers who have
no
> >>>>>knowledge of Maxwell and wave theory.
> >>>>>
> >>>>>Doug
> >>>>>
> >>>>>
> >>>>>
> >>>>>
> >>>>>At 12:01 PM 8/5/2005, steve weir wrote:
> >>>>>
> >>>>>
> >>>>>>Doug, well I am going to argue vehemently that until someone
repeals
> >>>>>>Maxwell that the wave description is fundamentally more accurate
than the
> >>>>>>fluid analogy. The E/M fields cause the electron drift in those
> >>>>>>wires. From the time you closed the switch the changing E/M
field that
> >>>>>>resulted propagated outward. Marconi found a useful purpose for
that
> >>>>>>phenomenon.
> >>>>>>
> >>>>>>The fluid analogy is certainly easy to understand, but what is
the point
> >>>>>>when it is so misleading? I can't tell you how many times
otherwise
> >>>>>>intelligent engineers that I have known have been thrown off
understanding
> >>>>>>PCB wave guides, because they were intent on following the DC
current loop
> >>>>>>of the fluid analogy.
> >>>>>>
> >>>>>>Teaching the fluid analogy requires that we later break that
teaching when
> >>>>>>we want to explain what happens at significant frequencies.
Consider for
> >>>>>>instance visualization of return current ( which is the original
subject
> >>>>>>matter ) when we transition planes in a PCB. If we think about
it as a
> >>>>>>fluid model we are easily misled into searching out a conduction
> >>>>>>path. For
> >>>>>>ready examples of this mass confusion, just look at some of the
> >>>>>>discussions
> >>>>>>on splitting-up grounds in the wrong ways for the wrong reasons,
with the
> >>>>>>wrong results. But if we simply consider waves to begin with,
then the
> >>>>>>behavior is easy enough to intuit out.
> >>>>>>
> >>>>>>Eric does a very nice job in his book explaining signal
propagation that
> >>>>>>does not rely on the fluid analogy. I think his approach is
very
> >>>>>>accessible.
> >>>>>>
> >>>>>>Regards,
> >>>>>>
> >>>>>>
> >>>>>>Steve
> >>>>>>At 11:21 AM 8/5/2005 -0700, Doug Brooks wrote:
> >>>>>>
> >>>>>>
> >>>>>>>With all due respect, Steve, if I have a battery connected to a
> >>>>>>>
> >>>>>>>
> >>>>>>transistor
> >>>>>>
> >>>>>>
> >>>>>>>through a switch, I can turn the transistor "on" and "off" with
the
> >>>>>>>switch. That is easy to explain using the electron flow concept
(which I
> >>>>>>>hesitate to call an analogy, it in fact describes the physics
involved).
> >>>>>>>
> >>>>>>>Is your description more complete AND also easier to
understand?
> >>>>>>>
> >>>>>>>And if it is the frequency with which I "flip" the switch that
bothers
> >>>>>>>you, that simply means that some of the parameters that were
not an
> >>>>>>>
> >>>>>>>
> >>>>>>issue
> >>>>>>
> >>>>>>
> >>>>>>>with slow "flipping" (inductance and capacitance, for example)
start
> >>>>>>>becoming more of an issue with faster "flipping!" But the basic
> >>>>>>>
> >>>>>>>
> >>>>>>nature of
> >>>>>>
> >>>>>>
> >>>>>>>what is happening (in particular where the electrons are
flowing) is not
> >>>>>>>changing, just speeding up. (How the electrons are flowing is
> >>>>>>>
> >>>>>>>
> >>>>>>speeding up,
> >>>>>>
> >>>>>>
> >>>>>>>the electrons themselves, of course, don't change speed!)
> >>>>>>>
> >>>>>>>Doug
> >>>>>>>
> >>>>>>>
> >>>>>>>
> >>>>>>>At 10:45 AM 8/5/2005, steve weir wrote:
> >>>>>>>
> >>>>>>>
> >>>>>>>>Doug, I have some real heartburn with some of those
representations,
> >>>>>>>>particularly the fluid analogy that speaks of current as the
flow of
> >>>>>>>>electrons. When I grew up current was defined as time
variation of
> >>>>>>>>electric flux. When an E/M field impinges a chunk of metal
the
> >>>>>>>>
> >>>>>>>>
> >>>>>>resulting
> >>>>>>
> >>>>>>
> >>>>>>>>interaction concentrates the field forming a wave guide. All
> >>>>>>>>
> >>>>>>>>
> >>>>>>practical
> >>>>>>
> >>>>>>
> >>>>>>>>wave guides leak, be they a microstrip over a plane, a
stripline, or
> >>>>>>>>whatever. Some, like a good semirigid coax leak only a little
tiny
> >>>>>>>>bit. When they leak too much creating excessive disturbance
in
> >>>>>>>>
> >>>>>>>>
> >>>>>>nearby wave
> >>>>>>
> >>>>>>
> >>>>>>>>guides, we have cross talk problems. I hope that this is what
you were
> >>>>>>>>trying to convey.
> >>>>>>>>
> >>>>>>>>Regards,
> >>>>>>>>
> >>>>>>>>
> >>>>>>>>Steve.
> >>>>>>>>
> >>>>>>>>A
> >>>>>>>>
> >>>>>>>>
> >>>>>>>Check out UltraCAD's new presentation videos and new skin
effect
> >>>>>>>calculator at http://www.ultracad.com
> >>>>>>>
> >>>>>>>
> >>>>>>>
>
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