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Principles of Communication
Analog Modulation System (3)
LC 5-6
Lecture 9, 2008-10-10
Lecture 9 2
Contents
Linear vs. Angle ModulationPhase modulation and frequency modulation Narrowband frequency modulationSpectrum of angle modulation systemPower of angle modulation systemAngle modulator
Lecture 9 3
Linear vs. Angle Modulation MethodsAmplitude modulation (AM) methods are also called linear modulation methods, although conventional AM is not linear in the strict sense.In phase modulation systems, the phase of the carrier is changed according to the variations in the message signal.In frequency modulation system, the frequency of carrier is changed by the message signal.PM and FM are special cases of angle modulation methods. They are obviously quite nonlinear.
Lecture 9 4
Representation of PM and FM signalsThe complex envelop is
( ) cos[ ( )]c cs t A t tω θ= +
( )( ) j tcg t A e θ=
The angle modulated signal is
For PM, the phase is directly proportional to the message signal
For FM, the phase is proportional to the integral of message signal
Dp is the phase sensitivity
Df is the frequency deviation constant
( ) ( )pt D m tθ =
1 ( )( ) ( ) ( ) ( )2 2
t ff d
Dd tt D m d f t m tdtθθ σ σ
π π−∞= ⇒ = =∫
Lecture 9 5
Comparison of PM and FM
( ) ( )tf
p fp
Dm t m d
Dσ σ
−∞= ∫
( )( ) p p
ff
D dm tm t
D dt=
Lecture 9 6
Example (1)
1 2
m(t)
PM
FM
21
( )( ) dm tm tdt
=
Lecture 9 7
Example (2)Message signal ( ) cos(2 )p mm t V f tπ=
Carrier cos(2 )c cA f tπ
In PM ( ) ( ) cos(2 ) cos(2 )p p p m p mt D m t D V f t f tθ π β π= = =
In FM
( ) ( ) sin(2 ) sin(2 )2
t f pf m f m
m
D Vt D m t dt f t f t
fθ π β π
π−∞= = =∫
βp is phase modulation index
βf is frequency modulation index
Lecture 9 8
Modulation Index
1 ( ) 1 1 ( )( ) [ ( )]2 2 2i c c
d t d d tf t t t fdt dt dtψ θω θ
π π π= = + = +
[ ]( ) ( ) cos ( ) , ( ) ( )cs t R t t t t tψ ψ ω θ= = +
fFB
β Δ=
The instantaneous frequency of s(t) is
If a bandpass signal is represented by
In FM, the peak frequency deviation from the carrier frequency is1 1max[ ( )] max[ ( )]
2 2d f f pF f t D m t D Vπ π
Δ = = =
The frequency modulation index is
Vp = max[m(t)]
We can extend the definition for a general non-sinusoidal signal
B is the bandwidth of the message signal.
