chapter 2 continuous-wave modulationcc.ee.ntu.edu.tw/~wujsh/10101pc/chapter2_101.10.16.pdf · 2.7...
TRANSCRIPT
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1
Chapter 2 Continuous-Wave Modulation
2.1 Introduction
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2
2.2 Amplitude Modulation
The output of the modulator
Where m(t) is the baseband signal , ka is the amplitude sensitivity.
frequencycarrier :
amplitudecarrier :
(2.1) )2cos()(
c
c
cc
f
A
tfAtc
(2.2) )2cos()(1)( tftmkAts cac
)( offreqency hightest theis where
(2.4) .2
(2.3) t allfor ,1 )( .1
tmW
Wf
tmk
c
a
X 1+kam(t) S(t)
Accos(2fct)
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3
Recall
1.Negative frequency component of m(t) becomes visible.
2.fc-W M(f) fc lower sideband
fc M(f) fc+W upper sideband
3.Transmission bandwidth BT=2W
(2.2) )2cos()()2cos()( tftmkAtfAts caccc
)( of Transform Fourier theis )( where
(2.5) )()(2
)()(2
)(
)()(2
1)2cos()(
)()(2
1)2cos(
tmfM
ffMffMAk
ffffA
fs
ffMffMtftm
fffftf
ccca
ccc
ccc
ccc
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4
Virtues and Limitations of Amplitude Modulation
Transmitter
Receiver
Major limitations
1.AM is wasteful of power.
2.AM is wasteful of bandwidth.
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2.3 Linear Modulation Schemes
Linear modulation is defined by
Three types of linear modulation:
1.Double sideband-suppressed carrier (DSB-SC) modulation
2.Single sideband (SSB) modulation
3.Vestigial sideband (VSB) modulation
component Quadrature)(
component phase-In)(
(2.7) )2sin()()2cos()()(
ts
ts
tftstftsts
Q
I
cQcI
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Notes:
1.sI(t) is solely dependent on m(t)
2.sQ(t)is a filtered version of m(t).
The spectral modification of s(t) is solely due to sQ(t).
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Double Sideband-Suppressed Carrier (DSB-SC) Modulation
The Fourier transform of S(t) is
(2.8) )2cos()()( tftmAts cc
(2.9) )()(2
1)(
ccc ffMffMAfs
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Coherent Detection (Synchronous Detection)
The product modulator output is
Let V(f) be the Fourier transform of v(t)
(2.10) )()cos('2
1)()4cos('
2
1
)()2cos()2cos('
)()2cos(' )(
tmAAtmtfAA
tmtftfAA
tstfAtv
ccccc
cccc
cc
(2.11) )( cos'2
1)(0 tmAAtv cc
filtered out
(Low pass filtered)
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Costas Receiver
I-channel and Q-channel are coupled together to
form a negative feedback system to maintain synchronization
The phase control signal ceases with modulation.
1
4
2 2 2 2
2 2
0
1 1cos sin ( ) ( )sin(2 )
4 8
( ) (sin2 2 )
c c
c
A m t A m t
A m t
(multiplier +
very narrow band LF)
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Quadrature-Carrier Multiplexing (or QAM)
Two DSB-SC signals occupy the same channel
bandwidth, where pilot signal (tone ) may be
needed.
)2sin()()2cos()()( 21 tftmAtftmAts cccc
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Single-Sideband Modulation (SSB)
The lower sideband and upper sideband of AM signal
contain same information .
The frequency-discrimination method consists of a
product modulator (DSB-SC) and a band-pass filter.
The filter must meet the following requirements:
a.The desired sideband lies inside the passband.
b.The unwanted sideband lies inside the stopband.
c.The transition band is twice the lowest frequency of
the message.
To recover the signal at the receiver, a pilot carrier or a stable oscillator
is needed (Donald Duck effect ).
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Vestigial Sideband Modulation (VSB) When the message contains near DC component
The transition must satisfy
(2.14)
(2.13) for 1)()(
:linear is response phase b.The
1)()(.a
fW B
W fWffHffH
ffHffH
νT
cc
cc
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Consider the negative frequency response: H f
c vf f cf c vf fcf Wc vf f cf
c vf f cf W
Here, the shift response │H(f-fc)│ is cH f f
2 c vf f 2 cf 2 c vf f 2 cf Wvf0vfW
13
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and │H(f+fc)│ is cH f f
vf 0 vf W2 c vf f 2 cf2 c vf f 2 cf W
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So, we get │H(f-fc)│ +│ H(f+fc)│ is
cH f f
2 c vf f 2 cf 2 c vf f 2 cf Wvf0vfW
cH f f
vf 0 vf W2 c vf f 2 cf2 c vf f 2 cf W
15
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Consider –W<f<W we get:
vfvfW 0 W
Which is equal to
W W
So, │H(f-fc)│ + │H(f+fc)│ =1 for -W<f<W
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± corresponds to upper or lower sideband
(2.15) )2sin()('2
1)2cos()(
2
1)( tftmAtftmAts cccc
HQ(f) m(t) m’(t)
(2.16) for )()( )( W f WffHffHjfH ccQ
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Television Signals (NTSC)
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2.4 Frequency Translation
Up conversion
f2=f1+fl , fl=f2-f1
Down conversion
f2=f1-fl , fl=f1-f2
cos(2 )A f t
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2.5 Frequency-Division Multiplexing (FDM)
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2.6 Angle Modulation
Basic Definitions:
Better discrimination against noise and interference
(expense of bandwidth).
