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From: mark@mips.COM (Mark G. Johnson)
Newsgroups: sci.electronics
Subject: Re: Transmission Lines - What Are They??
Message-ID: <31219@obiwan.mips.COM>
Date: 11 Nov 89 17:09:27 GMT
References: <868@encad.Wichita.NCR.COM>
Lines: 63

In article <868@encad.Wichita.NCR.COM> dkukral@encad.Wichita.NCR.COM (Dean Kukral) writes:
   >Advertisement: "At high speeds interconnections take on the 
   >characteristics of transmission lines and ..."
   >
   >What does this mean?  What is a transmission line?  What makes
   >an "interconnection" into a "transmission line?"

Transmission lines are physical structures which have certain properties
that allow the propagation of electromagnetic waves.  They are discussed
at length in college sophomore courses on "Electromagnetics", where they
are analyzed using Maxwell's equations.

The standard example of a transmission line is a coaxial cable; solving
the applicable Maxwell's equations is especially easy here because the
fields are contained in a small, geometrically simple (cylindrically
symmetric) region.

"Schaum's Outline Series" (a bunch of paperbacks that are sold at
bookstores as study aids and textbook supplements) has a great, inexpensive
book.  It's _Electromagnetics_, by Joseph A. Edminister.   They also
sell another one called _Transmission Lines_.

Another good source of introductory information is the "Motorola MECL
System Design Handbook".  It's one of Moto's databooks and they'll
give it to you free.
. . . . . . . . . . . . . . . .

One way to think about transmission lines is in the frequency domain.
A real-life cable or PCboard (trace + groundplane) or other pair of
electrical conductors has some physical, nonzero length.  If a signal
is injected into one end of that conductor pair it takes nonzero time
to propagate to the other end (velocity is <= speed of light in vacuum).

If the signal contains components of high enough frequency, then
the wavelength of the signal can be shorter than the length of the
conductor pair.  Now the conductor pair must be considered a waveguide
complete with reflections, propagation velocity, etc.

Note that this is only true for long conductors or very high frequency
signals, or both.  That's why transmission line effects are negligible
in opamp circuits etc.

Also note that a square wave can contain VERY high frequency components,
determined essentially by the risetime and falltime of its edges.  Doctor
Fourier tells us that a perfect squarewave, one with zero risetime and
zero falltime, has a frequency spectrum that extends to infinity.

However, real square waves have finite risetimes, thank goodness.  For
example a 74LS04 drives from 0.2 to 3.2 volts in about 15 nanoseconds.
The 15ns edge rate is low enough that transmission line effects are
negligible for wires less than about 7 feet long.

The newer digital circuits like the blazing fast ECL ones, have edge
rates of about 1ns.  This means that with ECL you have to consider
your "wires" ("interconnects") to be waveguides, reflections and all,
if they are more than 4 inches long.

And the newest CMOS like the FACT and 74AC series devices, have fast
edges too.  So beware.
-- 
 -- Mark Johnson	
 	MIPS Computer Systems, 930 E. Arques, Sunnyvale, CA 94086
	(408) 991-0208    mark@mips.com  {or ...!decwrl!mips!mark}