Newsgroups: sci.electronics
Path: utzoo!henry
From: henry@utzoo.uucp (Henry Spencer)
Subject: Re: Transmission Lines - What Are They??
Message-ID: <1989Nov12.013850.7756@utzoo.uucp>
Organization: U of Toronto Zoology
References: <868@encad.Wichita.NCR.COM>
Date: Sun, 12 Nov 89 01:38:50 GMT

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?"

The changeover happens when the signals being sent into the line get fast
enough to change in times roughly comparable to the propagation delay of
the line.  Consider a pair of wires with a 5kohm resistor on one end and a
switched +5V source on the other.  If the source switches on relatively
slowly, the circuit's behavior is very simple:  1mA of current flows.
Now, suppose the propagation delay is 10ns and the risetime of the
source is 1ns.  How much current flows 2ns after the source switches
on?  The source cannot "see" the resistor, because it is 10ns away and
there hasn't been time for a signal to get there and back.  It turns
out that the current is determined by the "characteristic impedance"
of the wire pair, which in turn is determined by things like wire size,
spacing, insulation, etc., plus some effect from nearby objects.  The
characteristic impedance is often rather less than 5kohm.  So "too much"
current is flowing.  When the current reaches the resistor, the voltage
produced across it will be more than 5V.  The excess voltage propagates
back toward the source.  When it arrives, the source will reduce its
output current due to the "back pressure", generally reducing it too
far.  When the reduced signal arrives at the resistor, the back pressure
will be reduced, and so forth.  The changes in voltage, and corresponding
changes in current, bounce back and forth until everything converges on
the 1mA steady state.

This happens even for slow signals, but when the risetime of the signal
is slow compared to the delays, the reflections are trivial disturbances
in the slow rise.  For fast signals, the reflections are conspicuous, and
they can confuse listening circuits.  For example, flip-flops that trigger
on rising edges may trigger more than once if reflections are strong.

The fix, in this example, is to adjust the resistor so that its resistance
equals the characteristic impedance of the line.  Then the right amount of
current flows from the start, and there are no reflections.  Unfortunately,
this happy situation can be arbitrarily hard to achieve in complicated real
circuits, and one generally has to settle for approximations.

Warning:  the above is oversimplified.  The reality is even worse.
-- 
A bit of tolerance is worth a  |     Henry Spencer at U of Toronto Zoology
megabyte of flaming.           | uunet!attcan!utzoo!henry henry@zoo.toronto.edu