Re: [SI-LIST] : Trace Impedance Selection-- 28 on Rambus

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From: Giorgio Ravesio (gravesio@flextel.it)
Date: Mon May 08 2000 - 23:25:08 PDT


George,
excuse me for the stupid question, but I really didn't grab what you said in
your e_mail.
Could you explain me how can you said that adding multiple loads to a 55-75 ohm
trace,
the impedence of the net drops down to 28 ohm?

Thanks in advance

Giorgio Ravesio
Tel +39-125-235307
gravesio@flextel.it
Flextel s.p.a.

"Harris, George" wrote:

> Why 28 ohms on Rambus?
> Because if you add multiple 2.2pf loads to a 55-75 ohm trace, the loaded
> impedance will be about 28 ohms. To minimize reflections when the wave
> transitions from the unloaded section of etch to the congested area where
> all the loads are, you need the unloaded etch to have the same impedance, 28
> ohms.
>
> -George
> George Harris
> Tel: 508-467-8893
> george.harris@compaq.com
> COMPAQ Computer Corporation
>
> -----Original Message-----
> From: opamps cybernetics [mailto:opamps@hotmail.com]
> Sent: Friday, May 05, 2000 1:41 PM
> To: si-list@silab.eng.sun.com
> Subject: Re: [SI-LIST] : Trace Impedance Selection
>
> Hi Gurus,
>
> One thing for sure is that larger characteristic impedance means thinnner
> trace on PCB which also means more fringing electric and magnetic fields. So
>
> more emission and susceptibility. Definitely space is something to be
> concerned.
>
> So what value of impedance you choose is determined by the above and the
> fact that the smaller characteristics impedance (thicker traces) require
> powerful drivers. So power is the an issue here.
>
> I do not know why Rambus choose the magic number of 28 Ohm. Does any body
> know why 28 Ohm in Rambus?
>
> Subas
>
> >Doug Brooks wrote:
> >
> > > OK, gurus....
> > >
> > > Lets say I have a trace on a board between two devices. There is nothing
> > > that determines what the characteristic impedance of the trace should
> >be.
> > > That is, there are no device considerations, no space considerations, no
> > > power considerations, etc. that would favor one value of characteristic
> > > impedance over another. It is my opinion that as long as the
> >characteristic
> > > impedance is constant over the length and properly terminated, the
> >choice
> > > of impedance value in this situation is completely arbitrary.
> > >
> > > Does anyone (having accepted the assumptions above) have a different
> > > opinion? In particular, are there any EMI reasons that would favor one
> > > value of characteristic impedance over another? (But don't tell me the
> > > assumptions above are wrong! That's not the question!)
> > >
> > > Doug Brooks
> > > .
> > > ************************************************************
> > > See our updated message re in-house seminars on our web page
> > > .
> > > Doug Brooks, President doug@eskimo.com
> > > UltraCAD Design, Inc. http://www.ultracad.com
> > >
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