Page 89 - Complete Wireless Design

Modulation

88 Chapter Two

its entire bandwidth, and then summing them together. However, this is not
necessary, as you will see below.
To precisely measure a digital signal’s power, the bandwidth must first be
found. In order to measure the power as accurately as possible, the 30-dB-
down points will be used instead of the normal 3-dB-down points normally
adopted to indicate a signal’s bandwidth (Fig. 2.42). This will allow for most of
the digital signal’s power to be measured within its entire communication
channel; any power below the 30-dB points can be discounted, and will gen-
erally be quite near the noise floor.
After we have obtained the signal’s true bandwidth, we can then go about
accurately measuring its average power level. Most quality spectrum analyz-
ers have the capability to make digital power measurements. The procedure is

1. Find the power menu on your model spectrum analyzer.
2. Select digital.
3. Input the digital channel’s center frequency.
4. Input the digital channel’s 30 dB bandwidth.
5. The digital signal’s true power will now be indicated on the spectrum ana-
lyzer’s screen.
Another, slightly less accurate, technique is utilized by spectrum analyzers
that do not possess the above automatic power measurement capabilities:
1. Measure the digital signal’s bandwidth at the 30-dB-down points.
2. Adjust the analyzer’s RBW setting to 1/20 of the signal’s 30 dB bandwidth.

Figure 2.42 A digital signal’s bandwidth for an average power measurement.

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