Page 90 - Complete Wireless Design

Modulation

Modulation 89

3. Set the analyzer’s SPAN to 1.25 times the signal’s 30 dB bandwidth.
4. Decrease the analyzer’s video bandwidth (VBW) setting to reduce the sig-
nal’s displayed noise.
5. Take the digital signal’s power measurement with the spectrum analyzer’s
frequency/amplitude MARKER placed at the center of the signal.
6. Now, find the true total average power of the digital signal by taking the
power in dBm (as measured in step 5), and adding a bandwidth correction
factor (BWCF):

signal BW 30 dB
BWCF 10 log
RBW
To find the true digital output power of the signal, calculate total digital
signal power (dBm) measured power in dBm (from step 5) BWCF
7. For a more accurate digital signal power measurement you can add another
correction factor that takes into account the internal RBW and log detection
stage losses, inherent in any spectrum analyzer, of approximately 2 dB. The
formula for digital power measurement now becomes:

True digital power (dBm) measured power (dBm) BWCF (dB) 2 dB

Constellation and eye diagrams. To view the degradation created by noise and
frequency instabilities in a digital signal, as well as other impairments, we can
employ constellation and eye diagrams.
To measure or view constellation or eye diagrams requires the ability to tap
into the digital receiver demodulator’s I and Q outputs, as well as the demod-
ulator’s timing clock (Fig. 2.43). The outputs of the demodulator’s I and Q
may be fed into an oscilloscope with an X-Y display that has the capability to
turn on a persistence function for a view of the I/Q outputs over time. Such a
setup will allow the operator to confirm the phase and amplitude differences
of the output signal—in the form of a constellation diagram—thus allowing
the viewing of the signal’s quality (lack of distortion, phase noise, or ampli-
tude instabilities). A perfect constellation diagram with no impairments is
shown in Fig. 2.44a.
Constellation diagrams display the digital modulation’s symbol patterns,
while eye diagrams (Fig. 2.44b) permit the transition of the symbols to be
viewed over time. Both measure the baseband signal’s modulation condition,
and whether impairments are degrading this expected pattern. In eye dia-
grams the eye itself is rounded, instead of square, because of the necessary
limiting of the baseband bandwidth by filters. The eye comprises two lines, one
at digital 1 and the other at digital 0, and is only a series of pulses displayed
on the phosphor of the test oscilloscope, with each pulse being sent out of the
receiver’s demodulator with noise and jitter added by the transmitter, the sig-
nal path, and the receiver. This makes each pulse slightly different from the

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