© 2010 the mcgraw-hill companies communication systems, 5e chapter 5: angle cw modulation a. bruce...
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© 2010 The McGraw-Hill Companies 5.1 Phase and frequency modulationTRANSCRIPT
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Communication Systems, 5e
Chapter 5: Angle CW Modulation
A. Bruce CarlsonPaul B. Crilly
(modified by J. H. Cho using Prof. W.J. Song’s lecture note)
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Chapter 5: Angle CW Modulatation
Phase and frequency modulation
Transmission bandwidth and distortion
Generation and detection of FM and PM
Interference
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5.1 Phase and frequency modulation
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Terms 5.1-1
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• Total instantaneous angle• Angle modulation = exponential modulation
• Phase modulation (PM)• Phase modulation index = phase deviation
• Instantaneous frequency vs. spectral frequency• Frequency modulation (FM)• Frequency deviation
• Zero-crossing rate
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Exponential Modulation ( 각변조 )
Phase modulation
).()(where),(cos)(
ttwttAtv
c
c
indexmodulationphase:
])(cos[)(
1)(for.shiftphasemax.:180)()(
txtwAtv
txtxt
ccph
Frequency Modulation (FM)
Phase Modulation (PM)
PM & FM Signals
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Def ) 순간 주파수 (instantaneous frequency) 의 정의
Frequency modulation
)(21)(
21)( t
dtdft
dtdtf c
oo
t
t
cc
tttdxft
txftdtd
fftxfftf
o
)()(2)(
)()(21
)()(
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PM signals
0Δ
CW signal:
( ) cos[ ( )]
PM: ( ) cos[ ( )]
where 180 , ( ) 1, and phase modulation index or phase deviation
c c c
c c c
x t A t t
x t A t x t
x t
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CW signal:
( ) cos[ ( )]
FM:
( ) cos 2 ( )
where ( ) 1, and frequency deviation
c c c
t
c c c
x t A t t
x t A t f x d
x tf
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FM and PM Signals
21Power: 2T cS A
Power is constant, and not a function of messagepower
0For PM ( ) 180t
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FM and PM Message content resides in zero crossings not amplitude Modulated waveform does not resemble message waveform Amplitude is constant we can use more efficient
nonlinear amplifiers
Note: FM and PM, ( ) , or effective message power increases with larger
values of , or
x t f
f
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Illustrative AM, FM, and PM waveforms
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Terms 5.1-2
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• Narrowband PM and FM (NBPM and NBFM)
• Single-tone modulation• Bessel function of the first kind of order n and argument beta
• Multitone modulation • Periodic modulation
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Narrowband PM & FM
])(!3
1)([
)(sin
])(!2
11[
)(cos)(
sin)(sincos)(cos)](cos[)(
3
2
ttA
tAv
tA
tAtv
twtAtwtAttwAtv
c
cq
c
ci
cccc
cc
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Narrowband Condition
따라서
.radian1)( t
)()()(
tAtvAtv
cq
ci
FM:)(PM:)(
)]([)(
0)(2
)(21)(
ffXjf
fXtFf
fffAjffAfV cccc
WBT 2Note this is true only when is small.)(t
Then
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Narrowband PM and FM
2
3
Quadrature-carrier form:
( ) ( )cos ( )sin1 where: ( ) cos ( ) 1 ( ) ...2
1 ( ) sin ( ) ( ) ...3!
with the narrowband case (
c ci c cq c
ci c c
cq c c
x t x t t x t t
x t A t A t
x t A t A t
t) 1 rad ( ) ( ) (t)ci c cq cx t A x t A
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The NBFM/NBPM spectra becomes:
1 ( ) ( ) ( )2 2
where ( ) ( )
c c c c cjX f A f f A f f
f t
The spectrum of narrowband FM and PM looks like that of AM!
