From: Phil Hobbs
Subject: Re: Who thinks this?
Date: Thu, 31 Oct 2002 22:14:05 -0500
Organization: IBM Global Services North -- Burlington, Vermont, USA
NNTP-Posting-Date: 1 Nov 2002 03:14:09 GMT
X-Mailer: Mozilla 4.61 [en] (OS/2; U)
"Christopher R. Carlen" wrote:
> I find many people have the following conception:
> A transmission line is like a capacitor. If you have a long line, and
> feed it a digital logic transition, the output signal at the other end
> will look like a RC exponential response. The slowness of that response
> will be related, for the most part proportionally, to the length of the
> cable and the output impedance of the driver.
> Furthermore, that putting a resistor in series with the output of a
> driver, and the input of the cable, will somehow slow the response even
> more, which seems logical if you think the line is capacitive.
> Of course all of these notions are very incorrect, assuming one is
> talking about an almost ideal line with a purely real complex
> propagation constant, and thus a purely real characteristic impedance,
> and a non-dispersing, non-distorting line, such as typical controlled
> impedance cables that we use every day.
> What do you think?
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.
IBM T. J. Watson Research Center