Digital Signal Processing Reference
In-Depth Information
As shown in Figure 8.1, the attenuation of the high-frequency components
cause the received pulse to become rounded and reduced in amplitude, and the
base of the pulse smears and becomes wider than the base of the transmitted pulse.
The progressive distortion of a 1-V, 400-ps pulse stream as it progresses down
a 1-meter (39-inch)-long transmission line is illustrated in Figure 8.2. The effect on
the pulse is evident when compared to the undistorted pulse. In fact, the pulse base
has become wide enough to collide with the rising edge of the succeeding pulse.
This effect (dispersion) can severely distort a pulse stream, which is of particular
concern in serial signaling. For instance, dispersion severe enough to cause a pair
of back-to-back pulses (such as a 0110 pattern) to interfere with one another may
not be great enough to cause significant interference when the pulses are widely
separated (for example, a 1001 pattern). This leads to “data-dependent” signaling
errors.
8.2.2 How Is Loss Specifi ed?
In signal integrity work, loss is defined as the ratio of the received voltage to the
transmitted voltage. As shown in (8.1), the ratio is given in terms of the common
logarithm and has units of decibels (dB).
⎛
V
⎞
Gain in dB
=×
20
log
received
(8.1)
⎜
⎟
10
⎝
V
⎠
transmitted
When calculating loss, decibels are used rather than a simple percent because it
is only necessary to add together the losses in decibels of the various parts to find
the total loss. As we will see, microstrip and stripline losses come in two parts, and
the total loss in dB is simply the sum of the two.
Input Pulse
1.0V
1m (39 inches)
0.8V
0.6V
0.4V
0.2V
0.0V
0 ns
5 ns
10 ns
15 ns
20 ns
Figure 8.1
Effects of loss created by a 1-meter (39-inch)-long microstrip on a sharp edged, 5-nS-
wide pulse (dotted curve). The pulse exiting the line (solid curve) is smaller, rounded, and wider at
its base.
