60 Chapter2
Making Measurements
Example 3: Modulation
Example 3: Modulation
What Is Modulation?
Modulation is the act of translating some low frequency or baseband
signal (voice, music, data) to a higher frequency. In the modulation
process, some characteristic of a carrier signal (usually amplitude or
frequency) is changed in direct proportion to the instantaneous
amplitude of the baseband signal.
More information about amplitude and frequency modulation can be
found in Agilent Technologies Application Note 150-1, literature
number 5954-9130.
Spectrum Analyzer Functions Used
The following procedure describes how to measure signals with AM and
FM types of modulation on them. It shows how to tune the signal onto
the spectrum analyzer display. It demonstrates using the frequency
count function and several marker functions to make measurements.
Stepping Through Modulation Measurements
Amplitude Modulation
1. Connect the signal source to the spectrum analyzer INPUT 50Ω.
2. Set the source to a carrier frequency of 100 MHz with amplitude
modulation of about 34 kHz.
3. Set the spectrum analyzer center frequency to 100 MHz. Set the
span to 500 kHz.
4. Figure 2-7 on page 61 illustrates the amplitude-modulated signal as
seen on the spectrum analyzer display. Note the carrier signal. To
determine its frequency, simply press
PEAK SEARCH.
5. Additional modulation information can be easily determined from
the carrier signal and a sideband. For example, the difference
between the carrier frequency and the sideband frequency can be
found by pressing
PEAK SEARCH, MARKER DELTA, and NEXT PEAK.
The markers read the frequency difference between the two signals,
which is equal to the modulating frequency. The marker also reads
the difference in amplitude. This difference in amplitude between
the two signals can be used to determine percentage of modulation
(refer to Figure 2-8 on page 61).
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