chapter 4
TRANSCRIPT
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Chapter 4
Amplitude (Linear) Modulation
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Outlines• Introduction• Base-band and Carrier Communication• Amplitude Modulation (AM):DSB-Large Carrier• Amplitude Modulation: Double sideband- Suppressed
Carrier (DSBSC)• Quadrature amplitude Modulation (QAM)• Single Sideband Modulation (SSB)• Vestigial Sideband (VSB)• Frequency mixing• Superhetrodyne AM radio.• Frequency division multilplexing (FDM).
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Introduction• Modulation is a process that causes a shift in
the range of frequencies of a message signal.
• A communication that does not use modulation is called baseband communication
• A communication that uses modulation is called Carrier communication
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Example of AM transmitter
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Example of AM (radio) Receiver
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Baseband and Carrier Communication
• Baseband signal: is message signal (information bearing signal) delivered by the information source or the input transducer .it is usually low frequency signal.
• Communication that uses modulation to shift the frequency spectrum of message signal is known as carrier communication.
– Amplitude modulation (AM),
– Frequency modulation (FM)
– Phase modulation (PM)
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Amplitude Modulation (AM) Double Sideband Large Carrier (DSB-LC)
( ) ( ) cos 2 cos 2AM c cs t m t f t A f t
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Another example of AM Waveform
( ) sin 2
( ) sin 2c
m
c t Ec f t
m t Em f t
( ) ( ( ))sin 2 cS t Ec m t f t
( ) sin 2
( ) sin 2c
m
c t Ec f t
m t Em f t
( ) ( ( ))sin 2 cs t Ec m t f t
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Modulation Index
• The amount of modulation in AM signal is given by its modulation index:
max min
max min
, min ( )pp
m E Eor m m t
A E E
When mp = A , =1 or 100% modulation.
Over-modulation, i.e. mOver-modulation, i.e. mpp >A >A , should be avoided, should be avoided
because it will create distortions.because it will create distortions.
max min,p pE A m E A m
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Effect of Modulation Index
<1
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>1
=1
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Effects of Modulation Index
= 1 > 1
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Sideband and Carrier Power
• Carrier Power
• Sideband Power • Total power
• Power efficiency
• For single tone modulation
2
2c
AP
sc
s
PP
P
22
22100%, [1 ]
2 tot cP P
2m
s
PP
tot c sp P P
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Modulation index
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Example• Conventional AM signal with a sinusoidal
message has the following parameters:
A=10, =0.5, fc= 1MHz, and fm= 1kHz
1. Find time-domain expression
2. Find its Fourier transform
3. Sketch its spectrum
4. Find the signal power, carrier power and the power efficiency
5. Find the AM signal bandwidth
( )Ams t
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Example• A given AM (DSB-LC) broadcast station
transmits an average carrier power output of 40kW and uses a modulation index of 0.707 for sine-wave modulation. Calculate
a) the total output power
b) the power efficiency
c) the peak amplitude of the output if the antenna is represented by a 50- resistance load.
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Generation of AM Signalsdiode as NLE or as switch
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Square-law modulator
2( ) ( ) ( )o i iv t av t bv t
' ( ) [ 2 ( )]cos 2o cv t aA Abm t f t
3cf B To avoid overlap the spectrum of
2 ( ) and ( )cm t M f f
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Switching modulator
• Assume
1
1
'
( ) ( ) ( ),
1 2 ( 1)( ) cos 2 (2 1)
2 2 1
2( ) [ ( )]cos 2
2
o i
n
cn
o c
v t v t w t where
w t f n tn
Av t m t f t
( ) ,and diode an ideal switchm t A
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Demodulation of AM signals
• AM signals can be demodulated by
– Envelope detector
– Rectifier detector
– Coherent (synchronous) detector.
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Envelope Detector
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Envelope Detector (Cont.)
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Rectifier Detector
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Coherent detector
cos(2 )cA f t
LPFV(t)
Local oscillator
2 ( ) cosA m t
( )Ams t
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Advantages/Disadvantages of Conventional AM (DSB-LC)
• Advantages– Very simple demodulation (envelope detector)– “Linear” modulation
• Disadvantages– Low power efficiency– Transmission bandwidth twice the message
bandwidth.
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Double-sideband suppressed carrier DSBSC
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The modulating signal m(t)
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DSBSC signal: m(t) cos(ct)
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Modulated signal m(t) cos(ct)
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Example.
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DSBSC Modulators
• DSBSC signal can be generated using several types of modulators:– Multiplier Modulators
– Nonlinear Modulators
– Switching Modulators
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Multiplier modulator
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Nonlinear Modulators
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Switching Modulators
( ) cos 2 ckm t f t
a b
BPFM(t)
+
-
v2
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Switching Modulators
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Diode-bridge electronic switch
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Series-bridge diode modulator
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Shunt-bridge diode modulator
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Ring Modulator
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Ring modulator
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Demodulation of DSBSC
( ) cos(2 )cm t f t
cos(2 )cA f t
LPFV(t)
Local oscillator
2 ( ) cosA m t
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Quadrature Amplitude Modulation (QAM)
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Transmitter
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Receiver
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QAM cont.
• Quadrature multiplexing is used in color television to multiplex the signals which carry the information about colors.
