Keysight N9020A - Page 145

145
Demodulating AM Signals
Measuring the Modulation Index of an AM Signal
6 Set the y-axis units to
volts.
• Press AMPTD Y Scale,
More, Y-Axis Units, V
(Volts).
7 Position the signal peak
near the reference level.
• Press AMPTD Y Scale, Ref
Level, (rotate front-panel
knob).
8 Change the y-scale type
to linear.
• Press AMPTD Y Scale,
Scale Type (Lin).
9 Set the analyzer in zero
span to make
time-domain
measurements.
a. Press SPAN X Scale, Zero
Span.
b. Press Sweep/Control,
Sweep Time, 5, ms
10Place the analyzer in
free run trigger mode.
• Press Trigger, Free Run.
11 Increase the sweep time
and decrease the VBW.
a. Press Sweep/Control,
Sweep Time, 5, s.
b. Press BW, Video BW, 30,
Hz.
The waveform is displayed as a flat
horizontal signal.
12Center 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.
a. Press AMPTD Y Scale, Ref
Level, (rotate front-panel
knob).
b. Press BW, Video BW,
100,
kHz.
c. Press Sweep/Control,
Sweep Time, 5, ms.
13Measure the modulation
index of the AM signal.
To measure the modulation index as % AM, read the trace as follows (see
Figure 10-3 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%.
Step Action Notes