In PM, the peak phase deviation is max[ ( )]p p pD m t D VθΔ = =
The phase modulation index is pβ θ= Δ
Lecture 10 9
Narrowband Frequency Modulation
( ) ( 0.56
t
fD m d πτ τ−∞
⎡ ⎤ <<⎢ ⎥⎣ ⎦∫ or )If , narrowband frequency modulation (NBFM)
cos ( ) 1
sin ( ) ( )
t
f
t t
f f
D m d
D m d D m d
τ τ
τ τ τ τ
−∞
−∞ −∞
⎡ ⎤ ≈⎢ ⎥⎣ ⎦⎡ ⎤ ≈⎢ ⎥⎣ ⎦
∫
∫ ∫
Q
( ) cos ( )d
cos cos ( )d sin sin ( )d
t
FM c f
t t
c f c f
s t A t D m
t D m t D m
ω τ τ
ω τ τ ω τ τ
−∞
−∞ −∞
⎡ ⎤= +⎢ ⎥⎣ ⎦⎡ ⎤ ⎡ ⎤= −⎢ ⎥ ⎢ ⎥⎣ ⎦ ⎣ ⎦
∫
∫ ∫
( ) cos ( ) sint
NBFM c f cs t t D m d tω τ τ ω−∞
⎡ ⎤∴ ≈ − ⎢ ⎥⎣ ⎦∫
Lecture 10 10
NBFM vs. AM
[ ]
( ) ( )1( ) sin2
( ) ( )( ) ( ) ( )2
c cc
c c
f c cNBFM c c
c c
F Fm t dt t
D F FS
ω ω ω ωωω ω ω ω
ω ω ω ωω π δ ω ω δ ω ωω ω ω ω
⎡ ⎤+ −⎡ ⎤ ⇔ −⎢ ⎥⎣ ⎦ + −⎣ ⎦⎡ ⎤− +
∴ = + + − + −⎢ ⎥− +⎣ ⎦
∫Q
[ ] [ ]1( ) ( ) ( ) ( ) ( )2AM c c c cS M Mω π δ ω ω δ ω ω ω ω ω ω= + + − + + + −
Lecture 10 11
ExampletAtm mm ωcos)( =
[ ]( ) cos cos( ) cos( )2m f
NBFM c c m c mm
A Ds t t t tω ω ω ω ω
ω= + + − −
[ ]( ) cos cos( ) cos( )2m
AM c c m c mAs t t t tω ω ω ω ω= + + + −
sAM(ω )
O-ω m ωω m
F(ω )
O-ω c-ω m -ω c -ω c+ω m ω c-ω m ω c ω c+ω m ω
sNBFM(ω )
O-ω c-ω m -ω c
-ω c+ω m ω c-ω m
ω c ω c+ω m ω
NBFM
LSB USBCarrier
AM
Carrier
USB
LSB
Lecture 10 12
Spectrum of Angle Modulated Signals
Due to the nonlinearity of angle-modulation systems the precise characterization of their spectral properties, even for simple message signals, is mathematically intractable.We will study the spectral characteristics of an angle modulation signal in the case of a sinusoidal signal
Example 5-2
Lecture 10 13
order.nth theof kindfirst theoffunction Bessel the:)(
)(21
21
1
1)(1where
)(
is envelopecomplex The
)sin()sin(
2/
2/
sin
2/
2/
sin2/
2/
sin)(
β
βθπ
θπ
ωω
π
π
θθβπ
π
θθβ
ω
ω
ωωβ
ωωβω
ωωβθ
n
ncnj
cnj
c
T
T mtjntj
cmm
T
T
tjntjc
m
T
T
tjn
mn
n
tjnn
tjc
tjc
J
JAdeAdeA
tdeeAT
dteeAT
dtetgT
c
eceAeAtg
mm
mm
mm
m
m
mmm
m
m
mm
=⎥⎦⎤
⎢⎣⎡==
=
==
===
∫∫
∫
∫∫
∑
−
−
−
−
−
−
−
−
−
−
∞
−∞=
Lecture 10 14
Angle Modulation by a Sinusoidal Signal
Bessel Function
1)( .3
2 , 0)(2/)(
1)(small very is When .2
)()1()( .1
2
1
0
=
>≈≈
≈
−=
∑∞
∞=
−
-nn
n
nn
n
J
nJJJ
JJ
β
βββ
ββ
ββ
Lecture 10 15
∑
∑∑
∞
−∞=
∗
∞
−∞=
∞
−∞=
−−=
>
−−+−=
−=−=
nmcnc
cc
nmnc
nmn
nfffJAfS
fffGffGfS
nffJAnffcfG
)()()(
0For )]()([)(
is signal modulated-angle theof spectrum The
)()()()(
isenvelopecomplex theofspectrum The
21
21
δβ
δβδ
Lecture 10 16
Carson’s Rule
Carson’s rule2( 1)TB Bβ= +
The effective bandwidth of an angle-modulated signal, which contains at least 98% of the signal power.
