[SI-LIST] Re: Question regarding current loop
- From: Faraydon Pakbaz <pakbazf@xxxxxxxxxx>
- To: LSMITH@xxxxxxxxxx
- Date: Fri, 28 Apr 2006 13:28:13 -0400
Larry;
As you said people like their Circuit Theory because it is easier. What
they should realize is that
Field theory can explain circuit theory and its own field theory. Circuit
theory alone can not explain
field theory. Circuit theory is subset of Field theory. It is like Newton
gravitational law vs Einstein
gravitational law(general relativity). At low speed of daily life nobody
needs Einstein, at near speed of
light Newton law will not work. However, the point of matter is Einstein
law works in both cases.
Circuit Theory advocates keep using the word Voltage & Ground which don't
exist.
There is no such a thing as Ground( ground is were potato and carrot grows)
There is no such a thing as voltage( only field and potential difference
between two point)
Regards;
Don Pakbaz
Silicon Solutions Engineering
IBM Systems & Technology Group
"Larry Smith"
<LSMITH@xxxxxxxxx
m> To
Sent by: <steven.d.corey@xxxxxxxxxxxxxx>,
si-list-bounce@fr <si-list@xxxxxxxxxxxxx>
eelists.org cc
Subject
04/28/2006 12:47 [SI-LIST] Re: Question regarding
PM current loop
Please respond to
LSMITH@xxxxxxxxxx
Steve - Yes, this is just semantics, perhaps I need to get a copy of
your lexicon. :) Over the years I believe we have expanded our
definition of "circuit theory" because the field concepts are just too
difficult to deal with.
At audio frequencies, there is no need to use field theory or time delay
concepts. I can analyze the signals at my speakers without ever
thinking about t-lines. Remember the 8MHz microprocessors? The TTL
circuits probably had a 5 nSec rise time and all you needed to know was
the output resistance (or current) and the load capacitance to analyze
timing. Trace length was not an issue unless it added significant R and
C. =20
We have plenty of good high speed circuit designers that deal with
internal logic circuits and even I/O drivers and receivers on chip. If
I talk to them about ports and S parameters there eyes will roll and
communications will have broken down. I have been asked the meaning of
the TDR and VNA acronyms as if they hold the keys to deep dark secrets.
Multi port scattering parameters are not in their vocabulary but they
are world class circuit designers in the transistor world. =20
What do these things have in common? They are all examples of circuits
with physical size less than 1/20th of a wavelength of the highest
frequency of interest. And, yes you can assume that a trace is a single
node with no time delay if it short enough compared to a wavelength. We
certainly don't use transmission lines to represent the trace that goes
from one transistor to another. (Well, maybe if we are going a
significant distance across the die, but then it is an RC t-line.. And
yes, on-chip signal integrity is becoming an issue.).
Back in the dark ages when I went to school, EE101 involved circuits
with RLC & G's and some voltage and current sources. In our sophomore
year we took the bran new transistor class that the profs were
struggling with because it replaced the vacuum tube class that had been
dropped from the previous year. Transmission lines were introduced in
our junior year after we had some field theory. Perhaps my definition
of circuit theory is a bit different than yours. Things have changed
for the better.. =20
The discussion that was going on between Scott and Doug seemed to be
based on language of field theory and circuit theory (the archaic
definition of the word). Does your lexicon have a word or phrase for
analysis of circuits that are less than 1/20th of a wavelength?
Regards,
Larry Smith
Altera Corporation
PS - From your definition of "circuit theory" it seems that the only
place you can talk about voltage and current is at a port. Yes, that
seems like a good way to look at it. My, how things have changed. :)
-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx]
On Behalf Of steven.d.corey@xxxxxxxxxxxxxx
Sent: Thursday, April 27, 2006 6:32 PM
To: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Re: Question regarding current loop
Hi Larry -- this may seem like pure semantics, but I would disagree with
your characterization of circuit theory. In my lexicon, circuit theory
describes system behavior in terms of what have traditionally been
called "circuit variables" (e.g., voltage, current, voltage waves, power
waves). I suppose field theory could analogously describe system
behavior in terms of its "field variables" (e.g., E,H fields).
Basically, field variables are defined at points, and circuit variables
are defined at ports.
In this sense, circuit theory encompasses concepts such as multiport
scattering parameters -- they're just a transformed version of impedance
or admittance parameters, which actually were covered in EE101. It also
includes delay between ports -- an example might be T-Line equations
relating circuit variables voltage and current (or even incident and
reflected voltage waves) at the ports of a T-Line network. PEEC
modeling is an example of a method that maps field interactions into
circuit variables without loss of generality.