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Single Sideband (SSB)
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SSB time representation
ˆ( ) ( ) cos 2 ( )sin 2 ,
:
:
1ˆ ( ) ( ) Hilpert transform of ( )
SSB c cS t m t f t m t f t
USB
LSB
m t m t m tt
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Selective filtering method
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Selective filtering method (Cont.)
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Phase–Shift Method
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Phase–Shift Method
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Hilbert transform
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Phase–Shift Method (Cont.)
• Advantages:
– Does not deploy bandpass filter.
– Suitable for message signals with frequency content down to dc.
• Disadvantage:
– Practical realization of a wideband 90o phase shift circuit is difficult.
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Demodulation of SSB Signals• Demodulation of SSB signals can be accomplished
by using a synchronous detector as used in the demodulation of normal AM and DSBSC signals.
• If we want to use an envelope detector, it can be shown that we must insert a pilot carrier signal Acos(2 fct) to the SSB signal,
where A >> m(t) and A >> m^(t).
• The pilot signal carries most of the transmission power which becomes inefficient.
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Example
• A DSB-LC signal is generated using a 1-kHz carrier and the input is m(t)= cos(200t). The modulation index is 80%. The lower sideband is attenuated (assume ideal filter). Find an expression for the resulting SSB-LC signal if it develops 0.58 W across a one-Ohm resistive load.
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Vestigial-Sideband Modulation (VSB)
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VSB modulator
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Demodulation of VSB• Demodulation of VSB signals can be
accomplished by using a synchronous detector.
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Vestigial-Sideband Modulation (VSB)
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VSB modulator
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Demodulation of VSB• Demodulation of VSB signals can be
accomplished by using a synchronous detector.
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Transfer function of LPF in VSB receiver
1( ) ,
( ) ( )LPFBPF c BPF c
H f f BH f f H f f
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VSB+C
• VSB modulated signals can also be detected by an envelope detector.
• As in the demodulation of a SSB signal, we need to send a pilot carrier signal, resulting an inefficient use of available transmitted power.
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Comparison of conventional AM, DSB-SC, SSB and VSB.
• Conventional AM: simple to modulate and to demodulate, but low power efficiency (50% max) and double the bandwidth
• DSB-SC: high power efficiency, more complex to modulate & demodulate, double the bandwidth
• SSB: high power efficiency, the same (message) bandwidth, more difficult to modulate & demodulate.
• VSB: lower power efficiency & larger bandwidth but easier to implement.
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Multiplexing• Multiplexing: combining a number of message
signals into a composite signal to transmit them simultaneously over a wideband channel.
• Two commonly-used types: time-division multiplexing (TDM) and frequency division multiplexing (FDM).
• TDM: transmit different message signals in different time slots (mostly digital).
• FDM: transmit different message signals in different frequency slots (bands) using different carrier frequencies.
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FDM
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Time Division Multiplexing
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TDM
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AM receiver for many radio stations ?
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Frequency mixing
• It is desired in communication system to translate the spectrum of the modulated signal up word or down word in frequency to be centered around desired frequency
0
0
: up conversio
: down conversio
l c
l c
c l
f f f
f f nf
f f n
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Superheterodyne AM Receiver
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• The RF amplifier amplifies the incoming signal and start the process of selecting the wanted station and rejecting the unwanted ones.
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The Mixer and the IF Amplifier
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Introduction to Carrier Acquisition
• Consider a DSB-SC demodulator where a received signal is m(t) cos(ct) and the local carrier is 2 cos[(c+) t+] . Find the LPF output if
a) =0, and
b) =0
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Carrier Acquisition• To ensure identical carrier frequencies at the
transmitter and the receiver, we can use quartz crystal oscillators, which are generally very stable.
• At very high carrier frequencies, the quartz- crystal performance may not be adequate, we can use the phased-locked loop (PLL)
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Phased-Locked Loop (PLL)
• Phase-locked loop is one of the most commonly used circuit in both telecommunication and measurement engineering.
• PLL can be used to track the phase and the frequency of the carrier component of an incoming signal.
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• A PLL has three basic components:
1. A voltage controlled oscillator
2. A multiplier
3. A loop filter H(s)
recovered carrier signal
vout(t)
vin(t) e0(t)x(t) Loop Filter
H(s)
Voltage-Controlled Oscillator (VCO)
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• In every application, the PLL tracks the frequency and the phase of the input signal. However, before a PLL can track, it must first reach the phase-locked condition.
• In general, the VCO center frequency differs from the frequency of the input signal.
• First the VCO frequency has to be tuned to the input frequency by the loop. This process is called frequency pull-in.
• Then the VCO phase has to be adjusted according to the input phase. This process is known as phase lock-in.
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How the PLL works?
)sin()( icin tAtv
)cos()( ocout tBtv
vout(t)
vin(t) e0(t)x(t) Loop Filter
H(s)
Voltage-Controlled Oscillator (VCO)
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Signal Squaring Method
( )2BPF
@ 2 c
PLL
2:1 Frequency divider
m(t) cos(c t)
k cos(c t)
c cos(2c t)
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Suggested Problems
• 4.2-1 4.2-2 4.2-3 4.2-4 4.2-6, 4.2-8
• 4.3-1 4.3-2 4.3-3 4.3-4 4.3-7 4.3-8
• 4.5-1 4.5-2 4.5-3 4.5-5, 4.5-6
• 4.6-1
• 4.8-1 4.8-2
• Read Section 4.9 (Television)