lec am modulation

Chapter 2:Modulation

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Communication System Chart

2

CommunicationSystem

Continuous Wave Digital Wave

Amplitude Modulation

(AM)

Pulse Modulation

(PM)

Angle Modulation

FrequencyModulation

(FM)

Analogue Pulse Modulation

Digital Pulse Modulation

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Modulation is defined as the process of modifying a carrier

signal (radio wave) systematically by the modulating signal

(audio)”

This process makes the signal suitable for the transmission and

compatible with the channel.

The resultant signal is called the modulated signal

MODULATION

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MODULATION

Modulated signal

Carrier signal (a transmitted electromagnetic pulse or wave high frequency of alternation on which information can be imposed by increasing signal strength, varying the base frequency, varying the wave phase, or other means)

Basebandsignal

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Demodulation is the act of extracting the original

information-bearing signal from a modulated carrier

wave

Types of Modulation

Three main type of modulations:

• Analog Modulation

▫ Amplitude modulation Example: Double sideband with carrier (DSBWC), Double

sideband suppressed carrier (DSBSC), Single sideband suppressed carrier (SSBSC), Vestigial sideband (VSB)

▫ Angle modulation (frequency modulation & phase modulation) Example: Narrow band frequency modulation (NBFM),

Wideband frequency modulation (WBFM), Narrowband phase modulation (NBPM), Wideband phase modulation (NBPM)

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Types of Modulation• Pulse Modulation

▫ Example: Pulse Amplitude Modulation (PAM), Pulse width modulation (PWM) , Pulse Position Modulation (PPM)

• Digital Modulation

▫ Modulating signal is analog Example: Pulse Code Modulation (PCM), Delta

Modulation (DM), Adaptive Delta Modulation (ADM), Differential Pulse Code Modulation (DPCM), Adaptive Differential Pulse Code Modulation (ADPCM) etc.

▫ Modulating signal is digital (binary modulation) Example: Amplitude shift keying (ASK), frequency Shift

Keying (FSK), Phase Shift Keying (PSK) etc.

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Modulation 1Analogue ModulationAmplitude Modulation

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Amplitude Modulation ~ DSBFC (Full AM)

“Amplitude Modulation is the process of changing the amplitude of the radio frequency (RF) carrier wave by the amplitude variations of modulating signal”

The carrier amplitude varied linearly by the modulating signal which usually consist of a range of a audio frequencies. The frequency of the carrier is not affected

Application of AM - Radio broadcasting, TV pictures(video), facsimile transmission

Frequency range for AM - 535 kHz – 1700 kHzBandwidth - 10 kHz

AMPLITUDE MODULATION

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Figure AM band allocation

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In amplitude modulation, the amplitude of the carrier varies proportional to the instantaneous magnitude of modulating signal

Assuming

Modulating signal : vm(t) = Vm cos wmt

vm(t) = instantenous value of the

sine wave voltage Vm(t) = peak value of the sine wave

Carrier signal : vc(t) = Vc cos wct

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AMPLITUDE MODULATION

modulatingSignal

vm(t)

Modulated Signal

Carrier waveVc cos wct

)cos()cos()( ttVVtv cmmcAM

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maxVminV

Envelope – the imaginary line on the carrier waveform

Vc max

maxVminV

V modulated signal vam

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ccccc f2 where)tcos(V)t(v

tVtv mmm cos)(

Carrier signal

Modulating signal

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The amplitude-modulated wave can then be expressed as

)cos()()cos()( ttvtVtv cmccAM

)cos()()( ttvVtv cmcAM

)cos()cos()( ttVVtv cmmcAM

tV

VtVtv m

c

mccAM cos1)cos()(

tmtVtv maccAM cos1)cos()(

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where notation m is termed the modulation index. It is

simply a measurement for the degree of modulation and

bears the relationship of Vm to Vc

c

ma V

Vm

Therefore the full AM signal may be written as

tmtVtv maccAM cos(1)cos()(

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Modulation Index m (Coefficient of Modulation/Modulation Factor/Degree of Modulation)

What is the degree of modulation required to establish a

desirable AM communication link?

Answer is to maintain m<1.0 (m<100%).

This is important for successful retrieval of the original

transmitted information at the receiver end.

Modulation Index m

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minmax

minmaxVV

VV

cVmV

ma

VmVcV

VmVcV

min

max

The modulation index can be determined by measuring the

actual values of the modulation voltage and the carrier

voltage and computing the ratio.

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maxVminV

Vm

Vc

maxVminV

Modulation Index m

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If the amplitude of the modulating signal is higher than the

carrier amplitude, which in turn implies the modulation index

. This will cause severe distortion to the

modulated signal.

%)100(0.1m

Modulation Index m

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The ideal condition for amplitude modulation (AM) is when

m=1, which also means Vm=Vc.

This will give rise to the generation of the maximum

message signal output at the receiver without distortion.

Modulation Index m

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)]cos()[cos(2/1coscos BABABA

tVm

tVm

tVtv mcca

mcca

ccAm )cos(2

)cos(2

)(cos)(

Carrier component

Upper sidebandcomponent

Lower sidebandcomponent

Using

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　　 The frequency spectrum of AM waveform contains 3 parts:

• A component at the carrier frequency fc

• An upper sideband (USB), whose highest frequency component is at fc+fm

• A lower sideband (LSB), whose highest frequency component is at fc-fm

• The bandwidth of the modulated waveform is twice the information signal bandwidth.

