From: Chris Carlen
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Subject: Re: Who thinks this?
References: <3DC1D056.firstname.lastname@example.org> <3DC1F17C.D3149AB3@SpamMeSenseless.us.ibm.com>
Date: Fri, 01 Nov 2002 05:38:11 GMT
NNTP-Posting-Date: Thu, 31 Oct 2002 21:38:11 PST
Phil Hobbs wrote:
> I'll leave the LC transmission line questions to others, and just make
> an observation about really fast interconnections: At very high speeds
> (2 Gb/s or higher), transmission lines on circuit boards really do look
> like distributed RC circuits, e.g. they slow down quadratically with
> distance. The reason is basically copper losses and dielectric
> absorption in common materials such as FR4 board. To get fast signals
> to go any distance on a backplane, you have to do all sorts of ugly
> preemphasis tricks to preserve bandwidth. In many instances, e.g. the
> IBM Regatta servers, fast logic signals are demultiplexed down into
> several lines each to cross the board or backplane, then multiplexed
> back up into the original logic line. Ugly but necessary--this isn't
> your dad's computer.
> One reason computer companies (such as my employer) are looking at using
> optical interconnections is that the bandwidth can be preserved over
> much longer distances.
Interesting. That indicates a case that is far from ideal, unlike what
I am conjecturing about.
I'd like to master the totally general simulation of transmission line
wave propagation, using the transfer function the whole way through.
But at this point I only know how to do steady state AC, and step/pulse
propagation with real reflection coefficients and Zo.
There's a lot to learn to really understand this subject. It's really a
case where if the mathematical tools aren't available, one will be stuck
in rules of thumb and what the other guy told me.
Christopher R. Carlen
Suse 7.3 Linux 2.4.10