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PHYSICAL DESIGN AND DEBUGGING
The following sections discuss the negative effects of a transmission line that occur when
operating at higher frequencies. In higher frequency design the reflection and cross talk
effects are inevitable;
it
is
impossible to design optimum systems without accounting for
these effects.
Later sections include a discussion
of
techniques that can minimize these
effects.
.
11.4.1 Transmission Line Effects
As a general rule, any interconnection
is
considered to be a transmission line when the
time required for the signal to travel the length of the interconnection
is
greater than
one-eighth of the signal rise time (True K.M.,
"Reflection: Computations and Waveforms,
The Interface Handbook, " Fairchild Corp., Mountain View, CA.,
1975,
Ch.
3). The rise
time can be either rise time or fall time, whichever
is
smaller, and it corresponds to the
linear ramp amplitude from
0% to 100%. Normally the rise times are specified between
10% to
90% or 20% to 80% amplitude points. The respective values are multiplied
by
1.25
or 1.67 to obtain the linear-ramp duration from
0%
to 100% amplitude.
For example in a
PCB using G-lO and polymide (the two main dielectric systems avail-
able for printed circuit boards) signals travel
. at approximately 5 to 6 inches per
nanosecond (ns).
If
trv/l < 8 then the signal path
is
not a transmission line but it
is
a lumped element
(True K.M.,
"Reflection: Computations and Waveforms, The Interface Handbook," Fair-
child Corp., Mountain View, CA.,
1975,
Ch. 3).
where
tr
= rise time 0%-100%
v = speed
of
propagation
(5
to 6 inches/sec)
I
= length of interconnection (one-way only)
The calculation
is
given
by:
6t,/1
::;;
8 so I
~
6trl8 =
(6x4X
1.67)/8 =
5.01
inches
This calculation
is
based on the fact that the maximum rise time of the signals for the
Intel386 DX processor
is
4 ns. For I > =
5.01
inches, interconnections
will
act
as
trans-
mission lines.
Every conductor that carries an AC signal and acts
as
a transmission line has a distrib-
uted resistance, an inductance and a capacitance which combine to produce the charac-
teristic impedance
(ZO).
The value
of
ZO
depends upon physical attributes such
as
cross-
sectional area, the distance between the conductors and other ground
or
signal
conductors, and the dielectric constant of the material between them. Because the char-
acteristic impedance
is
reactive, its effect increases with frequency.
11-9