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Tone Modulation
Let sin PM ( ) cos FM
with ( ) cos[ ( )]
( ) sin 2
PM/ FM
m m
m m
c c c
m
m
m m
A tx t A t
x t A t t
t f t
AA f f
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Consider FM
( ) cos[ ( )]
( ) cos 2 cos2
( ) 2 cos2 sin 2
with /
c c ct
cFM c c m m
t
m m m
m m
x t A t t
x t A t f A f d
t f A f d f t
A f f
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Narrowband modulation
1
( ) cos sin sin
= cos cos( ) cos( )2 2
looks like a tone modulated AM signal
c c c c m c
c cc c c m c m
x t A t A t t
A AA t t t
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NBFM with tone modulation (a) Line spectrum; (b) Phasor diagram
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Let
)FM(cos)PM(sin
twAtwA
mm
mm
FM
PMsin)(
ffA
Atwt
m
m
mm
Then
]sin)sinsin(cos)sin[cos(]sin)(sincos)([cos
)](cos[)(
twtwtwtwAtwttwtA
twAtv
cmcmc
ccc
cc
일반적으로 임의의 에 대한 FM 의 대역폭을 구하는 것은 불가능하다 .)(tx최고 주파수 mf
)(tx
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FM/PM spectra with an arbitrary index value
m
In quadrature-carrier form,
( ) [cos ( )cos sin ( )sin ]
with (t)= sin
( ) [cos cos cos sin sin sin ]
c c c c
c c m c m c
x t A t t t t
t
x t A t t t t
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Note)
Trigono. Fourier Series
deJ
n
tnwJtw
tnwJJtw
njn
moddn
nm
mevenn
nom
)sin(
21)(
0
sin)(2)sinsin(
cos)(2)()sincos(
Bessel Function of the First Kind of order and argumentn .
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)()1()(})cos{()(
])cos())[cos((
])cos())[cos((
cos)()(
nn
nn
mcnc
mcevenn
mcnc
mcoddn
mcnc
coc
JJtnwwJA
tnwwtnwwJA
tnwwtnwwJA
twJAtv
)(OJ
)(3 J
)(2 J
)(1 J
)(1 J)(2 J )(3 J
cf
mf
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Note) determines the magnitude of the Fourier Coeff.’s frequency spacing
)()1()( nn
n JJ
(decay rate of sideband harmonics)
mf
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Infinite Sideband !
따라서
Up to N sidebands
)(2)]([2
2
1
222
2
nno
cT
cT
JJAP
AP
1)(2)(1
22
n
no JJ
)](2)([2 1
222
N
nno
cN JJAP
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99% POWER
mT
mmT
T
N
wB
wNwB
NPP
2ThenNBFM,i.e.,1If
RulesCarson')1(22
199.0
Note)
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0 odd
even
Given the message is a single tone and periodic, we can write( ) as a trigonometric Fourier series
cos cos ( ) ( )cos
sin sin ( )sin
Where
c
m n mn
m n mn
x t
t J J n t
t J n t
J
( ) is a Bessel function of the first kind, order
Taking advantage of ( ) ( 1) ( )
( ) ( )cos( )
n
nn n
c c n c mn
n
J J
x t A J n t
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The FM/PM signal ( ) consists of a carrier with multiple sidebands that are spaced by an integer multiple of with the sideband having an amplitude of ( ).
c
m n
x t
J
Note: the lower sidebands alternate from + to -
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Magnitude of tone-modulated line spectra (a) FM or PM with ƒm fixed; (b) FM with Amƒ fixed
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Multitone Modulation
FM
twAtwAtx 2211 coscos)(
twfwAtwf
wAtw
dttxftwt
c
c
22
21
1
1 sinsin
)(2)(
][
}]{[
][)(
)sinsin
)sinsin(
)(
22
21
1
1
22
21
1
1
twfwA
jtwfwA
jtjw
ce
twfwA
twfwA
twj
ce
tjceFM
eeeAR
eAR
eARtv
c
c
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])cos[()()(
])()([
}])(}{)({[
2121
)(21
21
21
21
tkwnwwJJA
eJJAR
eJeJeAR
ckn k
nc
tkwnwwjk
n knce
tjkw
kk
tjnw
nn
tjwce
c
c
21
2
1
kwnwwkwwnww
c
c
c
Intermodulation component !
because FM is a nonlinearmodulation method.
