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ECC1015 Communication System I
Communication System I
Electronics and Communication EngineeringHanyang University
Haewoon Nam
Lecture 6
(ECC1015)
1
ECC1015 Communication System I
Modulation
• Modulation – The process by which some characteristic of a carrier wave is
varied in accordance with an information-bearing signal.– Continuous-wave modulation
• Amplitude modulation• Frequency modulation
• AM modulation family– Amplitude modulation (AM)– Double sideband-suppressed carrier (DSB-SC)– Single sideband (SSB)– Vestigial sideband (VSB)
2
ECC1015 Communication System I
Amplitude Modulation
• A sinusoidal carrier signal
• An amplitude-modulated signal
3
)cos()( tfAtc cc 2π=
)cos()]([)( tftmkAts cac 21 π+=
cA : carrier amplitude
cf : carrier frequency
ak : amplitude sensitivity
)(tm : message signal
)(tm )(ts
)cos( tfA cc 2π
Xak
+
(DC)1
(carrier)
ECC1015 Communication System I
Amplitude Modulation
• Without DC addition
• To avoid signal distortion when envelope detector is used, the following condition needs to be met.
4
)(tm )(ts
)cos( tfA cc 2π
Xak
(carrier)
Phase reversals
)cos()()( tftmkAts cac 2π=
1<|)(| tmka Therefore DC is added.
ECC1015 Communication System I
Amplitude Modulation
• Amplitude modulated signal
• Fourier transform of the signal
5
)cos()]([)( tftmkAts cac 21 π+= )cos()()cos( tftmkAtfA caccc 22 ππ +=
)]()([)]()([)( ccac
ccc ffMffMkAffffAfS 2 2 −+++−++= δδ
[Ref] Introduction to Analog and Digital Communications,by Haykin and Moher, Wiley
ECC1015 Communication System I
Amplitude Modulation
• A simple sinusoidal message signal
• Amplitude modulated signal
• To avoid signal distortion, the modulation factor must be kept below unity.
6
)cos()]cos([)( tftfAkAts cmmac 221 ππ+=
)cos()( tfAtm mm 2π=
)cos()]cos([ tftfA cmc 221 ππμ+=
ma Ak=μwhere is the modulation factor or percentage modulation.
ECC1015 Communication System I
Amplitude Modulation
• A simple sinusoidal message signal
7[Ref] Introduction to Analog and Digital Communications,
by Haykin and Moher, Wiley
ECC1015 Communication System I
Amplitude Modulation
• The average power delivered to a 1-ohm resistor
• The percentage of total power
8
cA 221=Carrier power
Upper side-frequency power cAμ281 2=
Lower side-frequency power cAμ281 2=
When the percentage modulationis less than 20 percent, the powerin one side frequency is less than
1 percent of the total power in the AM wave.
ECC1015 Communication System I
Amplitude Modulation
• An example
• The modulation factor
9
1=cA
50.=μ : under-modulation
Carrier frequency
Carrier amplitude
Modulation frequency
Hzfc 40.=Hzfm 050.=
: 100% modulation
: over-modulation
01.=μ 02.=μ
ECC1015 Communication System I
Amplitude Modulation
10[Ref] Introduction to Analog and Digital Communications,
by Haykin and Moher, Wiley
ECC1015 Communication System I
Amplitude Modulation
11[Ref] Introduction to Analog and Digital Communications,
by Haykin and Moher, Wiley
ECC1015 Communication System I
Amplitude Modulation
12[Ref] Introduction to Analog and Digital Communications,
by Haykin and Moher, Wiley
ECC1015 Communication System I
Envelope Detection
• Enveloping detector– The AM wave is narrowband, which means that the carrier frequency is
large compared to the message bandwidth– The percentage modulation in the AM wave is less than 100 percent– So that the capacitor C charges rapidly and thereby follows the applied
voltage up to the positive peak when the diodes is conducting .
