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– 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 DSSC is
y(t)=d•m(t)•sin(ω
c
In DSB-SC, the carrier signal is inverted whenever m(t) < 0. For QAM, the second carrier signal would be cos(ω
making it 90 degrees out of phase from the first carrier.
Frequency Modulation (FM)
Frequency modulation varies a carrier signal's frequency according to the modulating signal:
y(t)=A
•sin[(ω
+d•m(t) )•t]
c
c
where m(t) is the modulating signal and d is the frequency deviation. FM is called narrowband if the deviation is less
than 1% of the modulating signal's bandwidth, and wideband otherwise. You can approximate the modulated
signal's bandwidth with the following equations.
BW ≈ 2•(Modulating Signal Bandwidth) for narrowband FM
BW ≈ 2•(Deviation+Modulating Signal Bandwidth) for wideband FM
The top trace below represents the modulating signal; the bottom trace represents the modulated carrier.
Keysight EDU33210 Series User's Guide
t)
t),
c
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