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Chapter 5: Angle CW Modulatation
Phase and frequency modulation
Transmission bandwidth and distortion
Generation and detection of FM and PM
Interference
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5.2 Transmission bandwidth and distortion
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Bandwidth of Narrowband FM Approximate spectrum of narrowband
FM Message bandwidth vs. transmission
bandwidth
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Bandwidth of Tone Modulated FM Spectrum of Single-tone modulated
FM M significant sideband pairs B=2Mf_m
M(beta) vs. beta +2 Beta = A_m f_delta/f_m=< f_delta/f_m
Deviation ratio = f_delta/W
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Transmission BandwidthSeveral equations to determine , all are approximate
Let deviation ratio
1. 2 ( )
T
T
BfDW
B M D W
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2 2 12. 2 1
3. Carson's rule: 2( ) 2( 1) 1, 1
3. 2( 2 ) 2( 2) 2 10
TDW f DB W D
f W D W D D
B f W D W D
• In all approximations, the transmission bandwidth is proportional to twice the transmission bandwidth.
• The proportionality constants are different.
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Commercial FM radio bandwidth example
Commercial FM radio75 15 kHz, 15 kHz 55
using #3 above 2( 2) 2(5 2)15 210 kHz
using Carson's 2(
T
T
f W D
B D W
B D
1) 2(5 1)15 180 kHz
Assigned frequencies to the FM band are set up for 200 kHzT
W
B
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Station engineer has set constants so BT conforms to the FCC limits dictated by their license
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5.2.2 Linear and Nonlinear Distortion
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Wireless channel as an LTI system FM-to-AM conversion
Controlled nonlinear distortion and filtering to remove unwanted amplitude variation
Memoryless nonlinear system
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Limiter
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Nonlinear processing circuits (a) Amplitude limiter; (b) frequency multiplier
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Amplitude Limiter and Noise Reduction
FM signal processing using a limiter: Noiseless FM signal, (b) noisy FM signal, (c) limiter output with noisy input, (d) BPF output
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Output of Memoryless Nonlinear System to FM Input
V_in(t) = A_c cos{omega_ct+phi(t)} Weierstrass Approximation Theorem For every epsilon, there exists a
polynomial such that… V_out(t) = …
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Limiter for frequency multiplier
Limiter or some other nonlinear device generates harmonics
BPF selects which integer multiple of
Nonlinear device also changes frequency/phase deviation constants
'c cf nf
cf
' ' Limiter-BPF output and c cf nf f nf
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Chapter 5: Angle CW Modulatation
Phase and frequency modulation
Transmission bandwidth and distortion
Generation and detection of FM and PM
Interference
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5.3 Generation and detection of FM and PM
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Generation of FM and PM signals
Pros. Constant envelope more power efficient
nonlinear methods can be used longer battery life
Cons. Required to have frequency vary linearly with
the message amplitude. Not straightforward.
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5.3.1 Direct FM and Voltage Controlled Oscillator (VCO): Use a VCO!
0
0 1
Oscillator output frequency =
Oscillator tank circuit with resonant frequency of , , and ( )
v
f f
f f f L C C t
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00
0
( ) capacitance of the varactor diode which is affected by the applied voltage ( ) ( )
1Nominal value of 2
but since the total capacitance in the tuned circuit is + ( )
v
v
v
ouput
C tC t f x t
fLC
C C C t
f f
0 0 [ ( )]f f f x t
Important: frequency change must be linear with x(t) sets a limit on maximum frequency deviation
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How can we increase and still have change linearlywith ( )?
1. Different hardware, increase power supply, etc.
2. Frequency multiplier and indirect FM: recall: nonlinear frequency
f fx t
multiplier also multiplies f
Tripler0 , f f 03 , 3f f
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Important: Frequency multiplication is not the sameas hetrodyning
Hetrodyning is a linear process and does not affectthe frequency or phase deviation constants
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5.3.2 Phase modulators and indirect FM: Generate a NBFM, use frequency multiplier, and down-convert!
Convert a PM signal to an FM one by integrating The message signal
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Phase modulators and Indirect FM
2 1 1
2
( ) ( ) PM FM
( ) ( ) ( ) ( )
multiplier also multiples frequency deviation2
Output frequency: ( ) ( )
t
c
LO
x t x d
f t nf t nf t f x t
f nT
f t f t f
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5.3.4 Frequency detection
Produces output voltage that is proportional to the instantaneous frequency of the input the messagex(t).