The instantaneous frequency is
(2.19) )(cos)( tAts ic
constant is where
(2.22) 2)(
is )( carrier, dunmodulate anFor
(2.21) )(
2
1
2
)()(lim
)(lim)(
0Δ
Δ0Δ
c
cci
i
i
ii
t
tt
i
tft
t
dt
td
t
ttt
tftf
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1. Phase modulation (PM)
2. Frequency Modulation (FM)
(2.23) )(2cos
modulator theofy sensitivit phase :
)(2)(
tmktfAs(t)
k
tmktft
pcc
p
pci
(2.24)
π π
π π (2.26)
:frequency sensitivity of the modulator
compare (2.23) and (2.26)
0
0
( ) ( )
( ) 2 2 ( )
cos 2 2 ( )
i c f
t
i c f
t
c c f
f
p
f t f k m t
t f t k m d
s(t) A f t k m d
k
k m'
π0
2 ( )t
f(t) k m d
(2.25)
generating FM signal generating PM signal
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2.7 Frequency Modulation
FM is a nonlinear modulation process , we can not apply
Fourier transform to have spectral analysis directly.
1.Consider a single-tone modulation which produces a
narrowband FM (kf is small)
2.Next consider a single-tone and wideband FM
(kf is large)
deviationfrequency :Δ
(2.28) )2cos(
)2cos( )(
(2.27) )2cos()(let
mf
mc
mmfci
mm
Akf
tfff
tfAkftf
tfAtm
(deterministic)
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radian. one nlarger tha is , FM Wideband
radian. one ansmaller th is , FM Narrowband
(2.33) )2sin(2cos)(
(2.32) )2sin(2 )(
(2.31) index Modulation
(2.30) )2sin(2
)(2)( (2.25), Recall0
tftfAts
tftπft
f
f
tff
ftπf
dft
mcc
mci
m
m
m
c
t
ii
(2.19) =>
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Narrowband FM
(2.35) )2sin()2sin()2cos()(
)2sin()2sin(sin
1)2sin(cos
small, is Because
)34.2( )2sin(sin)2sin()2sin(cos)2cos(
)2sin(2cos)(
tftfAtfAts
tftf
tf
tftfAtftfA
tftfAts
mcccc
mm
m
mccmcc
mcc
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The output of Fig 2.21 is
s(t) differs from ideal condition in two respects:
1.The envelope contains a residual AM.
(FM has constant envelope)
2.i(t) contains odd order harmonic distortions
For narrowband FM, ≤ 0.3 radians.
)2sin()()2cos()(' tfdmkAtfAts cfccc
)!7!5!3
(sin753
xxx
xx
β
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(2.37) )(2cos)(2cos2
1)2(cos
)2cos()2(cos)2(cos
(2.2) )2(cos)(1 )(
)2cos()( , wavemodulating sinusoidal with AMFor
(2.36) )(2cos)(2cos2
1)2(cos
(2.35) )2)sin(2(sin)2(cos)(
(2.35) Recall
AM
tfftffAtfA
tftfAktfA
tftmkAts
tftm
tfftffAtfA
tftfAtfAts
mcmcccc
mccacc
cac
m
mcmcccc
mcccc
Narrow band FM
AM
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Wideband FM (large β)
(2.40) )2exp()(~
)]2sin(exp[)(~by defined envelopecomplex theis )(~ andpart real thedenotes Re where
(2.38) ))(2exp()(~Re
))2sin(2exp(Re)(
sincosexp
(2.33) )2sin(2cos)(
n
mn
mc
c
mcc
mcc
t nfjcts
tfjAts
ts
tfjts
tfjtfjAts
xjx(jx)
tftfAts
(2.39)
Complex Fourier Transform
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(2.41)
Let (2.42)
12
12
12
12
( )exp( 2 )
exp sin(2 ) 2 )
2
exp ( sin )2
m
m
m
m
f
n m mf
f
m c m mf
m
cn
c f s t j nf t dt
f A j f t j nf t dt
x f t
Ac j x nx dx
2
(2.43)
Define the th order Bessel function of the first kind as
A3, x
(2.44)
22 2
2( ) 0)
1( ) exp ( sin )
2
( )
( ) ( )
n
n c n
c n
n
n
d y dyx x n y
dx dx
J j x nx dx
c A J
s t A J
(2.45)exp( 2 )m
j nf t
(
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(2.49) )()()(2
)(
is )( of ransform Fourier tThe
(2.48) )(2cos)(
(2.47) )(2exp)(Re)(
mcmcnc
mcnc
mcnc
nfffnfffJA
fS
ts
tnffJA
tnffjJAts
Figure 2.23 Plots of Bessel functions of the first kind for varying order.
