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Chapter 5 Tutorial
Modulation
5
Modulation
Amplitude Modulation (AM) The 33500 Series implements two forms
of AM. The first form is double-sideband full-carrier (DSB-FC). DSB-FC
has an ITU designation of A3E and is the type used in AM broadcasting.
The second form is double-sideband suppressed-carrier (DSSC). Many
modern communications systems employ DSSC on each of two carriers
that have the same frequency but a 90-degree phase difference. This is
called quadrature amplitude modulation (QAM).
The equation for DSB-FC is
y(t)= [(1/2)+(1/2)·d·m(t)]·A
c
·sin(ω
c
t)
where
• m(t) is the modulating signal
•A
c
is the carrier amplitude
• ω
c
is the carrier frequency of the carrier
• d is the “modulation depth,” or fraction of the amplitude range is used
by the modulation
For example, a depth setting of 80% varies the amplitude from 10% to
90% of the amplitude setting (90% - 10% = 80%) with either an internal
or a full-scale (±5-Volt) external modulating signal. You may set depth
as high as 120%, as long as you do not exceed the instrument’s maximum
output voltage of (±5 V into 50 Ω, ±10 V into high impedance).
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