Amplitude Modulation HO Colour

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Amplitude Modulation

Module I

Syllabus – Module IIntroduction–communication process, source of

information, communication channels; Modulation –need, band width requirements – electromagneticspectrum. Amplitude modulation – principles – visualconcepts, modulation factor and percentage ofmodulation, mathematical relationship, componentphasors, frequency spectrum, band selection.p , f q y p ,Amplitude modulators – ISB modulators – VSBmodulation. AM transmitters – low level, high level –SSB systems – comparisons, mathematical analysis, SSBgeneration –SSB transmitters – filter method, phaseshift method, third method. AM receivers – TRF receivers,Super heterodyne receiver, Double Super heterodynereceiver – SSB receiver – BFO, envelope detection, multi-channel Pilot carrier.

International Telecommunications Union Band DesignationsBand Number Frequency Range Designations

2 30Hz-300Hz ELF (Extremely low frequencies)

3 0.3KHz-3KHz VF (Voice frequencies)

4 3KHz-30KHz VLF (Very low frequencies)

5 30KHz-300KHz LF (Low frequencies)

6 0.3MHz-3MHz MF (Medium frequencies)

7 3MHz-30MHz HF (High frequencies)

8 30MHz-300MHz VHF (Very high frequencies)

9 0.3GHz-3GHz UHF (Ultra high frequencies)

1/12/20103

10 3GHz-30GHz SHF (Super high frequencies)

11 30GHz-300GHz EHF (Extremely high frequencies)

12 0.3THz-3THz Infrared light

13 3THz-30THz Infrared light

14 30THz-300THz Infrared light

15 0.3PHz-3PHz Visible light

16 3PHz-30PHz Ultra violet light

17 30PHz-300PHz X Rays

18 0.3EHz-3EHz Gamma Rays

19 3EHz-30EHz Cosmic Rays

International Telecommunications Union Band Designations

Frequency Applications

ELF (Extremely low frequencies) Low frequency telemetry

VF (Voice frequencies) Telephone Channels

VLF (Very low frequencies) Submarine communications

LF (Low frequencies) Marine & Aeronautical navigation

MF (Medium frequencies) Commercial AM broadcasting

HF (High frequencies) Short wave radio, Amateur radio broadcasting

VHF (Very high frequencies) Commercial FM and TV broadcasting

UHF (Ultra high frequencies) TV, mobile communications, navigation systems

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SHF (Super high frequencies) Microwave and satellite radio communications

EHF (Extremely high frequencies) Used for very sophisticated, specialized applications

Infrared lightHeat seeking guidance systems, electronic photography and astronomy

Infrared light

Infrared light

Visible light Optical fibre systems

Ultra violet light

Not used for electronic communicationsX Rays

Gamma Rays

Cosmic Rays

MultipliersFactor Name Symbol

1024 yotta Y

1021 zetta Z

1018 exa E

1015 peta P

Factor Name Symbol

10-1 deci d

10-2 centi c

10-3 milli m

10-6 micro P

1012 tera T

109 giga G

106 mega M

103 kilo k

102 hecto h

10 deca da

10-9 nano n

10-12 pico p

10-15 femto f

10-18 atto a

10-21 zepto z

10-24 yocto y

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Wired CommunicationsTelegraphFixed line telephoneCableWired networksInternetFiber communicationsCommunication bus inside computers to communicate between CPU and memory

Wireless CommunicationsSatellite TVCordless phoneCellular phoneWireless LAN, WIFIWireless MAN, WIMAXBluetoothUltra Wide BandWireless LaserMicrowaveGPSAd hoc/Sensor Networks

Wireless CommunicationsA code is used to designate the types of signals that can be transmitted by radio and wire.The code is made up of a capital letter, a number, and another capital letter as shown in the table.Examples of codes:

A3F: amplitude-modulated analog TVJ3E: SSB voiceF2D: FSK dataG7E : phase-modulated voice, multiple signals

