page 7.1
CHAPTER 7
AMPLITUDE MODULATION
Transmit information-b earing message or baseband signal voice-music
through a Communications Channel
Baseband = band of frequencies representing the original signal
for music 20 Hz - 20,000 Hz, for voice 300 - 3,400 Hz
write the baseband message signal mt $ M f
Communications Channel
Typical radio frequencies 10 KHz ! 300 GHz
write ct= A cos2f ct
c
ct = Radio Frequency Carrier Wave
A = Carrier Amplitude
c
fc = Carrier Frequency
Amplitude Mo dulation AM
! Amplitude of carrier wavevaries a mean value in step with the baseband signal mt
st= A [1 + k mt] cos 2f t
c a c
Mean value A .
c 31
page 7.2
Recall a general signal
st= at cos[2f t + t]
c
For AM
at = A [1 + k mt]
c a
t = 0 or constant
k = Amplitude Sensitivity
a
Note 1 jk mtj < 1or [1 + k mt] > 0
a a
2 f w = bandwidth of mt
c 32
page 7.3
AM Signal In Time and Frequency Domain
st = A [1 + k mt] cos 2f t
c a c
j 2f t j 2f t
c c
e + e
st = A [1 + k mt]
c a
2
A A
c c
j 2f t j 2f t
c c
e + e st =
2 2
A k
c a
j 2f t
c
+ mte
2
A k
c a