Lecture 10 17
Power in an Angle Modulation Signal
[ ]
[ ]
2 2 2
2 2
( ) cos ( )
1 1 cos 2 ( )2 2
c c
c c c
s t A t t
A A t t
ω θ
ω θ
= +
= + +
The normalized power of angle modulation signal:
If the carrier frequency is large so that s(t) has negligible frequency content in the region of dc, the second term is negligible
2 21( )2 cs t A=
Thus the power contained in the output of an angle modulator is independent of the message signal.
Lecture 10 18
Angle ModulatorsDirect FM – Varactor Diodes
0
00 0
00 0
12
00
12
0
0
0
Assume that ( ) and 1
1 1,2 2 ( )
1 12 ( ) 2
1 [(1 ) 1]2
[(1 ) 1]
1[(1 ) 1]2
1 ( )2
c
c
c
c
c
c c
CC K m tC
f fLC L C C
f f fL C C LC
CCLC
CfC
CfC
f K m tC
π π
π π
π−
−
ΔΔ = <<
= =+ Δ
Δ = − = −+ Δ
Δ= + −
Δ= + −
Δ≈ − −
= −
Simple LC circuitLarge frequency offsetBut, unstable
Lecture 10 19
Angle ModulatorsIndirect Method - narrowband frequency modulation (NBFM)
443442143421termsideband
cc
termcarrier
cccc
ctj
c
ttAtAttyttxtstjAeAtg
t
ωθωωωθ
θθ
sin)(cossin)(cos)()()](1[)(
valuesmallatorestrictedis)(When )(
−=−=+≈=
Lecture 10 20
Angle ModulatorsIndirect Method – Narrowband-to-Wideband Conversion
( ) cos[ ( )]( ) cos[ ( )]( ) 2cos( )( ) cos[( ) ( )] cos[( ) ( )]( ) cos[( ) ( )]
i c c
n c c
LO
m c c LO c c LO
o c c LO
s t A t ts t A n t n tf t ts t A n t n t A n t n ts t A n t n t
ω θω θ
ωω ω θ ω ω θω ω θ
= += +
== + + + − += − +
The central idea is that frequency multiplier changes both the carrier frequency and the deviation ratio by a factor of n, whereas the mixer changes the effective frequency but does not affect the deviation ratio.
FM DemodulationDifferentiator and Envelope Detector
Zero Crossing DetectorUses rate of zero crossings to estimate fi
Phase Lock Loop (PLL)Uses VCO and feedback to extract m(t)
Lecture 10 21
∫++=′t
fcfcc dmktftmkfAts0
])(22sin[)](22[)( ττππππ
Theorem
Lecture 10 22
( ) ( )
signal. modulating theof PDF theis )( where
222
)(
by edapproximat is signal WBFM theof PSD normalized them(t), ofbandwidth theis B and
12
)](max[
)(cos)(
wheresignaling, For WBFM
2
⋅
⎥⎥⎦
⎤
⎢⎢⎣
⎡⎟⎟⎠
⎞⎜⎜⎝
⎛−−+⎟
⎟⎠
⎞⎜⎜⎝
⎛−=
>=
⎥⎦⎤
⎢⎣⎡ += ∫ ∞−
m
cf
mcf
mf
c
ff
t
fcc
f
ffD
fffD
fDAf
BtmD
dmDtAts
πππ
πβ
σσω
P
Example 5-3
Lecture 10 23
Main Points of Angle ModulationAn angle-modulation signal is a nonlinear function of the modulation, and consequently, the bandwidth of the signal increases as the modulation index increases.The discrete carrier level changes, depending on the modulating signal, and is zero for certain types of modulating waveforms.The bandwidth of a narrowband angle-modulated signal is twice the modulating signal bandwidth (the same as that for AM signaling).The real envelope of an angle-modulated signal is constant and consequently does not depend on the level of the modulating signal.
Lecture 10 24
Lecture 9 25
HomeworkLC 5-21, 5-24, 5-38, 5-40, 5-44