FCC Emission Classifications

Symbol

Letter

Type of modulation Symbol

Letter

Type of modulationFIR

ST

N Unmodulated carrier

TH

A Manual telegraphy

A Double sideband full carrier B Automatic telegraphy

B Independent sideband full carrier C Facsimile

C Vestigial sideband full carrier D Data, telemetry

H Single sideband full carrier E Telephony

J Single sideband suppressed carrier

F Television

R Single sideband reduced carrier W No information

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HIR

D

R Single sideband reduced carrier W No information

F Frequency modulation

G Phase modulation

D AM and FM simultaneously

SECO

ND

0 No modulating signal

1 Digitally keyed carrier

2 Digitally keyed tone

3 Analog sound or video

7 Two or more digital channels

8 Two or more analog channels

9 Analog and digital EE 541/451 Fall 2007

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Need for modulation1. To Reduce Antenna Size

2. To Remove Interference

3. To Reduce Noise

4. To Perform Multiplexing

5. To Match the Passband

Characteristics of the Channel

6. To Increase Range

Antenna SizeFor efficient transmission and reception ofelectromagnetic signals the transmitting and receivingantennas must have lengths equal to a quarterwavelength of the frequency used.As an example consider an AM broadcast system withmaximum audio frequency 5KHz.q yIf this message audio signal is to be transmitted withoutmodulation, the height of the antenna required will be

4l λ=

cWheref

λ =83 10 /velocity m s×→

frequency→

Antenna Size

When the same audio signal is modulated on a carrier of say 10 mHz

4clf

=8

3

3 104 5 10

×=

× ×15Km=

CLEARLYIMPRACTICAL

say, 10 mHz

4clf

=8

6

3 104 10 10

×=

× ×7.5m=

ANTENNA LEGNTH IS REALISTIC

Removal of InterferenceThe frequency of audio signal is from 20Hz to 20 KHzSuppose several radio stations are transmitting audiosignals without modulation in this frequency range.Then these programmes will get hopelessly mixed up.

T id thi h t hift th i di id lTo avoid this we have to shift the individualtransmissions to different portions of the electromagneticspectrum.Each station is allocated a band of frequencies and theaudio signal is to be shifted to this band by modulation.

Removal of Interference

STATION1800KHz-810KHz



AUDIO20Hz-20KHz

AUDIO20Hz-20KHz

AUDIO20Hz-20KHz

Reduction of Noise

Most of the man made noise produced by automobileignition, heavy machinery etc has frequency contentmore in the base band region.

If the message spectrum is shifted to relatively quieterIf the message spectrum is shifted to relatively quieterportions of the electromagnetic spectrum it will bepossible to attain better signal to noise ratio.

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MultiplexingWhen we have to send different signals through thesame communication channel without mutualinterference, we may use a process called multiplexing.

Different signals are shifted to different amounts alongthe frequency spectrum so that they do not interfere withthe frequency spectrum so that they do not interfere witheach other.

Matching the characteristics of the channelDifferent channels have different pass bands .If we have to pass a particular signal through a specifiedchannel we have to shift the frequency of the signal tothe pass band of the channel.

Amplitude ModulationAmplitude modulation is a system of modulation in whichthe amplitude of the carrier is made proportional to theinstantaneous amplitude of the modulating signal.Let the carrier voltage and the modulating voltage berepresented by

cosc c cv V tω= carrier signal⇒

The ratio of modulating signal amplitude Vm and thecarrier amplitude Vc is defined as modulation index, m.

c c c

cosm m mv V tω=g

modulating signal⇒

m

c

Modulation i ex mn VdV

=

Amplitude ModulationDuring the process of modulation the amplitude of thecarrier is modified by the instantaneous amplitude of themodulating signal.So the amplitude of the modulated signal is

c mA V v= +c m

cosc m mV V tω= + m

cm c

Vm V VV

m= ⇒ =

cosc c mV mV tω= +

( )1 cosc mV m tω= +

Amplitude ModulationThe modulated signal is also a sinusoid and itsinstantaneous value is given by

cos cv A tω=

( )1 cos cosc m cV m t tω ω= +

The above equation may be expanded by thetrigonometric identity

cos cos cosc c c c mV t mV t tω ω ω= +

( ) ( )2 cos cos cos cosA B A B A B= + + −

Amplitude Modulation

The equation of an amplitude modulated wave containsthree terms:

