Multimedia University: ETM2046 Analog & Digital Communications
Tutorial 2
AM
1. An audio signal which mathematical description is 25cos ( 2π 1000t ) modulates a carrier
signal described as 75cos ( 2π 150000t ) .
(a) Sketch the time domain modulated waveform and its one-sided frequency spectrum if
the modulated waveform is a DSB-LC (conventional AM or Full AM) modulated
signal.
(b) Calculate the modulation index and the percentage of modulation.
(c) Sketch the one-sided frequency spectrum if the modulated waveform is a DSB-SC
signal instead.
2. A 90 kHz bandwidth is to accommodate 6 Full AM stations simultaneously. What
maximum modulating frequency, fm, must each station be limited to? Assume that the
maximum modulating frequency is the same for all the 6 stations.
3. An antenna transmits a DSB-LC signal having a total power content of 15 kW. Determine
the power transmitted at the carrier frequency and at each of the sidebands when the
percent modulation is 85% (modulation index is 0.85).
4. The demodulation of double sideband suppressed carrier (DSB-SC) signal with carrier
frequency fc is shown in Figure 1. The message signal m(t) is bandlimited to fm as shown
in Figure 2. The DSB-SC signal is given by s DSB (t ) = m(t ) cos 2πf c t .
s DSB (t ) LPF r(t)
× v(t)
fcut
c(t ) = cos 2πf c t
Figure 1. DSB-SC demodulation.
|M(f)|
M(0)
-fM 0 fm f
Figure 2. Spectrum of m(t), |M(f)|.
(i) Draw the double-sided spectrum of s DSB (t ) and v(t).
(ii) What is the range of values for fcut so that the modulating signal, m(t), can be
recovered completely?
Chang YC (2010)
Multimedia University: ETM2046 Analog & Digital Communications
(iii) If the multiplying local carrier is c(t ) = cos(2πf c t + θ ) , find the recovered signal
r(t).
5. An SSB transmission contains 800W. This transmission is to be replaced by a Full AM
signal with the same power content. Determine the power content of the carrier and each
of the sidebands when the percent modulation is 85%. From the result above, state one
advantage of SSB over Full AM.
Chang YC (2010)
