Specifications

94 Chapter 11
Demodulating AM and FM Signals
Measuring the Modulation Index of an AM Signal
Demodulating AM and FM Signals
Measuring the Modulation Index of an AM
Signal
This procedure demonstrates how to use the spectrum analyzer as a
fixed-tuned (time-domain) receiver to measure the modulation index as
a percent AM value of an AM signal. 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).
Step 1. Follow steps 1 through 7 of the procedure “Measuring the Modulation
Rate of an AM Signal” on page 92.
Step 2. Turn off all markers and place the analyzer in free run trigger mode:
Press
Marker, Off.
Press
Trig, Free Run.
Step 3. Increase the sweep time and decrease the VBW so that the waveform is
displayed as a flat horizontal signal:
Press
Sweep, Sweep Time, 5, s.
Press
BW/Avg, Video BW, 30, Hz.
Step 4. Center the flat waveform at the mid-point of the y-axis and then widen
the VBW and decrease the sweep time to display the waveform as a sine
wave:
Press
AMPLITUDE Y Scale, Ref Level.
Press
BW/Avg, Video BW, 100, kHz.
Press
Sweep, Sweep Time, 5, ms.
Step 5. Measure the modulation index of the AM signal:
To measure the modulation index as % AM, read the trace as follows
(see Figure 11-2 for display examples): 100% AM extends from the top
graticule down to the bottom graticule. 80% AM (as in this example) is
when the top of the signal is at 1 division below the top graticule and 1
division above the bottom graticule. To determine % AM of your signal
count each y-axis division as 10%.