( ) ( )cos cos cos2 2

c cc c c m c m

mV mVv V t t tω ω ω ω ω= + + + −

( ) cosc ci V tω Unmodulated carrier⇒( ) c c

( )( ) cos2

cc m

mVii tω ω+

( )( ) cos2

cc m

mViii tω ω−

Upper sideband⇒

Lower sideband⇒

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Amplitude ModulationThis equation reveals that the AM signal has threefrequency components as below

Carrier frequency ωc of amplitude Vc

Upper side band (ωc + ωm) having amplitude (mVc /2)Lower side band (ωc - ωm) having amplitude (mVc /2)

Amplitude modulation

EE 541/451 Fall 2007

Time-frequency-amplitude plot Amplitude modulation

28

Under modulated (<100%) 100% modulated Over Modulated (>100%)Vm=VC Vm>VC

Vm<VC

Spectrum of amplitudemodulated signal

cos cos cosc c c m cv V t mV t tω ω ω= +

1 32

{ } ( )( ) cos2

c cj t j tcC c c

VX F V t F e eω ωω ω −⎧ ⎫= = +⎨ ⎬⎩ ⎭

2 ( )cj t Fce ω πδ ω ω←⎯→ − 2 ( )cj t F

ce ω πδ ω ω− ←⎯→ +

cosc cFourier transform of V tω

[ ]( ) 2 ( ) 2 ( )2

cC c c

VX ω πδ ω ω πδ ω ω= − + +

( ) ( )c c c cV Vπ δ ω ω π δ ω ω= − + +

( ) ( ) ( )C c c c cX V Vω π δ ω ω π δ ω ω= − + +

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cVπcVπ

cosc cSpectrum of V tω

cω− cω0

cosc mFourier transform of mV tω{ }( ) cosm C mLet X F mV tω ω=

( ) ( ) ( )X m V m Vω π δ ω ω π δ ω ω+ +

As in the previous case

( ) ( ) ( )m c m c mX m V m Vω π δ ω ω π δ ω ω= − + +

{ } { }cos cos ( )cosc m c cThen FT mV t t FT x t tω ω ω=

( )⎭⎬⎫

⎩⎨⎧ += − tjtj cc eetxFT ωω)(

21

⎬⎫

⎨⎧

⎬⎫

⎨⎧ tjtj ωω )(1)(1

cos cosc m cSpectrum of mV t tω ω( ) cosc mLet x t mV tω=

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⎭⎬⎫

⎩⎨⎧+

⎭⎬⎫

⎩⎨⎧= − tjtj cc etxFTetxFT ωω )(

21)(

21

By frequency shifting property,

{ } { }{ }

F x(t) ( ), F x(t)e ( )

F x(t)e ( )

c

c

j tc

j tc

If X X

X

ω

ω

ω ω ω

ω ω−

= = −

= +

cos cosc m cSpectrum of mV t tω ω

{ } 1 1( )cos ( ) ( )2 2c m c m cFT x t t X Xω ω ω ω ω= − + +

( ) ( ) ( )m c m c mBut X mV mVω π δ ω ω π δ ω ω= − + +

{ })cos(FT x tt ω{ }

{ }

{ }

1 ( ) ( )2

1 ( ) (

)cos

)2

(

c m c c m c

c m c c

c

m c

mV mV

mV m

FT x t

V

t

π δ ω ω ω π δ ω ω ω

π δ ω ω ω π ω

ω

δ ω ω

= − − + + −

+ − + + + +

{ }

( ( ) ( ( )2 2

( ( ) ( (

( )

