Re: [SI-LIST] : What's your favorite Screwy SI Concept?

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From: Lee Ritchey (leeritchey@earthlink.net)
Date: Tue Jan 11 2000 - 08:09:10 PST


This quote doesn't explain or prove the rule. When I was asked to do a book review on Mark's book, I asked him for the supporting analytical or laboratory
evidence that the rule was valid and over what range of frequencies it was valid as well as in what circumstances. I got no answer, except that Mike King
said so.

For that reason, I could not endorse it. None of use who say we are engineers should do it either, until we see the science that goes with the rule.
After all, we get paid to do engineering which is applying the scientific method to the design of useful products.

Imagine how we would react if our doctor did something to us because "someone gave him a rule of thumb to follow." I bet we'd have him in court in a f
lash.

The physicians' creed might well apply here. "First, do no harm" If we cannot explain why the rule is valid, perhaps we may do harm when we use it.

This applys to all of the "rules of thumb" that circulate in our community.

In my experience, the only place rules of thumb always work is a butcher shop, and then only in favor of the butcher when he puts his thumb on the scales.

Rules of thumb do not constitute engineering.

PRDEWASTHALEE wrote:

> Hi Jay and ALL,
>
> I think I can answer Q1. What is 20H rule?
>
> Reference: Printed Circuit Board Design Techniques for EMC Compliance
> by Mark I Montrose, IEEE Press
>
> Section 2.2: Important Points Only(I hope they are the ONLY imp points!)
>
> 1. When using High Speed Logic and clocks, power planes can couple
> RF energy to each other and thus radiate RF current into free space.
> To minimise this coupling effect, all power voltage planes must be
> physically smaller than the closest ground plane per the 20-H rule.
> < footnote: The "20-H" rule was defined by W. Michael King >
>
> 2. Use of the 20-H rule increases the intrinsic self-resonant frequency
> of the PCB. (!?)
>
> 3. To implement the 20-H rule, determine the physical distance spacing
> between the power plane and it's nearest ground plane. This distance
> spacing includes the thickness of the core, prepreg filler, and
> isolation separation as specified in the PCB fabrication drawing.
>
> 4. If this distance is H, make the power plane 20*H smaller than the
> ground plane.
>
> *******
> At the end of the chapter reference given is:
> 1. Technical Definition and explanation provided by W.Michael King
> (PERIOD!)
>
> In the acknowledgements amongst other *Celebrities* is the name of DOUG
> SMITH,
> for providing Technical review of the material. If he is the same as the
> one
> on this list we may get some clarity over this.
> *******
>
> Also as you(Jay) say you are relatively new, there was this rule
> discussed
> to great
> length (and perhaps enough depth) some months back. You might get it in
> the
> Archieves.
>
> BTW, Michael's mail is an exception to the normal sane mail traffic on
> this
> list.
>
> Good luck,
> - Dewa!
> <prdewa@wipinfo.soft.net>
>
> ***********************************************************************
>
> > -----Original Message-----
> > From: owner-si-list@silab.eng.sun.com
> > [mailto:owner-si-list@silab.eng.sun.com]On Behalf Of Jay Chesavage
> > Sent: Sunday, January 09, 2000 2:55 PM
> > To: si-list@silab.eng.sun.com
> > Subject: Re: [SI-LIST] : What's your favorite Screwy SI Concept?
> >
> >
> > I'm kind of new to this alias. Is this how things are normally
> > explained in this forum?
> >
> > 1) exactly what is the 20H rule.
> > 2) would a subscriber to this rule kindly say something specific
> > about its validity (theoretical or measured)
> > 3) would a naysayer to this rule kindly offer a counterindication
> > of its validity (theoretical or measured).
> >
> > In this manner, the bandwidth of the thousands of people following this
> > alias could be better used. Maybe some different people can contribute.
> >
> > Many thanks.
> >
> > -Jay
> >
> > On Sat, 8 Jan 2000, Michael Vrbanac wrote:
> >
> > > Lee,
> > >
> > > I can explain it and have but I won't as I explained before. I
> > have had laboratory
> > > evidence but could not retain it as it was left at a previous
> > place of employment.
> > > Sorry! That was in accordance with my work agreement. And, of
> > course, as
> > > many have said to me before... "that's such a convenient
> > excuse!" Again, sorry!
> > >
> > > As a consolation for those disappointed, I will give only one
> > final hint. For those
> > > who love simplicity, it indeed is. For those who love the
> > complex, an important
> > > piece of it can be seen in a section in a highly revered tome
> > written by a well-
> > > respected author but it is not in a form that you would normally expect.