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Frequency detector = Discriminator
1. FM-to-AM conversion
2. Phase-shift discrimination
3. Zero-crossing detection
4. Frequency feedback→ phase locked loops (Chap 7)
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5.3.4.1 FM to AM conversion
FM signal: ( ) cos 2 2 ( )
Taking the derivative with respect to time:
( )
( ) 2 [ ( )]sin 2 2 ( )
2 [ ( )]sin 2 ( )
( ) looks like an A
t
c c c
c
t
c c c c
c c c
c
x t A f t f x d
dx tdt
x t A f f x t f t f x d
A f f x t f t t
x t
M signal
we can recover ( ) with an envelope detectorx t
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FM to AM conversion Take derivative of FM signal
Use an envelope detector
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FM Detection Waveforms
(a)frequency detector with limiter and FM to AM conversion(b) waveforms
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FM to AM methods - Derivative
Slope detector via a BPF
Balanced discriminator
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Derivative function( ) ( )Recall ( 2 ) ( ) ( )
we can use the edge of a BPF or LPF as a differentiator
dv t dv tj f V f jKV fdt dt
Allows an AM receiver with a BPF to detect an FM signal
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Balanced discriminator To get the maximum response from the BPF we combine two BPF-envelope detectors to get a balanced discriminator
(b) circuit, (c) voltage to frequency characteristic
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5.3.4.2 Phase shift discriminator
1
11
1 1 1
( ) ( )( )
Assume that is small compared to variation in ( )1( ) ( ) ( )
FM wave ( ) ( ) ( ) 2 ( )
dv t v tv tdt t
t v t
v t v t v t tt
t t t t t f t x t
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Phase-shift discriminator
1
1 1
1 1 1
Multiplier output: cos ( ) sin ( )
1 1 sin 2 ( ) ( ) sin ( ) ( )2 2
LPF output1 ( ) sin ( ) ( ) ( ) ( ) ( )2
c c
c
D
t t t t t
t t t t t t t
y t t t t t t t t t
( ) ( )D Dy t K f x t
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Chapter 5: Angle CW Modulatation
Phase and frequency modulation
Transmission bandwidth and distortion
Generation and detection of FM and PM
Interference
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5.4 Interference Occurs when another signal is received concurrently in
the receiver’s bandpass Multipath: multiple versions of the transmitted signal
with different delays can cause interference Effects can be affected by the types of modulation and
detectors used. Interference: generally not random Sometimes can be canceled out Is not the same as random noise
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Interfering sinusoidsLet ( ) be a received signal consisting of the desired component at
and some interfering sinusoid at frequency with
c c i
i
v tf f f f f
f W
( ) cos2 cos[2 ( ) ]
Let , , and be the interferer's amplitude, carrier frequency, and phase respectively.
Let and ( ) 2
c c i c i i
i c i
ii i i
c
v t A f t A f f t
A f f
A t f tA
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Interfering sinusoid in envelope-phase form
2
1
Putting ( ) in envelope phase form, we have:
( )cos 2 ( )
( ) 1 2 cos ( )
sin ( ) ( ) tan1 cos ( )
v c v
v c i
iv
i
v t
A t f t t
A t A t
ttt
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The interfering sinusoid produces both amplitude and phasemodulation
If interference is relatively small 1
( ) 1 cos(2 ) looks like an AM signal with single tone message
( ) sin(2 ) looks like an FM or PM signal wit
v c i i
v i i
A t A f t
t f t
h single tone message
This is why nearby AM/FM signals with unsuppressed carriers generate a disproportionate amount of obnoxious background “whistles.”
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1
If interference is relatively small 1
( ) 1 cos(2 ) looks like an AM signal with single tone message
( ) 2 phase corresponds to a shifted c
v i i i
v i i
A t A f t
t f t
arrier freq. plus constant
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Demodulated output with interference
Assuming 1, and
(1 cos2 ) AM with envelope detector( ) sin 2 PM
sin 2 FM
i
D i
D D i
D i i
f W
K f ty t K f t
K f f t
Observe how interference level of FM depends on spacingof interference carrier frequency
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Interference level as function of interference frequency spacing
Note how with FM interference is reduced if the interferencefrequency spacing is reduced.
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Deemphasis and Preemphasis Filtering
We exploit the property of FM that causes the interference level to be reduced as fi ↓ by deemphasis filtering of the high frequencies at detection we preemphasize the highfrequencies at the transmitter.
at the transmitter end we predistort the signal
1( )( )pe
de
H fH f
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Chapter 5: Angle CW Modulatation
Phase and frequency modulation
Transmission bandwidth and distortion
Generation and detection of FM and PM
Interference