Communication System Icontents.kocw.net/KOCW/document/2014/hanyang/namhaeun/6.pdfECC1015 Communication System I Communication System I Electronics and Communication Engineering Hanyang

ECC1015 Communication System I

Communication System I

Electronics and Communication EngineeringHanyang University

Haewoon Nam

Lecture 6

(ECC1015)

1


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


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)



• 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.



• 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



• 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.



• A simple sinusoidal message signal

7[Ref] Introduction to Analog and Digital Communications,

by Haykin and Moher, Wiley



• 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.



• 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














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


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


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


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 ++−=



• DSB-SC





• Coherent detection (synchronous demodulation)

– Using the trigonometric identity

– Through the low pass filter



)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 −++=



• 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 +=



• Same examplein Slide 9









• 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




Announcement and Assignment

• Reading assignment– Amplitude Modulation

23

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Communication System Icontents.kocw.net/KOCW/document/2014/hanyang/namhaeun/6.pdfECC1015 Communication System I Communication System I Electronics and Communication Engineering Hanyang