))

cos

c cc m c m

c c

cFTmV mV

mV m

x t

V

tπ πδ ω ω ω δ ω ω ω

π πδ ω ω ω δ ω ω ω

ω

= − + + − −

+ + + + +

cos cosc m cSpectrum of mV t tω ω

( ( ) ( ( ))2 2c m c mδ ω ω ω δ ω ω ω+ + − + + +

Resultant spectra

2cmVπ

ω ω

2cmVπ

mc ωω +mc ωω −ωω +−ωω −− 0cω− cω

0

mc ωωmc ωωmc ωω +−mc ωω −−

cVπcVπ

cω−cω

0

ADD THESE TWO

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Resultant spectra

2cmVπ

ω ω

2cmVπ

mc ωω +mc ωω −ωω +−ωω −−

cVπcVπ

0

2cmVπ

2cmVπ

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cω− cω mc ωωmc ωωmc ωω +−mc ωω −− 0

signal modulated tonesingle of Spectrum

Spectrum of multiple frequency modulation

X(ω)Multiple frequency modulating signal

Resultant t

cω− cω0

spectrum

Double sideband full carrier AM modulation

Usually the spectrum of the base band signal iscontinuous as in the case of audio signals whichoccupy a continuous band of frequencies from20 Hz to 20 KHz.The spectrum of the base band signal and theresultant AM modulated wave is as shown:


mωmω−cVπ cVπ

cω− cωmc ωω +−

cω− cω

mc ωω +mc ωω −mc ωω −−

cVπ cVπ


There is a replication of the spectrum of theoriginal signal centered around +ωc and –ωc.These are called upper sideband and lowersideband.When ωc< ωm there will be overlap of the spectraand x(t) cannot be recovered from the modulatedsignal.For effective signal recovery ωc> ωm

Modulation index

mV

mVcV

max min

2mV VV −

=

max mcV V V= −

EE 541/451 Fall 2007

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8

Modulation Index

max min

2mV VV −

=

max mcV V V= − max minmax 2

V VV −⎡ ⎤= − ⎢ ⎥⎣ ⎦max min

2V V+

=

m

c

Modulation i ex mn VdV

= max min

max min

V VV V

−=

+

max min

max min

V VmV V

−=

+

Power RelationsThe total power in the modulated wave is

t carr LSB USBP P P P= + +2 2 2

carr LSB USBV V VR R R

= + +R R R

Where the voltages are rms converted valuesand R is the resistance in which power is

. .dissipated e g antenna resistance

Power Relations2

carrcarr

VPR

=( )2

2cV

R=

2

2cVR

=

2V 2 2m V

222cmV⎡ ⎤

⎢ ⎥⎣ ⎦LSBLSB

VPR

=8

cm VR

=2

22

2c

USBUSB

mVVP

R R

⎡ ⎤⎢ ⎥⎣ ⎦= =

2 2

8cm V

R=

2R

⎢ ⎥⎣ ⎦=

Power Relations

t carr LSB USBP P P P= + +2 2 2 2 2

2 8 8c c cV m V m VR R R

= + +

2 22 22

2 2 4 24c c cmV

RmV V

R R= + +

2 2

4 4c c cP P Pm m= + +

2

12 c

m P⎛ ⎞= +⎜ ⎟⎝ ⎠

Power Relations

2

12t c

mP P⎛ ⎞= +⎜ ⎟⎝ ⎠

2

1tP m

2

12t c

mP P⎛ ⎞= +⎜ ⎟⎝ ⎠

2

1tP m1

2t

cP= +

1When m power is maximum=1.5max cP P=

12

t

cP= +

Current relationsIn many situations it is easier to measure currents, so it will be worthwhile to express the preceding equations in terms of currents.

