Page 92 - Complete Wireless Design

Modulation

Modulation 91

Figure 2.44 (a) Constellation diagram; (b) eye diagram.

last and, as the received signal is degraded, the eye will begin to close—and
the BER will increase.
To view a constellation diagram, attach the I to the X input of the scope and
the Q to the Y input (which will normally be channel 1 and channel 2, respec-
tively), and turn on the X-Y mode and the display’s persistence function. Now
attach the symbol clock to the external trigger (turn on EXT TRIGGER). The
constellation should now be visible, along with faint lines joining the various
points. The lines are the actual symbol transitions between the constellation
points, called the symbol trajectories. To obtain just the constellation points
(without these symbol transition lines), a BNC connector on the back plate of
some oscilloscopes can be connected to the demodulator’s symbol clock so that
the oscilloscope’s electron beam will be turned on only at the moment of sam-
pling. This will blank the transitions between constellation dots by triggering
the beam only at the constellation point instants.
Common impairments, and how they look in a constellation diagram, are
shown in Fig. 2.45.
To view an eye diagram, keep the same setup as above, but turn off the
scope’s X-Y function. Set its HORIZONTAL TIMEBASE to obtain 3 symbol
times per 10 divisions, and then view the eye diagram. The eye should be open;
in fact, the height of the eye can be considered the noise margin of the receiv-
er’s output, while the left and right corner of each eye indicate the amount of
frequency jitter present. The wider the eye, the less jitter, while the taller the
eye, the less noise.

BER tests. The bit error rate test measures the ratio of bad bits to all the trans-
mitted bits, frequently over a complete range of input/output powers. When

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