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-
Properties of ( )
1. ( ) ( 1) ( ), for all (2.50)
2.If is small
( ) 1
( )2
( ) 0 2 (2.51)
3. ( ) 1
Observation o
0
1
2
n
n
n n
n
n
J
J J n
J
J
J n
J
f FM
1.An FM signal contains components.
2.For small , the FM signal is effectively composed of a carrier and
a single pair of side freqencies at narrowband FM
3.The am
2 3 ,c m m m
c m
f , f , f , f
f f
plitude of carrier depends on
A (2.54)
22 21
( )2 2
cc n
P A J
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Example 2.2
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Transmission Bandwidth of FM signals
With a specified amount of distortion , the FM signal is
effectively limited to a finite number of significant side
frequencies.
A.Carson’s rule
, = , (2.55)1
2 2 2 (1 )T m m
m
fB f f f f f
f
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34
B.
, ( ) 0.01 , maxmax max2 2
T m n T
fB n f J B n
Universal curve for evaluating the 1 percent bandwidth of an FM wave
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Example 2.3
In north America, the maximum value of frequency deviation is fixed at 75kHz for commercial FM broadcasting by radio. If we take the modulation frequency W=15kHz, which is typically the “maximum” audio frequency of interest in FM transmission, we find that corresponding value of the deviation ratio is
Using Carson’s rule of Equation (2.55) , replacing by D , and replacing fm by W , the approximate value of the transmission bandwidth of the FM signal is obtained as
BT=2(75+15)=180kHz
On the other hand , use of the curve of Figure 2.26 gives the transmission bandwidth of the FM signal to be
BT=3.2 =3.2x75=240kHz
In practice , a bandwidth of 200kHz is allocated to each FM transmission . On this basis , Carson’s rule underestimates the transmission bandwidth by 10 percent , whereas the universal curve of Figure 2.26 overestimates it by 20 percent.
515
75D
f
f
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Generation of FM signals
(2.56)
( )
The frequency multiplier output
(2.58)
21 2
0
0
( ) ( ) ( ) ( )
cos 2 2 ( )
'( ) 'cos 2 2 ( )
'( ) ( )
nn
t
c c f
t
c c f
i c f
v t a s t a s t a s t
s t A f t k m d
s t A nf t nk m d
f t nf nk m t
(2.59)
Frequency Multiplier
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Varactor diode VCO FM modulator
32-1
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Crosby Direct FM Transmitter
32-2
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Demodulation of FM signals
The frequency discrimination consists of a slope circuit
followed by an envelope detector
Consider Fig 2.29a , the frequency response of a slope
circuit is
elsewhere ,022
),2
(2
22 ),
2(2
)(1
Tc
Tc
Tc
Tc
Tc
Tc
Bff
Bf
Bffaj
Bff
Bf
Bffaj
fH
(2.60)
33
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34
1 1( ) 2 ( ) , 0c
H f f H f f
2 2( ) 2 ( ) , 0c
H f f H f f
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Appendix 2.3 Hilbert Transform
Fourier Transform-frequency-selective
Hilbert Transform-phase-selective
(±900shift)
Let g(t)G(f)
Denote the Hilbert transform of g(t) as
(A2.32) )(ˆ1
)(
ansformHilbert tr inverse The
(A2.31) )(1
)(ˆ
dt
gtg
dt
gtg
35
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j ft
f
f f
f
g t
G f j f G f
(A2.33)
(A2.34)
The Fourier transform of is
(A2.35)
1sgn( )
1 0
sgn( ) 0 0
1 0
( )
ˆ( ) sgn( ) ( )
H(f) g(t) )(ˆ tg
36
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Properties of the Hilbert Transform
(time domain operation)
If g(t) is real
)(ˆ)g( 0)(g)g(3.
