Page 190 - Fundamentals of Communications Systems

6.12 Chapter Six

2 1
0.8
1.5 0.6
G m (f)
1 0.4
0.2
0.5
0
0 −30 −20 −10 0 10 20 30
0.25
−0.5 0.2
G x c (f) 0.1
−1 0.15
−1.5
0.05
−2 0
0.081 0.083 0.085 0.087 0.089 0.091 −30 −20 −10 0 10 20 30
Time, t, sec Normalized Frequency, f/f m
(a) Time signal (b) Energy spectrum
Figure 6.12 LC-AM with m(t) = sin(2π f m t).

shows the modulated signal for large values m(t) and envelope comes close to zero. The
energy spectrum of the signal is shown in Figure 6.13(b). Note, the bandwidth of the
carrier modulated signal is 5 kHz and the large carrier is evident in the plot.

6.2.1 Modulator and Demodulator
The modulator for LC-AM is very simple. It is just one arm (the in-phase
one) of a quadrature modulator where a DC term has been added to the mes-
sage signal. Figure 6.14 shows a common implementation of this modulator.

5 −20
4
3 −30
2 −40
1
0 Energy Spectrum, G x c (f) −50
−1
−2 −60
−3
−70
−4
−5 −80
0.163 0.165 0.168 0.170 0.172 −1 −0.8 −0.6 −0.4 −0.2 0 0.2 0.4 0.6 0.8 1 4
Time, t, sec Frequency, f, Hz × 10
(a) The time domain signals (b) The bandpass energy spectrum
Figure 6.13 The resulting LC-AM signal for a computer generated voice signal. f c = 7 kHz.