> > > After seeing that, and considering its implications, compelling
> > supporting evidence
> > > can be seen in many texts. ( Those of you who already know, don't give
> > > it away! You'll spoil the learning experience for everyone
> > else.) Ok, one more
> > > hint and its the very last... and this I will credit to Michael
> > Chan.... think of WHY
> > > someone might have needed to do something like this, what they needed to
> > > accomplish, and where it just might make some sense.
> > >
> > > Have fun! <grin> Hopefully, the search might prove fruitful
> > for many looking for it
> > > even beyond learning about the 20H rule and where it really
> > applies. And maybe...
> > > just maybe, we can "unscrew" one "screwy rule" AND we might just all be
> > > in agreement about it!!!
> > >
> > > Once you figure it out, you will probably agree with me when I
> > say that the 20H rule
> > > may not provide significant benefit for every application in
> > every design but it
> > > does have its uses.
> > >
> > > Michael
> > >
> > > Lee Ritchey wrote:
> > >
> > > > Michael,
> > > >
> > > > If you cannot explain the 20h rule, do you have any laborator
> > evidence to support it?
> > > >
> > > > Lee
> > > >
> > > > Michael Vrbanac wrote:
> > > >
> > > > > Hi, Michael,
> > > > >
> > > > > That is my point exactly. The "screwy rule" was driven by some need
> > > > > at some time and place and more than likely be viewed negatively in
> > > > > an industry segment where the need to use it is much less.
> > The funny
> > > > > "human thing" to do is to therefore deny the existence of
> > anything we've
> > > > > never seen or attempted to simulate.
> > > > >
> > > > > As I have, over time, attempted to "thoroughly explain"
> > what I believe
> > > > > the 20H rule is "doing and what it is meant to do", I never
> > seem to get
> > > > > folks to accept that explanation for whatever reason even
> > though there
> > > > > have been designs which have benefitted from it. So I don't offer
> > > > > explanations anymore. Now I just let folks figure it out
> > for themselves. Its
> > > > > a whole like trying to argue about "grounding
> > methodologies" with someone
> > > > > who is intently opposed to your position. You can't
> > possibly create any
> > > > > scenario to convince them otherwise. So the discussion is
> > useless especially
> > > > > in a public forum. Perhaps you might understand what I mean if I
> > > > > asked you to defend exactly why it wouldn't work and let the process
> > > > > go from there.
> > > > >
> > > > > I was serious about the point about being careful of what
> > one claims as
> > > > > "non-applicable" or "nonsense" from a global sense (i.e.
> > all disciplines and
> > > > > sub-disciplines) unless it clearly violates Maxwell
> > Equations. By doing
> > > > > so, it amounts to a claim of infallibility both in
> > reasoning and test vehicle
> > > > > methodology and measurement. That's a pretty arrogant position and
> > > > > I try to stay away from that. The best anyone can say is
> > that, based on
> > > > > their testing and particular methodology and measurement,
> > the principle
> > > > > will or will not be applicable for that particular situation.
> > > > >
> > > > > Again, the creation of some of the "screwy rules" came
> > from a need to
> > > > > address a certain type of problem and it was apparently successful
> > > > > enough to gain a widespread audience in its application. Was it all
> > > > > hoopla or was it a practical solution for a particular
> > problem that many
> > > > > of us still don't understand? Either one is possible....
> > you must decide.
> > > > >
> > > > > Michael E. Vrbanac
> > > > >
> > > > > "Chan, Michael" wrote:
> > > > >
> > > > > > My 2 cents is to find out how a "screwy rule" being drawn out. A
> > > > > > rule ( no matter it is good or bad ) has to come out from
> > some initatives
> > > > > > and/or motivatives. I don't think a rule can come out
> > from nowhere. I
> > > > > > believe
> > > > > > a rule based on solid fundamental concepts cannot be
> > wrong by that much and
> > > > > > any derivative from real world observations can be
> > corrected and/or
> > > > > > improved.
> > > > > >
> > > > > > I believe building some test structures in order to see
> > whether a rule will
> > > > > > break or not cannot help to try to understand how a rule
> > being drawn up and
> > > > > > whether a rule make sense or not.
> > > > > >
> > > > > > For example, take the famous " 20H RULE " as a case. Can
> > anyone, especially
> > > > > > the originator(s) ( if there is some ) of the 20H Rule
> > explains the physical
> > > > > >
> > > > > > concepts behind how this rule is being drawn up?
> > > > > >
> > > > > > Michael Chan
> > > > > >
> > > > >
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