13
)cos()(ˆ tfAtm mm 2π≈
)cos()]cos([)( tftfAkAtr cmmac 221 ππ+=
)]cos()(ˆ tfAkAAtr mmacc 2π+≈
Envelope detection (filtering)
DC removal
ECC1015 Communication System I
Envelope Detection
• Example
• Conditions for envelope detector
14
kHzfkHzW
FCkR
rR
c
l
f
s
20 frequency Carrier 1 bandwidth Message 010 eCapacitanc 10 resistance Load 25 resistance Forward 75 resistance Source
==
=Ω=
Ω=Ω=
μ.
csf f
CRr 1)( <<+
Diode resistance
Charging time constant
WCR
f lc
11 <<<<
Discharging time constant
ECC1015 Communication System I
Virtues and Limitations of AM
• Advantage– Simple receiver
• Envelope detection (a simple low pass filter) allows the information recovery• Disadvantage (practical limitation)
– Amplitude modulation is wasteful of transmitted power• The transmission of the carrier wave therefore represents a waste of power
– Amplitude modulation is wasteful of channel bandwidth• It requires a transmission bandwidth equal to twice the message bandwidth
• Three modifications of amplitude modulation– Double sideband-suppressed carrier (DSB-SC) modulation
• The transmitted wave consists of only the upper and lower sidebands– Single sideband (SSB) modulation
• The modulated wave consists only of the upper sideband or the lower sideband
– Vestigial sideband (VSB) modulation• One sideband is passed almost completely and just a trace, of the other
sideband is retained
15
ECC1015 Communication System I
Double Side Band Suppressed Carrier
• Theory– DSB-SC (product modulation) consists of the product of the message
signal and the carrier wave,
– Fourier transform of s(t)
– Its only advantage is saving transmitted power, which is important enough when the available transmitted power is at a premium
16
)8.3()()2cos( )()()(
tmtfAtmtcts
cc π==
)9.3()]()([21)( ccc ffMffMAfS ++−=
ECC1015 Communication System I
Double Side Band Suppressed Carrier
• DSB-SC
17[Ref] Introduction to Analog and Digital Communications,
by Haykin and Moher, Wiley
ECC1015 Communication System I
Double Side Band Suppressed Carrier
• Coherent detection (synchronous demodulation)
– Using the trigonometric identity
– Through the low pass filter
18[Ref] Introduction to Analog and Digital Communications,
by Haykin and Moher, Wiley
)cos()cos()()cos()()( φππφπυ +′=+′= tfAtfAtmtfAtst cccccc 222)}cos(){cos()()( φφπυ ++′= tfAAtmt ccc 421
)cos()()( φυ 21ccO AAtmt ′=
)cos(21)cos(
21)cos()cos( 212121 θθθθθθ −++=
ECC1015 Communication System I
Double Side Band Suppressed Carrier
• Coherent detection (synchronous demodulation)– The recovery of the message signal m(t) can be accomplished by first
multiplying s(t) with a locally generated sinusoidal wave and then low-pass filtering the product.
• The quadrature null effect– The zero demodulated signal, when occurs for Φ=±π/2– The phase error Φ in the local oscillator causes the detector output to be
attenuated by a factor equal to cos Φ
19
)2cos(21
21)(cos2 θθ +=
ECC1015 Communication System I
Double Side Band Suppressed Carrier
• Same examplein Slide 9
20[Ref] Introduction to Analog and Digital Communications,
by Haykin and Moher, Wiley
ECC1015 Communication System I
Double Side Band Suppressed Carrier
21[Ref] Introduction to Analog and Digital Communications,
by Haykin and Moher, Wiley
ECC1015 Communication System I
Double Side Band Suppressed Carrier
• Costas receiver– Consists of two coherent detectors supplied with the same input signal
• Two local oscillator signals that are in phase quadrature with respect to each other
• I-channel : in-phase coherent detector• Q-channel :
quadrature-phase coherent detector
– Phase control in the Costas receiver ceases with modulation,
• Which means that phase-lock would have to be re-established with the reappearance of modulation
22[Ref] Introduction to Analog and Digital Communications,
by Haykin and Moher, Wiley
ECC1015 Communication System I
Announcement and Assignment
• Reading assignment– Amplitude Modulation
23
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