Clearly you can't assume that a signal trace is a single circuit node
with zero delay. Nor would I advocate the charge-hose concept of a
posse of electrons running full tilt down a transmission line, climbing
down to the ground plane, and hot-footing it back home. It's also been
pointed out in the past on this list that you can't expect to measure a
unique voltage between two points several wavelengths apart. None of
these is an inherent shortcoming of circuit theory, but each is an
example of misapplication of circuit theory in a high-speed environment.
You definitely need to understand the fields and their interactions to
be able to build a reliable high-speed circuit that is a good neighbor
in the FCC sense. You may even use a field solver to figure out the
port behavior of part of your circuit. But at the end of the day you
have to present a certain voltage to the input port of your receiver,
complete with enough current to charge or discharge its input circuitry.
In other words, we're stuck with circuit theory... Rather than
discarding it, I would say that we're actually learning better what it
is and how to use it for high-speed design.
-- Steve
------------------------------------------------------------------------
-
Steven D. Corey, Ph.D.
Principal Engineer
Tektronix - Enabling Innovation
=3D20
http://www.tdasystems.com
http://www.tektronix.com
=3D20
email: steven.corey@xxxxxxxxxxxxx
phone: (503) 627-6816
fax: (503) 627-2260
------------------------------------------------------------------------
-
=3D20
>-----Original Message-----
>From: si-list-bounce@xxxxxxxxxxxxx=3D20
>[mailto:si-list-bounce@xxxxxxxxxxxxx] On Behalf Of Larry Smith
>Sent: Thursday, April 27, 2006 12:24 PM
>To: scott@xxxxxxxxxxxxx; doug@xxxxxxxxxx
>Cc: si-list@xxxxxxxxxxxxx
>Subject: [SI-LIST] Re: Question regarding current loop
>
>Scott - Your recent post was really interesting, I enjoyed reading it.
>Thanks for taking the time to write it up. I think that what we are
>discussing here is the field theory view of the world and the circuit
>theory view of the world.=20
>
>In circuit theory, Kirchhoff has given us laws that discuss loop
>voltages and branch currents (EE101). There is no concept of time
>across a node. With low speed signals, the near and far ends of traces
>are connected with copper and considered to be the same node.=20
>Simple RC
>time constants can be used to evaluate the time delay from driver to
>receiver. (That is so 1980's... S parameters need not apply.) Never
>the less, these are the circuit theory concepts that we use in
>evaluating local circuits where there is not a significant
>time delay in
>the physical size of the circuit.
>
>The concepts in your post are based on field theory. A trace is not a
>node but a transmission line delay element, or else a string of nodes
>separated by L's and C's with equations like v=3DL*di/dt and
>i=3DC*dv/dt .
>Circuit theory still works but it is running out of gas and concepts
>break down.
>
>The dividing line between circuit theory and field theory is when the
>length of the trace becomes a significant portion of the rise time. A
>good rule of thumb for this dividing line would allow the
>trace to be no
>longer than 1/3 of a rise time for data lines, 1/5 of a rise time for
>clock signals and 1/7 of a rise time for instrumentation. That way the
>near end will have a chance to influence the far end 2, 3 and 4 times
>during the rise time respectively. Using the relationships
>freq=3D0.35/t_rise and period=3D1/frequency, we find that
>trace may be
>1/8.6, 1/14.3 and 1/20 of a wavelength for data, clocks and
>instruments,
>listed in order of increasing accuracy requirements.=20
>
>When the physical size of the circuit (trace length) becomes more than
>1/20th of a wavelength long, simple circuit theory (charge
>hose analogy)
>begins to break down and you must use field theory concepts to
>understand what is happening.=20
>
>It is kind of interesting that you can start with the concepts (1) that
>1/20th of a wavelength is where circuit size becomes important=3D20
>and (2) a
>trace becomes a transmission line when it is more than 1/7th of a rise
>time long and then work backwards to find that frequency content is
>0.35/t_rise. It's just another interesting way of looking at this.
>
>Regards,
>Larry Smith
>Altera Corporation
>
>
>
>
>
------------------------------------------------------------------
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 FAQ wiki page is located at:
http://si-list.org/wiki/wiki.pl?Si-List_FAQ
List technical documents are available at:
http://www.si-list.org
List archives are viewable at: =20
//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
=20
------------------------------------------------------------------
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 FAQ wiki page is located at:
http://si-list.org/wiki/wiki.pl?Si-List_FAQ
List technical documents are available at:
http://www.si-list.org
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 FAQ wiki page is located at:
http://si-list.org/wiki/wiki.pl?Si-List_FAQ
List technical documents are available at:
http://www.si-list.org
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:
