通信原理第三章3 Analog modulation system

where m’(t) is the A.C. component in m(t), and
m '( t ) 1
The output of the modulator is called the amplitude modulation signal.
There are discrete carrier components in the frequency spectral density of an AM signal.

r '( t ) H (f )
baseband signal m(t)
co s 0t
The input of the detector s(t)
The local carrier is
s ( t ) m '( t ) c o s 0 t
c ( t ) c o s [( 0 ) t ]
0 m 0 0
0
m
H ( f f0 ) H ( f f0 ) C
f fm
Contents
3.1 Introduction 3.2 Linear Modulation
3.3 Nonlinear Modulation
3.4 Brief Summary
3.3 Nonlinear Modulation
The output of the modulator is called the Doublesideband suppressed carrier AM.
There is no discrete carrier components in the frequency spectral density of a DSB signal.
PM
Contents
3.1 Introduction 3.2 Linear Modulation
3.3 Nonlinear Modulation
3.4 Brief Summary
3.2 Linear Modulation
New words:
carrier
multiplier rectifier
bandpass filter
which refers there are frequency and phase differences between the transmitter carrier and receiver carrier. The output of the product is
r '( t ) m '( t ) c o s 0 t c o s [( 0 ) t ]
New words:
modulating signal anti-jamming modulated signal modulator
linear modulation
nonlinear modulation
amplitude modulated signal
single-sideband signal
Cha.3 Analog Modulation System
12-2012
Peng Yanni
Contents
3.1 Introduction 3.2 Linear Modulation
3.3 Nonlinear Modulation
3.4 Brief Summary
3.1 Introduction
Amplitude demodulation Since the envelope of the AM signal is the same as the shape of modulating signal, envelope detector can be used to demodulate AM signal. Rectifier
3.2.3 Single-sideband Modulation (SSB) SSB modulation Two sidebands contain the same information, so we can only transmit any one of them by suppressing the other one. or S '( f ) S '( f )
S '( f )
f0
0
f
f0
Most part of the power in an AM signal is occupied by the carrier, but the carrier itself doesn’t contain any information of the baseband signal.
Hence, the output of demodulator is
m (t ) 1 2 m '( t ) c o s ( t )
We can make a conclusion that only if the local carrier has no frequency and phase error, the demodulated output signal has no dision
phase modulation frequency deviation
instantaneous frequency (phase)
frequency modulation frequency modulation index
narrowband frequency modulation
broadband frequency modulation
3.3.1 Basic Principles Nonlinear modulation is also called angle modulation, where modulating signal is carried on the phase of the carrier. Angle modulation enables frequency and phase of the carrier to vary with the modulating signal. Assume the carrier: c ( t ) A c o s ( t ) A c o s ( t ) where d ( t ) / d t
A 2
[ M ( f f 0 ) M ( f f 0 )]
S '( f )
0
f
f0
0
f
f0
3.2.1 Amplitude Modulation (AM) Amplitude modulation AM signal s’(t) may be expressed as
s '( t ) [1 m '( t )] A c o s 0 t
Hence, the output of detector is
v 0 ( t ) m '( t ) A n c ( t )
So, the input SNR
ri E {
1 2
[1 m '( t )] A / n ( t )}
2 2 2
And the output SNR
ro E [ m ' ( t ) A / n c ( t ) ]
modulated signal s(t)
carrier c(t)
c ( t ) A c o s ( 0 t 0 )
AM linear modulation Two kinds analog modulation systems DSB-SC SSB VSB FM nonlinear modulation
Upper-sideband
Upper-sideband lower-sideband
f0
S '( f )
0
f
f0
lower-sideband
DSB demodulation We need a coherent carrier in the demodulator, which makes the circuits rather complex. received signal s(t)
The envelope of y(t):
V y (t )
{ [1 m '( t )] A n c ( t )} n ( t )
2 2
s
Under the condition of large SNR,
V y ( t ) [1 m '( t ) ] A n c ( t )
2 2 2
The ratio of input and output SNR is
ro ri
E[
2 m ' (t ) [1 m '( t )]
2
2
]
Since most part of power in signal before detection is occupied by the carrier, which makes no contribution to the useful detected signal, the SNR has decreased after detection.
f0
0
f
f0
f0
0
f
f0
which is called the Single-sideband Modulation.
SSB demodulation We need a coherent carrier in the demodulator, either. received signal s(t)