Digital Signal Processing Reference
In-Depth Information
13.2.1 How Is a Differential Pair Observed?
A probe with a ground clip is used to make single-ended oscilloscope measurements
in the laboratory. The ground clip sets the reference for the measurement by specify-
ing the location where zero volts is defined. This is also how SPICE simulations are
customarily observed, since, unless told otherwise, SPICE calculates and displays its
results with respect to node 0, the system ground.
In contrast, a differential probe is used to measure the differential signal. The
displayed signal is truly differential: The difference between
RI
+ and
RI
appearing
in Figure 13.1 is measured rather than its value relative to ground. An example of
a differential signal is shown in Figure 13.2.
We see that each of the signals swing between 0V and 1V with respect to
ground, but the difference signal swings from
−
1V.
Because an oscilloscope only allows a few transitions at a time to be displayed,
it is difficult to observe the interaction between data bits occurring at significantly
different places in a long stream of data. Setting the oscilloscope time base to si-
multaneously display all the bits does not provide the resolution necessary to dis-
tinguish a single pulse. Rather than an ordinary oscilloscope, a device that captures
and then overlays successive pulses (bits) is used. One example of a device that can
do this is the digital signal analyzer such as the one shown in Figure 13.3.
+
1V to
−
13.2.2 What Is an Eye Diagram?
A digital signal analyzer creates an
eye diagram
[8-12] such as that shown in Figure
13.4 by capturing a data stream and overlying successive pulses. Ideally, all of the
rising edges should exactly overlay, as should all of the falling edges. The space in
between should be totally clear (void of any transitions). The
data mask
defines the
minimum acceptable width and height of this opening and is a specification that the
link must meet for proper operation.
1V
0V
−
1V
(a)
1.0V
DO
−
0.5V
DO+
0.0V
(b)
Figure 13.2
(a) The differential signal (
DO
+) - (
DO
−
) swings between
±
1V even though (b) when
measured respect to ground the signals switch from 0 to 1V.