# sideband is a component above and below centre frequency# Every sideband contains all the original message, but not the carrier

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Amplitude Modulation

Various forms of Amplitude Modulation

• Conventional Amplitude Modulation (Alternatively known as Full AM or Double Sideband Large carrier modulation (DSBLC) /Double Sideband Full Carrier (DSBFC)

• Double Sideband Suppressed carrier (DSBSC) modulation

• Single Sideband (SSB) modulation

• Vestigial Sideband (VSB) modulation

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DSBFC Frequency Spectrum

With single frequency fm

B = Maximum freq. - minimum freq.

= (fc+fm)-(fc-fm) = fc+fm-fc+fm = 2fm

fC fc+fmfc-fm

2fm

cV

2c

a

Vm2

ca

Vm

freq

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If fm consists of a range frequencies f1 to f2, the component of the sidebands become:

Upper sideband (USB) range is from (fc+f1) to (fc+f2)

Lower sideband (LSB) range is from (fc-f2) to (fc-f1)

f1 f2fc-f2 fc-f1 fc+f1 fc+f2

Amplitude,V Amplitude,V

Baseband signal lower sideband upper sideband

Modulatedsignal

freq freq

Amplitude Modulation ~ DSBFC (Full AM)

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The previous modulated signal (DSBFC) has two drawbacks; it waste power and bandwidth Power sent as the carrier contains no information and each sideband carries the same information independently

Amplitude Modulation ~

Double Sideband Suppress Carrier (

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fc-fm fc+fm

LSB USBfreqfreq

Frequency spectrum of a DSBSC system

LSBUSBtotal pPP

Total power in DSBSC

Although, the power is improved, the bandwidth remain unchanged,

that is BW = 2B = 2 fmax

Amplitude Modulation ~ DSBSC The double sideband suppressed carrier (DSBSC) is introduced to eliminate carrier hence improve power efficiency It is a technique where it is transmitting both the sidebands without the carrier (the carrier is being suppressed)

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The suppressed carrier is further improved by sending only one sideband

This not only uses less power but also only half of the bandwidth and it is called single sideband suppressed carrier (SSBSC)

Amplitude Modulation ~ SSBSC

• As both DSB and standard AM waste a lot of power and occupy large bandwidth, SSB is adopted

• SSB is a process of transmitting one of the sidebands of the standard AM by suppressing the carrier and one of the sidebands (only transmits upper or lower sideband of AM)

• Reduces bandwidth by factor of 2 There are two possible of SSBSC

the lower sideband VLSB = Vm cos (wc-wm)t

the upper sideband VUSB = Vm cos (wc+wm)t

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Frequency spectrum of a SSB system

LSBUSBtotal pPP Total power in SSB

fc

LSB USBLSB

fc

USB

Amplitude Modulation ~ Single Sideband (SSB)

SSB Applications:

• SSB is used in the systems which require minimum bandwidth such as telephone multiplex system and it is not used in broadcasting

• Point to point communications at frequency below 30 MHz – mobile communications, military, navigation radio etc where power saving is needed

32Amplitude Modulation ~ Single Sideband (SSB)

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VSB is a technique AM transmission where the carrier, one sideband and a part of the other sideband are transmitted

VSB application:

VSB is mainly used in TV broadcasting for their video transmissions. TV signal consists of:

Audio signal – is transmitted by FMVideo signal – is transmitted by VSB

Amplitude Modulation ~ Vestigial Sideband

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A video signal consists of range of frequencies and maximum frequency is as high as 4.5Mhz.

If it is transmitted using the conventional AM system, the required bandwidth is 9.0 Mhz (B=2fm). But according to the standardization, TV signal is limited to 6MHz only.

So, to reduce to 6Mhz bandwidth, a part of the LSB is not transmitted. In this case SSB transmission is not applied as it is very difficult to suppress a sideband accurately at high frequency.


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Carrier for video

Audio Signal(FM)

Upper sidebandLowerSideband

fc-1.25 fc fc+4.54.5 MHz

Carrier for audio

Frequency spectrum of a Vestigial Sideband


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Conclusion

Only sidebands contain the information

Lower and upper sideband are identical. Only one sideband is enough to recover the original signal

Carrier component does not contain any information but constitute 2/3 of the total power, at full modulation (ma=1)

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Advantages and Disadvantages of AM

Advantages: simple with proven reliability low cost

Disadvantages: wastage of power as most of the transmitted power are in the carrier component which does not contain information. When ma=1, 2/3 of the power is wasted AM requires a bandwidth which is double to audio frequency Noisy

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The total transmitted power in

AM is the sum of the carrier

power and the power in the

sidebands.

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2a

c

SBc

LSBUSBctotal

mP

PP

pPPP

Carrier power :

R

VP cc 2

2

Sideband power: 4

2ca

LSBUSBSB

PmPPP

2

2ca

LSBUSBSB

PmPPP

AM Power Distribution

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2

2ca

LSBUSBSB

PmPPP

Thus, at optimum operation (m = 100%), only 33% of power is used to carry information

From previous equation, total current flow in AM is

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As most of the signals are complex and can be represented by combination of various sine waves, m can be determined by

Thus, total power for this complex signal is

......23

22

21 mmmmm effa

]2

1[2eff

cT

mPP

lec am modulation

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