2

2t t

c c

P I RP I R=

2 2

12

t

c

I mI

⎛ ⎞= = +⎜ ⎟⎝ ⎠

2

12

t

c

I mI= +

2

12t c

mI I= +

2

2 1t

c

ImI

⎛ ⎞⎡ ⎤⎜ ⎟= −⎢ ⎥⎜ ⎟⎣ ⎦⎝ ⎠

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9

Effect of non-linear resistance The relationship between voltage and current in a linear resistance is

i bv=Conductancei

v

Effect of non-linear resistance The relationship between collector current and basevoltage in a transistor working in its linear region is

i a bv= +

dc collector Transconductance

In a non-linear resistance the relationship betweenvoltage and current can be represented as

dc collectorcurrent

Transconductance of transistor

2 3i a bv cv dv higher powers= + + + +

Effect of non-linear resistance In most of the practical non-linear resistances, the coefficients are such that

a b c d higher order terms>> >> >> >>

Effect of non-linear resistance i

Positive c Negative c

v2i a bv cv= + +

a b c d higher order terms>> >> >> >>

Effect of non-linear resistance The initial portion the curve is almost linear since thevalues of c,d etc are very small.At larger values of v the term containing c becomessignificant.Now current become proportional to the square of thevoltage.voltage.Similarly at higher values of voltage the terms containingd,e etc becomes significant.Now current become proportional to higher powers ofvoltage.For most applications only the terms up to the squareterm is large enough to be taken in to consideration.

Effect of non-linear resistance SoLet us apply this equation to the non-linear portion of thegate voltage – drain current characteristics of an FET.If two voltages are applied simultaneously to the gate,

2i a bv cv= + +

( ) ( )21 2 1 2i a b v v c v v= + + + +

Let the two input voltages be sinusoidal

( ) ( )1 2 1 2

( ) ( )2 21 2 1 2 1 22a b v v c v v v v= + + + + +

1 1 sinv V tω= 2 2 sinv V tρ=

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Effect of non-linear resistance ( )1 2sin sini a b V t V tω ρ= + +

( )2 2 2 21 2 1 2sin sin 2 sin sinc V t V t VV t tω ρ ω ρ+ + +

1sin sin cos( ) cos( )2

A B A B A B= − − +

1 cos2 A

U

S2 1 cos2sin

2AA −

=

1 2sin sini a bV t bV tω ρ= + +

( )1 2 cos( ) cos( )cVV t tω ρ ω ρ+ − − +

( ) ( )2 21 2

1 11 cos2 1 cos 22 2

cV t cV tω ρ+ − + −

E

Effect of non-linear resistance 2 2

1 2 211 si12

n2

sini a cV tt bVbVcV ρω⎛ ⎞= + + + +⎜ ⎟⎝ ⎠

2 21 12 2V t V t⎛ ⎞⎜ ⎟

Term I Term II Term III

2 21 2cos2 cos2

2 2cV t cV tω ρ⎛ +− ⎞

⎜ ⎟⎝ ⎠

2 1 21 cos( ) cos( )cVV t cVV tω ρω ρ+ − +−

Term IV

Term V Term VI

Effect of non-linear resistance Term I DC component

Term II Carrier

Term III Modulating signal

Term IV Harmonics of the carrier and modulating signal

Term V Lower sideband

Term VI Upper sideband

When two frequencies are passed through anon linear resistance the process of amplitude−modulation takes place

Medium power AM modulator

1 1 : 1T =

Vcc

RFCSingle frequencym odulating signal

2 1 : 1T =

unm odulated carrier

1Q

1R 1C


cV0

0.7

ci

outVVcc

0

Vcc

Vcc

Vm Vcc

Vout

Vcc

Vcc

2Vcc Vout

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11

i

cV0

0.7

ci

outV Vcc

0

2Vcc


When the amplitude of the carrier exceeds the barrierpotential of the base-emitter junction of the transistor( say, 0.7V), Q1 turns on and collector current flows.When the amplitude of the carrier drops below 0.7V Q1turns off and collector current ceases.Q1 switches between saturation and cutoff controlled bythe carrier signal.Collector current flows for less than 1800 of each carriercycle and class c operation is achieved.When Q1 is off collector voltage is Vcc and when itconducts, collector voltage drops.

