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PHYSICAL DESIGN AND DEBUGGING
11.4.2 Impedance Mismatch
As mentioned earlier, the impedance of a transmission line
is
a function of the geometry
of the line, its distance from the ground plane, and the loads along the line. Any discon-
tinuity in the impedance
will
cause reflections.
Impedance mismatch occurs between the transmission line characteristic impedance and
the input or output impedances of the devices that are connected to the line. The result
is
that the signals are reflected back and forth on the line. These reflections can atten-
uate or reinforce the signal depending upon the phase relationships. The results of these
reflections include overshoot, undershoot, ringing and. other undesirable effects.
At
slower edge rates, the effects of these reflections are not severe. However at faster
rates, the rise time of the signal
is
short with respect to the propagation delay. Thus it
can cause problems
as
shown in Figure
11-9.
Overshoot occurs when the voltage level exceeds the maximum (upper) limit of the
output voltage, while undershoot occurs when the level exceeds the minimum (lower)
limit. These conditions can cause excess current on the input gates which results in
permanent damage to the device.
The amount of reflection voltage can be easily estimated. Figure 11-10 shows a system
exhibiting reflections.
..
...
~
o
>
Expected Output
Signal
Undershoot
~~---T-ime---~--~~~={)
Figure 11-9. Overshoot and UndershOot Effects
11-12
231732i11-9