Specifications

92 Chapter 11
Demodulating AM and FM Signals
Measuring the Modulation Rate of an AM Signal
Demodulating AM and FM Signals
Measuring the Modulation Rate of an AM
Signal
This section demonstrates how to determine parameters of an AM
signal, such as modulation rate and modulation index (depth) by using
frequency and time domain measurements (see the concepts chapter
“AM and FM Demodulation Concepts” on page 157 for more
information). Using the ESA built-in AM demodulator, you can also
tune-in and listen to an AM signal (“Demodulating an AM Signal Using
the ESA Series” on page 96).
To obtain an AM signal, you can either connect a source transmitting
an AM signal, or connect an antenna to the analyzer input and tune to a
commercial AM broadcast station. For this demonstration an RF source
is used to emulate an AM signal.
Step 1. Connect an Agilent ESG RF signal source to the analyzer INPUT. Set
the ESG frequency to 300 MHz and the amplitude to
−10 dBm. Set the
AM depth to 80%, the AM rate to 1 kHz and turn AM on.
Step 2. Preset the analyzer and then set the center frequency, span, RBW and
the sweep time:
Press
Preset, Factory Preset (if present).
Press
FREQUENCY Channel, Center Freq, 300, MHz.
Press
SPAN X Scale, Span, 500, kHz.
Press
BW/Avg, Res BW, 30, kHz.
Press
Sweep, Sweep Time, 20, ms.
Step 3. Set the y-axis units to volts:
Press
AMPLITUDE Y Scale, More, Y-Axis Units, Volts.
Step 4. Position the signal peak near the reference level:
Press
AMPLITUDE Y Scale, Ref Level, (rotate front-panel knob).
Step 5. Change the y-scale type to linear:
Press
AMPLITUDE Y Scale, Scale Type (Lin).
Step 6. Set the analyzer in zero span to make time-domain measurements:
Press
SPAN X Scale, Zero Span.
Press
Sweep, Sweep Time, 5, ms.
Step 7. Use the video trigger to stabilize the trace:
Press
Trig, Video.
Since the modulation is a steady tone, you can use video trigger to
trigger the analyzer sweep on the waveform and stabilize the trace,
much like an oscilloscope. See Figure 11-1.