ECC1015 Communication System I

Communication System I

Electronics and Communication EngineeringHanyang University

Haewoon Nam

Lecture 5

(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)

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ECC1015 Communication System I

Amplitude Modulation

• A sinusoidal carrier signal

• An amplitude-modulated signal

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)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.

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)(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

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)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.

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)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

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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

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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 .

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)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

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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

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ECC1015 Communication System I

Announcement and Assignment

• Reading assignment– Amplitude Modulation

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