Medium power AM modulatorWhen a modulating signal is applied to the collector inseries with DC supply voltage, it adds to and subtractsfrom Vcc.Assuming that maximum peak modulating signal voltageis Vcc, the output voltage waveform swings from amaximum value of 2 Vcc to 0 volts.In this way, the low frequency modulating signal issuperimposed on the high frequency carrier.Since Q1 operates nonlinear, the collector waveformcontains the two original frequencies, fm and fc and theirsum and difference frequencies fc ± fm and high orderharmonics.


3C

Single frequencymodulating signal

1 1 : 1T =

Vcc

1C

L

2T

gC

2 1 : 1T =

unmodulated carrier 1R

1Q

2C

NC

1LbcC

i

cV0

0.7

ci

outv

2Vcc+

2Vcc−

Medium power AM modulatorThis circuit is also a collector modulator with a maximumpeak modulating signal amplitude Vcc.Operation is similar to the first circuit except for theaddition of a tank circuit in the collector of Q1.Because the transistor is operating between saturationand cutoff, collector current is not dependent on basedrive voltage.The output voltage is a symmetrical DSBFC AM signalwith an average voltage zero.Maximum positive peak amplitude equals to 2Vcc andthe maximum negative peak amplitude equals -2Vcc.

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Medium power AM modulatorThe positive half cycle of the output waveform is producedin the tank circuit by the fly wheel effect.When Q1 is conducting C1 charges to Vcc+Vm(a maximum of 2Vcc), and when Q1 is OFF, C1 dischargesthrough L1.When L1 discharges, C1 charges to a minimum value of2Vcc-2Vcc.

This produces the positive half-cycle of the AM envelope.The resonant frequency of the tank circuit is equal to thecarrier frequency, and the bandwidth extends from fc-fm tofc+fm.As a result, modulating signal, harmonics, and all thehigher order cross products are removed from thewaveform.

Medium power AM modulatorR1 and C2 form a clamper circuit that produces a reverseself bias, and this bias determines the turn on voltage forQ1.C3 is a bypass capacitor that looks like a short to themodulating signal voltages, preventing it from enteringthe DC power supply.Cbc is the base-collector junction capacitance of Q1.At high frequencies, this capacitance becomessignificant, and due to the positive feedback introducedby this capacitor, Q1 may oscillate.Therefore a signal of equal amplitude and frequency and1800 out of phase must be fed back to the base toprevent oscillations.

Medium power AM modulatorCN, which is called a neutralizing capacitor, is used forthis purpose.Cg is an RF bypass capacitor whose function is toprevent the carrier from leaking in to the power supply.

High level and Low level modulation The location of the point in a transmitter where themodulation occurs determines whether the transmitter islow level or high level.If the modulation takes place at any point prior to theoutput element of the final stage of the transmitter, wecall it low level modulation.That is, prior to the۩ Collector of the output transistor in a transistorised

transmitter.۩ Drain of the output FET in an FET transmitter.۩ Plate of the output tube in a vacuum tube transmitter.

High level and Low level modulation Low level modulation requires less modulating signal powerto achieve a high percentage of modulation.Since all the amplifiers that follow the modulator stage mustbe linear amplifiers with lesser efficiency, a low levelmodulator is not suitable for high power applications.If the modulation takes place in the final element of the finalstage where the carrier signal is at its maximum amplitudewe call it high level modulation.High level modulation requires a much higher amplitudemodulating signal to achieve a reasonable percentage ofmodulation.Here the final modulating amplifier must supply all thesideband power.

Amplitude modulated transmitters (Low level)

RF CarrierOscillator

BufferAmplifier

ModulatingSignalSource

BandpassFilter Preamplifier

ModulatingSignalDriver

CarrierDriver

Modulator

LinearIntermediatePower amp

Linearfinal

Power ampBandpass

FilterCouplingNetwork

BandpassFilter

Antenna

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Amplitude modulated transmittersThe source of the modulating signal is generally anacoustical transducer such as a microphone, a magnetictape or a CD.The band pass filter is used to band limit the input signalsuch that all frequency contents outside the intendedbaseband is blocked.The function of the preamplifier is to raise the amplitudeof the source signal to a usable level while producingminimum non-linear distortion and adding as little noiseas possible.The driver for the modulating signal is also linearamplifier that amplifies the information signal to anadequate level to drive the modulator.