)( is )(ˆ of transform2.Hilbert
spectrum magnitude same thehave )( and )(ˆ.1
- tgtdttt
tgtg
tgtg
37
(take H.F of and
compare with A2.32)
( )g t
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For a band-pass system , we consider
x(t) X( f )
X( f ) is limited within ± W Hz
W fc
(A2.48)
The complex evelope of x(t) is
(A2.49)
( ) ( )cos(2 ) ( )sin(2 )
( ) ( ) ( )
I c Q c
I Q
x t x t f t x t f t
x t x t j x t
band pass
system, fc
x(t) y(t)
H(f)
fc
f
2B
(A2.50)( ) ( )cos(2 ) ( )sin(2 )I c Q c
h t h t f t h t f t 38
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)'(2)'(~
, )( from )(~
obtain can We
(A2.55) 0 , )(2)(~
with tolimited is )(H~
and
)()(*
real is )( Since
(A2.54) )(*~
)(~
)(2
(A2.53) toansformFourier trApply
(A2.53) )2exp()(*~
)2exp()(~
)(2
)*2 , ( have we(A2.52) From
functions pass-low are )(h~ and )(,)(
(A2.52) )2exp()(~
Re)(
)( oftion representacomplex The
(A2.51) )()()(~
response implusecomplex theDefine
c
c
c
cc
cc
QI
c
QI
ffHfHfHfH
ffHffH
fBBff
fHfH
th
ffHffHfH
t fjtht fjthth
zzvjuvz
tthth
t fjthth
th
tj hthth
39
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band-pass h(t)
system
x(t) y(t)
(A2.57)
(A2.58)
Define the pre-envelope of as
( ) Re ( )exp( 2 )
( ) ( )
( )
( ) ( ) ( ),
( ) ( ) sgn( ) ( )
2 ( )
( ) (0)
0
cy t y t j f t
h x t d
h t
h t h t j h t
H f H f f H f
H f f
H f H
A
0
0 ( 2.37)
0
f
f
:)(th
Hilbert T. of )(th
40 A A2.59)2.58 ( ) Re ( ) Re ( ) (y t h x t d
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1 1
-
ˆRecall ( ) ( )
( ) Re ( )
( ) Re ( )
Re ( ) ( )
ˆ ˆRe [ ( ) ( )][ ( ) ( )]
ˆ ˆ( ) ( ) ( ) ( )
h( )x(t- )d
h t h t jh t
h t h t
x t x t
h x t d
h jh x t jx t d
h x t d h x t d
1 1
ˆ( ) ( ) ,
ˆ( ) ( ) ( ) ( )
( ) ( ) ( ) ( )
2 ( ) ( )
2 Re[ ( )]Re[ ( )]
u
t u
h u x t d du
h x t d x d h u du
h x t d h u x t u du
h x t d
h x t d
dd
t
, t
To prove (A2.60)
41
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)(~)(~
)2exp(Re2
1
))(2exp()(~)2exp()(~
Re2
1
)()(Re2
1
(A2.59) )(Re)(Re)(
becomes (A2.58)
dtxhtfj
dtfjtxfjh
dtxh
dtxhty
c
cc
42
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)(
}{ (1)
)(2
222
c
tnffj
tfjtnfjtnfj
nff
dte
dteeeF
c
ccc
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)()(1
0),(1
0),(1
0,1
0,1
0,
0,
, 令, }{ (2)
)(2
)(2
22
22
222
cc
c
c
kfn
fj
kfn
fj
kn
fj
kfj
kn
fj
kfj
tfjtnfjtnfj
nfffn
f
n
nfn
f
n
nfn
f
n
ndken
ndken
nn
dkee
nn
dkee
n
dkdtkntdteeeF
c
c
c
c
ccc
{
{
{ =
=
=
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43
)2exp(factor he without t(t)h~ and
(t)y~ (t),x~ functions lowpass equivalent by the
systems and signals bandpassrepresent can We
(A2.63) )(~*)(~
)(~2or
(A2.62) )(~)(~
)(~2
have we(A2.61) and (A2.57) Comparing
tfj
txthty
dtxhty
c
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(A2.68) )()()()((t)2y
(A2.67) )()()()((t)2y
(A2.66) )(~)(~)(~let
(A2.65) )()()()(
)()()()(
(A2.64) )()()()()(~2
Q
I
txthtxth
txthtxth
tyjtyty
txthtxthj
txthtxth
tjxtxtjhthty
QIIQ
QQII
QI
QIIQ
QQII
QIQI
44
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45
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Procedure for evaluating the response
of a band-pass system
)2exp()(~ Re)( .4
)(~*)(~
)(~2Obtain .3
)2exp()(~
Re)( .2
)2exp()(~ Re)(
)(~by )( Replace 1.
tfjtyty
txthty
tfjthth
tfjtxtx
txtx
c
c
c
46
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To simplify the analysis
1. shift to the right by to align to the band-pass frequency
2. set , for (2.61)
Recall
1
1 1
1
( )
( ) 2 ( ) 0
2 ( )2
( )
c
c
Tc
H f f
H f f H f f
Bj πa f f
H f j
(2.60)
elsewhere
From (2.60) and (2.61), we get
(2.62)
elsewhere
1
2 2
2 ( )2 2 2
0
4 ( )( ) 2 2 2
0
T Tc c
T T Tc c c
T T T
B Bf f f
B B Bπa f f f f f
B B Bj a f f
H f
47
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48
t
cc f
t
c f
s t
s t A f t k m d
s t A j k m d
s t
1
Recall FM signal ( )
The complex envelope is
(2.63)
Let denote the complex envelop
0
0
( ) cos 2 2 ( )
( ) exp 2 ( )
( )
T T T
y t h t x t
S f H f S f
B B Bj a f S f f
e of the slope ckt. response output.
Recall (A2.63) 2 , we have
upper arm of Fig 2.30 in text)
elsewhere
11
( ) ( ) ( )
1( ) ( ) ( ) (
2
2 ( ) ( )2 2 2
0
T
tf
T c f
T
d s ts t a j B s t
dt
ks t j B aA m t j k m d
B
(2.64)
(2.65)
From (2.63) and (2.65) , we have
(2.66)
1
10
( )( ) ( )
2( ) 1 ( ) exp 2 ( )
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(2.67) 2
)(22cos)(2
1
)2exp()(~Re)(
0
11
t
fc
T
f
cT
c
dm kt ftmB
kaA B
t fjtsts
0sin 2 2 ( )
t
cf
f t k m d
is a hybrid-modulated signal (amplitude , frequency)
However, provided that we choose 1, for all
using an envelope detector, we have
1
1
( )
2( )
2( ) 1 ( )
f
T
f
T c
T
s t
km t t
B
ks t B aA m t
B
(2.68)
The bias term can be removed by a second frequency
discriminator with ( ) , where ( )2 2 1( ).
