通信原理英文PPT学习教案.pptx

会计学
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通信原理英文
2
Purpose of modulation
Frequency spectrum movement --- for accommodating the requirement of channel transmission or for combining several signals for multi-channel transmission
Improvement of anti-jamming ability
Classification of analog modulation:
Linear modulation: AM, SSB, DSB, VSB, …
Nonlinear modulation (angular modulation): FM, PM
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(a) Frequency spectral density of input signal
S (f)
f0
f
-f0
0
M(f)
f
0
(b) Frequency spectral density of output signal
Linear modulation
Basic concept
Assume: the carrier is: c(t) = Acos0 t = Acos2 f0t
modulating signal is a energy signal m(t), its spectrum is M(f )
carrier: c(t)
multiplication result: s(t)
filter output: s(t)
“” is used to express Fourier transform:

where,

H(f)
s(t)
Modulated signal
s(t)
Modulating
signal
m(t)
Acos0t
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AM
Basic principles
Let: m(t) = [1+m(t)]， |m(t)|  1，
m(t)|max = m －modulation index, then we have the AM signal
s(t) = [1+m(t)]Acos0t，
where, [1+m(t)]  0, ., the envelope of s(t) is non-negative
+1 =

 =
m(t)
1
0
1+m(t)
1
0
1+m(t)
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Frequency density
Contain discrete carrier component
When m(t) is cosinusoidal, and m＝100％,
sum of the two side band power ＝ half of carrier power



Figure Waveform and spectrum of modulated signal
m(t)
s(t)
M(f)
C(f)
c(t)
A
-A
t
fm
-fm
f0
-f0
2fm
S (f)
2fm
-f0
f0
f
f
f
t
t
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Reception of AM signal: envelope detecton
Principle:
Characteristics: assume the input voltage is
where
is the input noise voltage of the detector
The envelope of y(t):
For large S/N:
Rectifier
Low-pass filter
Figure Envelope detector
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After detection (. component has been filtered):
Signal to noise power ratio of output signal: