RE: [SI-LIST] : Definition of Hi-Speed (UC)

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From: Jack Olson (JackOlson@NVISION1.com)
Date: Fri Nov 05 1999 - 17:48:14 PST


Well, I can't answer a question like what "happened" to education for PCB
layout, because I haven't seen much of it available.
I have a "basic" electronics engineering degree and they didn't teach
ANYTHING about layout! Since my job is 100% layout I've used just about
every available way to learn more about it but it ain't easy. Even the
supposed experts intersperse provable results with lore and rules of thumb
of questionable or even detrimental value. You could spend ALL your time
poring through these lists and reading books and magazines, but some of us
have to get some WORK done and our learning time is limited. Then when you
attend these conferences one guy teaches decoupling is a waste and then the
next guy says you need 100 in parallel (just one example)

onward thru the fog...
Jack
(but I'm not here to discuss philosophy, sorry for going off-grid)

                -----Original Message-----
                From: Adrian Shiner [mailto:adrian.shiner@virgin.net]
                Sent: Tuesday, November 02, 1999 11:56 AM
                To: si-list@silab.eng.sun.com
                Cc: eric
                Subject: Re: [SI-LIST] : Definition of Hi-Speed (UC)

                What has happened to the education sytems in various
countries? ECL logic
                was in use in the late 1960's, complete with all these
characteristic fast
                rise time issues!!

                From reading many inputs to theis excellent forum for
discussion, I wonder
                if some of the PCB layout guys have the formal basic
engineering knowledge
                of the relevant subjects which are needed in this endevour.

                Adrian

                ----- Original Message -----
                From: Eric Bogatin <eric@bogent.com>
                To: <si-list@silab.eng.sun.com>
                Cc: eric <eric@bogent.com>
                Sent: 01 November 1999 18:52
                Subject: RE: [SI-LIST] : Definition of Hi-Speed (UC)

> The definition of High Speed I use in the classes I teach
is that high
                speed
> is when the interconnects are no longer transparent to the
signals. In
                this
> regime, the electrical effects of the interconnects will
affect signal
> quality and if you don't take the interconnects into
account, system
> performance will be degraded.
>
> As has been pointed out by many on this list, this
criteria depends more
                on
> the rise time of signals than on the clock frequency. In
my last article
                on
> ChipCenter (click on this link if you want to read it:
> http://www.chipcenter.com/signalintegrity/ ), I wrote
about the coming
> crisis when even jelly bean parts soon will have sub 0.5
nsec rise times,
> even if they are used in circuits with only 10 MHz clocks.
>
> However, sometimes managers don't really understand about
rise time, and
> just think in terms of clock frequency. They just want a
number and want
                you
> to fill in the blank with your best guess.
>
> So, given the caveat of the importance of rise time over
clock frequency,
> here's the rule of thumb I sometimes use to pick a number:
>
>
> The starting clock frequency for high speed is F > 200
MHz/L, where L is
                the
> longest length of the board, in inches.
>
>
> If your product is a 10 inch board, like a PC mother
board, high frequency
> is any clock frequency above about 20 MHz. If your product
is a PC card
> (PCMCIA format), 4 inches in a side, high frequency is
when the clock is
                50
> MHz and above.
>
>
> Here are the assumptions behind this "rule of thumb":
>
> 1. the rise times of signals are about 1/10 the clock
period
>
> 2. transmission line effects dominate (ie, the
interconnect is not
> transparent) when TD > 1/3 x rise time
>
> 3. the substrate is FR4 and the speed of a signal is about
6 inches/nsec.
>
>
> Note, as with all rules of thumb, if you are trying to
make a distinction
> between is 25 MHZ or 35 MHz really the "onset" of high
speed, you don't
                want
> to use a rule of thumb. But, if you want to know is 10 MHz
or 50 MHz high
> speed, this rule of thumb is a pretty quick way of getting
an answer.
>
> For those interested, I have added a paper I wrote on
"Rules of Thumb I
                Have
> Known and Loved" to my collection of downloads for
November. You are
                welcome
> to download a pfd version of this by visiting my web site
at:
> <http://www.bogatinenterprises.com/>
>
> --eric
>
> Eric Bogatin
> BOGATIN ENTERPRISES
> Training for Signal Integrity and Interconnect Design
> 26235 W. 110th Terr.
> Olathe, KS 66061
> v: 913-393-1305
> f: 913-393-1306
> pager: 888-775-1138
> e: eric@bogent.com
> web: <http://www.bogatinenterprises.com/>
>
>
> > -----Original Message-----
> > From: owner-si-list@silab.eng.sun.com
> > [mailto:owner-si-list@silab.eng.sun.com]On Behalf Of
Beal, Weston
> > Sent: Monday, November 01, 1999 11:22 AM
> > To: 'si-list@silab.eng.sun.com'
> > Subject: RE: [SI-LIST] : Definition of Hi-Speed (UC)
> >
> >
> > The best definition I've ever heard of High-Speed was
from someone on
                this
> > list. Speak up or I'll take the credit.
> >
> > High-Speed is any design that has faster edge rates or
clock rates than
> > you've designed before.
> >
> > It's new to you, so you need to study and analyze it.
> >
> > Regards,
> > Weston
> >
> >
> > -----Original Message-----
> > From: Lum Wee Mei [mailto:lweemei@dso.org.sg]
> > Sent: Sunday, 31 October, 1999 10:54 PM
> > To: 'si-list@silab.eng.sun.com'
> > Subject: [SI-LIST] : Definition of Hi-Speed (UC)
> >
> > Pardon me for asking this stupid question because I am
at a
> > loss of how
> > to explain hi-speed to my boss. He thinks that hi-speed
is
> > as simple and
> > straightforward as resistance = V/I and nothing else.
> > Hi-speed should be
> > some circuits that need to operate at xxMHz or more.
> > Anything less than
> > xxMhz is not hi-speed.
> >
> > I would appreciate anyone of you experts out there who
can
> > enlighten me
> > in a simple and easy to understand definition so that my
> > boss can
> > understand.
> >
> > Regards.
> >
> >
> >
> >
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