T c B aA
H f H f H f
49
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(2.71) )(4
)(~)(~)(
(2.70) )(2
1)(~
(2.69) )(~
)(~
210
2
12
tmaA k
tststs
tmB
kaA Bts
fHfH
cf
T
f
cT
Balanced Frequency Discriminator
Let the transfer function of the second branch of Fig 2.30
be (complementary slope circuit)
50
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FM Stereo Multiplexing
Two factors which influence FM stereo standards
1.Operation within the allocated FM channels.
2.Compatible with monophonic radio receiver.
(2.72) )2cos()4cos()()()()()( t fKt ftmtmtmtmtm ccrlrl 51
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Figure 9-40. FM stereo generation block diagram.
Stereo FM
51-1
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In Figure 9-40, audio signals from both left and right mircrophones are combined in an linear matrixing network to produce an L+R signal and an L-R signal.
Both L+R and L-R are signals in the audio band and must be separated before modulating the carrier for transmission. This is accomplished by translating the L-R audio signal up in the spectrum.
As seen in Figure 9-40, the frequency translation is
achieved by amplitude-modulating a 38-kHz subsidiary carrier in a balanced modulator to produce DSB-SC.
Stereo FM
51-2
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Stereo FM transmitter using frequency-division multiplexing.
Stereo FM Transmitter
51-3
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Stereo FM transmitter: (a) block diagram; (b) resulting spectrum.
SAC: Subsidiary Communication Authorization
Stereo FM Transmitter
51-4
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The stereo receiver will need a frequency-coherent 38-kHz
reference signal to demodulate the DSB-SC.
To simplify the receiver, a frequency- and phase-coherent
signal is derived from the subcarrier oscillator by frequency
division (÷2) to produce a pilot.
The 19-kHz pilot fits nicely between the L+R and DSB-SC L-
R signals in the baseband frequency spectrum.
Stereo FM
51-5
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As indicated by its relative amplitude in the baseband
composite signal, the pilot is made small enough so that
its FM deviation of the carrier is only about 10% of the
total 75-kHz maximum deviation.
After the FM stereo signal is received and demodulated to
baseband, the 19-kHz pilot is used to phase-lock an
oscillator, which provides the 38-kHz subcarrier for
demodulation of the L-R signal.
A simple example using equal frequency but unequal
amplitude audio toned in the L and R microphones is used
to illustrate the formation of the composite stereo (without
pilot) in Figure 9-41.
Stereo FM
51-6
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Figure 9-41. Development of composite stereo signal. The 38 kHz alternately multiplies L-R signal by +1 and –1 to produce the DSB-SC in the balanced AM modulator (part d). The adder output (shown in e without piot) will be filtered to reduce higher harmonics before FM modulation.
Stereo FM
51-7
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Spectrum of stereo FM signal. SCA: Subsidiary communication authorization (commercial-free program)
Stereo FM
51-8
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51-9
Reference : G. M. Miller “Modern Electronic Communication” 5th Edition, Prentice Hall
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2.8 Nonlinear Effects in FM Systems
1.Strong nonlinearity, e.g., square-law modulators ,
hard limiter, frequency multipliers.
2.Weak nonlinearity, e.g., imperfections
Nonlinear input-output relation
(2.73) )()()()(
32
3210 tvatvatvatv iii
Nonlinear
Channel (device)
vi(t) v0(t)
52
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(2.75) )(36cos4
1
)(24cos2
1
)(2cos)4
3(
2
1
(2.74) )(2cos
)(2cos)(2cos)(
)(2)(
)(2cos)(
signal FMFor
3
3
2
2
3
31
2
2
33
3
22
210
0
tt fAa
tt fAa
tt fAaAaAa
tt fAa
tt fAatt fAatv
dm kt
tt fAtv
cc
cc
cccc
cc
cccc
t
f
cci
53
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WfffB mT 2222 , rule sCarson'
W W W W f4f2
fc 2fc
In order to seperate the desired FM signal from the second
harmonic , we have
2
(2.76)
The output of the band-pass fil
(2 )
3 2
c c
c
f f W f f W
f f W
ter is
no effect to
An FM system is extremely sensitive to phase nonlinearities.
Common type of source : AM-to -PM conversion.
3
0 1 3
3'( ) ( )cos 2 ( ) ( ( ))
4c c c
v t a A a A f t t m t
54
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2.9 Super Heterodyne Receiver
(Carrier-frequency tuning , filtering , amplification , and demodulation)
fIF=fLO-fRF (2.78)
A FM system may use a limiter to remove amplitude variations.
55
AM radio receiver
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Commercial FM Broadcast、
Allocations and Sidebands
56
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2.10 Noise in CW modulation System
1.Channel model: additive white Gaussian noise (AWGN)
2.Receiver model: a band-pass filer followed by an ideal demodulator
The PSD of w(t) is denoted by .
20N
57
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(2.81) (SNR)
(SNR)merit of Figure
output at the noise ofpower average
signal ddemodulate theofpower average)SNR(
ratio noise-to-signaloutput The
)( ofpower average
)( ofpower average)SNR(
ratio noise-to-signal channel The
(2.80) )()()(
ison demodulatifor signal filtered The
(2.79) )2sin()()2cos()()(
:tionrepresenta noise narrowbandin noise filtered The
C
O
O
C
tn
ts
tntstx
t ftnt ftntn cQcI
58
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2.11 Noise in Linear Receiver Using Coherent Detection
The DSB-SC system
C,DSB
(2.83)
(SNR)
(baseband) (2.84)
C:system dependent scaling factor
2 2
0
2 2
0
( ) cos(2 ) ( ) ( ) ( )
( )
2
2
c c M
W
MW
c
c
s t CA f t m t m t S f
P S f df
PC A
WN
C A P
WN
59
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detector.coherent by the rejected completely is )( 2.
output.receiver at the additive are )( and )( 1.
:indicates (2.86)
(2.86) )(2
1)(
2
1)(filter pass-Low
componentsfrequency high
)4sin()(2
1)4cos()()(
2
1
)(2
1)(
2
1
)2()cos()(
(2.85) )2sin()()2cos()()()2cos(
)()()(
tn
tntm
tntmCAty
t ftnt ftntmCA
tntmCA
t ftxtv
t ftnt ftntmt fCA
tntstx
Q
I
Ic
cQcIc
Ic
c
cQcIcc
60
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problem! Serious
bandwidth. and eperformancbetween off- tradeNo 2.
SC-DSB ofmerit of figure same thehas SSBCoherent 1.
(2.88) 1(SNR)
(SNR)
(2.87) 2
2
4)SNR(
2
12)
2
1(power ))(
2
1( noise average The
2Let
4power ))(
2
1( signaloutput average The
SC-DSBC
O
0
22
0
22
SCDSBO,
002
22
WN
PAC
NW
PAC
WNWNtn
WB
PACtmCA
cc
I
T
cc
61
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2.12 Noise in AM Receivers Using Envelope Detection
)2cos()()2cos(
(2.89) )2cos()(1)(
t ftmkAt fA
t ftmkAts
caccc
cac
(2.91) )2sin()()2cos()()(
)()()(
:filter theofoutput At the
(2.90) 2
)1()SNR(
0
22
AMC,
t ftnt ftntmkAA
tntstx
WN
PkA
cQcIacc
ac
62
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(2.92) )()()(
)( of envelope)(
21 22
c tntntmkAA
txty
QIac
(2.95) 1(SNR)
(SNR)
(2.94) 2
(SNR)
1 2.
2
.1
)()()(
)( )()( Assume
2
2
C
O
0
22
AMO,
0
2
Pk
Pk
WN
PkA
k
WNA
tntmkAAty
tntntmkAA
a
a
AM
ac
a
c
Iacc
QIacc
63
(carrier power > noise power)
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Define the pre-demodulation SNR as
The average power of the modulated signal
SNR pre-de = The average noise power at the input of the demodulator
modulated Band pass
signal m(t)
s(t) filter
n(t) (SNR)pre-de (SNR)o
The Bandwidth of the bandpass filter is then the average noise power at the input of
the demodulator is
f
demodulator
(f)SN
20N
For an AM system SNR
If =2W SNR
To
ac
To
ac
BN
pkA
BN
pkA
2
)1(2)1( 2222
depre
AM
depre
AM
WN
pkA
o
ac
4
)1(22
Supplements
TB TB
TB
To BNTB
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Supplements For a DSB-SC system,
SNR
為與課本一致加
depre
SCDSB
WN
pAC
BN
PAC
o
c
To
c
4
222
22
2C
For an FM system
SNR
If using Carson’s rule, we have
=2Δf+2fm>> fm =w
depre
FM
To
c
To
c
BN
A
BN
A
2
2 22
TB
For the purpose of comparing different CW modulation systems, we define
The average power of the modulated signal
(SNR)c= The average power of channel noise in the message band
Message signal with LP filter
the same power as output
modulated wave
noise
n(t)
The equivalent baseband transmission model.
with bandwidth w
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Supplements
More precisely, we may express the DSB-SC
as m(t) S‘(t)
cos(2πfc t+θ)
θ is uniformly distributed over ﹝0, 2π﹞
S'(t)=Ac m(t) cos(2πfc t+θ)
At the receiver we may write
S(t)=C Ac m(t) cos(2πfc t+θ)
w
wmm
cmc
cc
cc
x
ss
dffSPR
PACRAC
tmEtfEAC
tftmCAE
dffS
RtSEP
)()0(
22)0(
)()2(cos
))2cos()((
)(
)0()(
2222
2222
2
2
The average noise power in –w<f<w
w
won WNdf
NP
2
0
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For convenience we write the modulated signal
as θ不出現
Since is ergodic and we take as a sample function
Supplements
SNRc=
=
= =
The average power of the modulated signal
The average power of channel noise in the message band
The average power of S(t)
The average power of channel noise in the message band
Ps
Pn WN
PAC
o
c
2
22
)2cos()()( tftmCAtS cc
)2cos( tfc )2cos( tfc
2
)0(
22
22
PAC
RACP
c
mcs
[time average of [ ]] )2(cos 2 tfc
WN
PAC
WN
PACSNR cc
c
0
22
0
22
2
2
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64
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Threshold Effect
65
noise power > carrier power
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2.13 Noise in FM Receivers
The discriminator consists of a slope network and an
envelope detector.
)2sin()()2cos()()(Let t ftnt ftntn cQcI
(2.130) )()2(cos)()( tt ftrtn c
(2.131) )()()( is envelope The 21
22
tntntr QI
(2.132) )(
)(tan)( is phase The 1
tn
tnt
I
Q
(1.114) 2 0 ,2
1)(
(1.115) 0 ),2
exp()(
.2over ddistribute uniform is )( and d,distributeRayleigh is )( where
2
2
2R
Ψf
rrr
rf
tΨtr
66
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The incoming FM signal s(t) is defined by
(2.133) )(22cos)(0
t
fcc dm kt fAts
(2.134) )(2)( where
0 dm kt
t
f
(2.135) )(2cos tt fA cc
(2.136) )(2cos)()(2cos
)()()(
outputfilter bandpass At the
tt ftrtt fA
tntstx
ccc
)137.2( )()(cos)(
)()(sin)( tan)()(
)( where
1
tttrA
tttrtt
trA
c
c
67
r
cA
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Note that the envelope of x(t) is of no interest to us (limiter)
(2.141) )()( sin)( 2
1)(
where
tttrdt
d
Atn
c
d
)138.2( )()(sin)(
)()(
)( Because
ttA
trtt
trA
c
c
(2.139) )()(sin)(
)(20
ttA
trdm k
c
t
f
noise additivemessage
(2.140) )()(
)(
2
1)(
2.40) (Fig isoutput tor discrimina The
tntmk
dt
tdtv
df
68
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(2.142) )(sin)(2
1)( ttr
dt
d
Atn
c
d
(2.143) )(sin)()(
have we, )( and )( of definition From
ttrtn
ttr
Q
(2.144) )(
2
1)(
dt
tdn
Atn
Q
c
d
as )(simplify may We
signal. message oft independen is )(then
),2 (0,over ddistributeuniformly is )()( Assume
tn
tn
tt
d
d
The quadrature component appears
69
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From (2.140)
The average output signal power = kf2P
Recall
noise is enhanced at high frequency
fjdt
d TF
2.
nQ(t) nd(t)
dt
d
Ac2
1
)( fSQN )( fS
dN
(2.145) )()(2
2
fSA
ffS
Qd N
c
N
70
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Assume that nQ(t) has ideal low-pass characteristic
with bandwidth BT
(2.146) 2
, )(2
2
0 T
c
N
Bf
A
fNfS
d
(2.147) , )(
output receiver At the
2
If
2
2
0
0Wf
A
fNfS
WB
c
N
T
71
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effect quieting noise 1
(2.148) 3
2
)( ofpower Average
2
2
3
0
2
2
0
0
c
c
W
Wc
A
A
WN
dffA
Ntn
(2.149) 2
3)SNR(
3
0
22
FM,WN
PkA fc
O
,FM
The average power of is ,
the average noise power in message bandwidth is
SNR (2.150)
2
0
2
0
2
,
( )2
( )2
(2.29) ( ) ( )
c
cC
f m o FM
As t
WN
A
WN
f k A SNR f
(2.151) 3
)SNR(
)SNR(2
2
FMW
Pk f
C
O
72
when increasing carrier power
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)2sin(2cos)( t f
f
ft fAts m
m
cc
2.4) Example (from 3
1
)SNR(
)NR( , AM tocompare
C
O
AM
S
)2cos()( sideboth
)2sin()(2 , may write We0
t fk
ftm
dt
d
t ff
fdm k
m
f
m
m
t
f
2
2
2
)( is load) 1 (across )( ofpower average The
fk
fPtm
,FM
0
3 3From (2.149), SNR
2 2 2 2
3
0
( )( ) ,
4 4c c
OA f A f
N W N W W
(2.152) 2
3)(
2
3
)SNR(
)SNR( 22
FM
W
f
C
O
FM. widebandand FM narrowbandbetween n transitio theas 5.0 Define
471.03
2
e.performancbetter has FM , 3
1
2
3When 2
Example 2.5 Single-Tone Modulation
73
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FM Threshold Effect (When CNR is low)
When there is no signal, i.e., carrier is unmodulated.
The composite signal at the frequency discriminator input
(2.153)
( ) tan
Occasionally,
1
( ) ( ) cos(2 ) ( )sin(2 )
( )
( )
cc I c Q
Q
c I
x t A n t f t n t f t
n tt
A n t
'
may sweep around the origin , ( >
(t) increases or decreases 2
The discriminator output is equal to
1 ( ) )
( )
2
cP r t A
t
nQ(t)
r(t)
x(t)
Ac
P1
0 P2
nI(t) 74
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Figure 2.44 Illustrating impulselike components in (t)
d (t)/dt produced by changes of 2 in (t); (a) and (b)
are graphs of (t) and (t), respectively.
75
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A positive-going click occurs , when
, , 0
A negative-going click occurs when
, , 0
The carrier-to -noise ratio is defin
( )( ) ( ) ( ) ( )
( )( ) ( ) ( ) ( )
c
c
d tr t A t t d t
dt
d tr t A t t d t
dt
ed by
(2.154)
The output signal-to-noise ratio is calculated as
1. The average output signal power is calculated assuming
a sinusoidal modulation which produces . (noise free)
2.
02
2
c
T
T
A
B N
Bf
The average output noise power is calculated when no
signal is present (The carrier is unmodulated). 76
2
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avoided bemay effects threshold
(2.155),202
or 202
When 0
2
0
2
NBA
NB
AT
c
T
c
Figure 2.45 Dependence of
output signal-to-noise ratio on
input carrier-to-noise ratio for
FM receiver. In curve I, the
average output noise power is
calculated assuming an
unmodulated carrier. In curve
II, the average output noise
power is calculated assuming
a sinusoidally modulated
carrier. Both curves I and II
are calculated from theory.
77
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The procedure to calculate minimum
1. Given and W, determine
(using Figure 2.26 or Carson's rule)
2. Given , we have 20
Capture Effect:
The receiv
2
0 0
( 20)
2
c
T
cT
A
B
AN B N
er locks onto the stronger signal
and suppresses the weaker one.
78
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FM Threshold Reduction (tracking filter)
• FM demodulator with negative feedback (FMFB)
• Phase locked loop
Figure 2.46
FM threshold extension.
Figure 2.47
FM demodulator with
negative feedback.
79
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Pre-emphasis and De-emphasis on FM
Figure 2.48 (a) Power spectral density of noise at FM receiver output.
(b) Power spectral density of a typical message signal.
Figure 2.49 Use of pre-emphasis and de-emphasis in an FM system. 80
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Figure 2.50 (a) Pre-emphasis filter.
(b) De-emphasis filter.
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The main difference between FM and PM is in the
relationship between frequency and phase.
f = (1/2).d/dt.
A PM detector has a flat noise power (and voltage) output
versus frequency (power spectral density). This is
illustrated in Figure 9-38a.
However, an FM detector has a parabolic noise power
spectrum, as shown in Figure 9-38b. The output noise
voltage increases linearly with frequency.
If no compensation is used for FM, the higher audio
signals would suffer a greater S/N degradation than the
lower frequencies. For this reason compensation, called
emphasis, is used for broadcast FM.
Preemphasis for FM
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Figure 9-38. Detector noise output spectra for (a). PM and (b). FM.
Preemphasis for FM
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A preemphasis network at the modulator input
provides a constant increase of modulation index mf
for high-frequency audio signals.
Such a network and its frequency response are
illustrated in Figure 9-39.
Preemphasis for FM
Fig. 9-39. (a)Premphasis network, and (b) Frequency response.
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With the RC network chosen to give = R1C = 75s in North America (150s in Europe), a constant input audio signal will result in a nearly constant rise in the VCO input voltage for frequencies above 2.12 kHz. The larger-than-normal carrier deviations and mf will preemphasize high-audio frequencies.
At the receiver demodulator output, a low-pass RC network
with = RC = 75s will not only decrease noise at higher audio frequencies but also deemphasize the high-frequency information signals and return them to normal amplitudes relative to the low frequencies.
The overall result will be nearly constant S/N across the 15-
kHz audio baseband and a noise performance improvement of about 12dB over no preemphasis. Phase modulation systems do not require emphasis.
Preemphasis for FM
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Preemphasis and deemphasis: (a) schematic diagrams; (b) attenuation curves
Pre-emphasis and De-emphasis on FM
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Example of S/N without preemphasis and deemphasis.
Pre-emphasis and De-emphasis on FM
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Example of S/N with preemphasis and deemphasis.
Pre-emphasis and De-emphasis on FM
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Dolby dynamic preemphasis
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Figure 2.55 Comparison of the noise performance of various CW modulation
systems. Curve I: Full AM, = 1. Curve II: DSB-SC, SSB. Curve III: FM, = 2.
Curve IV: FM, = 5. (Curves III and IV include 13-dB pre-emphasis, de-
emphasis improvement.) 91
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In making the comparison, it is informative to keep in
mind the transmission bandwidth requirement of the
modulation systems in question. Therefore, we define
normalized transmission bandwidth as
W
BB T
n
Table 2.4 Values of Bn for various CW modulation schemes
FM
AM, DSB-SC SSB
Bn
2 5
2 1 8 16
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李家同教授-我的恩師
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