B. P. Lathi, Zhi Ding - Modern Digital and Analog Communication Systems-Oxford University Press (2009)

198 AMPLITUDE MODULATIONS AND DEMODULATIONS
Figure P.4.2-8
b
C
a
5000 J -+ 2 cos 40,000nt
2 cos 20,000m
A slightly modified version of this scrambler was first used commercially on the 25-mile radiotelephone
circuit connecting Los Angeles and Santa Catalina island.
Figure P.4.2-9
M(f)
Low-pass
filter
0-15 kHz
y(t)
---- (Scrambled output)
2 cos 30,000nt
4.2-10 A DSB-SC signal is given by m(t) cos (2rr) 10 6 t. The carrier frequency of this signal, 1 MHz,
is to be changed to 400 kHz. The only equipment available consists of one ring modulator,
a bandpass filter centered at the frequency of 400 kHz, and one sine wave generator whose
frequency can be varied from 150 to 210 kHz. Show how you can obtain the desired signal
cm(t) cos (2rr x 400 x to 3 t) from m(t) cos (2rr)10 6 t. Determine the value of c.
4.3-1 Figure P4.3-l shows a scheme for coherent (synchronous) demodulation. Show that this scheme
can demodulate the AM signal [A + m(t)] cos (2rrfct) regardless of the value of A>
Figure P.4.3-1
[A + m(t)] cos w c f
Lowpass ,__.....,
0 _
ut _
put
filter
4.3-2 Sketch the AM signal [A + m(t)] cos (2rrfct) for the periodic triangle signal m(t) shown in
Fig. P4.3-2 corresponding to the modulation indices (a) µ, = 0.5; (b) µ, = l; (c) µ, = 2; (d)
µ, = oo. How do you interpret the case of µ, = oo?
Figure P.4.3-2
10
-10 , t-