Amplitude modulated transmittersThe RF carrier oscillator can be an LC oscillator like Hartley or Colpitts or a crystal oscillator.The buffer amplifier is a low gain, high input impedance linear amplifier.Its function is to isolate the oscillator from the high power amplifiers and to reduce frequency or amplitude p q y pvariations.The carrier driver provides enough current drive to operate the modulator.A band pass filter is used at the output of the modulator to remove the unwanted frequency components produced during modulation.

Amplitude modulated transmittersThe intermediate and final power amplifiers are eitherlinear class A or class B push pull amplifiers.The antenna coupling network matches the outputimpedance of the final power amplifier to thetransmission line and to the antenna.

Amplitude modulated transmitters( high level)

RF Carrier Buffer

ModulatingSignalSource

BandpassFilter Preamplifier

ModulatingSignalDriver

Carrier AM ModulatorCarrier

ModulatingSignal

Poweramp

Oscillator Amplifier Driver And outputPoweramp

BandpassFilter

CouplingNetwork

Poweramp

Antenna

Amplitude modulated transmittersThe modulating signal is processed in the same manner asin a low level transmitter except for the addition of a poweramplifier.With high level modulators the power of the modulatingsignal must be considerably higher than that required forlow level modulators.The carrier is at full power at the point in the transmitterwhere modulation occurs, and consequently requires a highamplitude modulating signal to produce 100% modulation.The carrier oscillator, buffer, driver etc are the same as inlow level modulation but an additional power amplificationstage is introduced.The final power amplifier is also the modulator.

AM band allocation

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14

Evolution of SSBWhen a carrier is amplitude modulated by a single sinewave, the resulting signal consists of three maincomponents:

The original carrier frequencyThe upper sideband frequencyThe lower sideband frequency

The carrier component remains constant in amplitude andThe carrier component remains constant in amplitude andfrequency and conveys no information .The two sidebands are images of each other and the sameinformation is conveyed by both of them.So the carrier and one of the sidebands are redundant.If the carrier is suppressed such type of modulation iscalled Double Side Band Suppressed Carrier modulation.

Evolution of SSBThe ordinary AM is called Double Side Band Full Carriermodulation.When the carrier and one of the sidebands aresuppressed such modulation is called Single Side BandSuppressed Carrier modulation.By suppressing the carrier and/or side bands transmittedpower and band width can be reduced considerably.But the transmitters and receivers for these systems arecomplex and expensive when compared to DSBFC.These systems are not compatible with the DSBFCreceivers which are widely used.

Types of AM modulationDSBFC

DSBSC

SSBFC

SSBRC

SSBSC

VSB

ISB

DSBFCUpper

Sideband

Lower Sideband

Unattenuated carrier

cω− cωMc ωω −− Mc ωω +− Mc ωω +Mc ωω −

DSBSCUpper

Sideband

Lower Sideband

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SSBFCUpper

SidebandUpper Sideband


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15

SSBFC

Lower Sideband

Lower Sideband


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SSBRCUpper


Reduced carrier

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SSBRC

Lower Sideband

Lower Sideband

Reduced carrier

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SSBSCUpper


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SSBSC

Lower Sideband

Lower Sideband

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ISBUpper

Sideband

Lower Sideband

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Ch1 Ch1Ch2 Ch2

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VSB

Vestige ofUpper

Sideband

Vestige ofUpper

Sideband

Lower sideband

Lower sideband

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DSBFC

Vestige ofLower

Sideband

Vestige ofLower

Sideband

Upper sideband

Upper Sideband

1/12/201093


DSBSC in frequency domain

Suppressed carrier AM signal (DSB)Full carrier AM signal

1

-0.5

0

0.5

1

1.5

2

Vol

tage

(V)

1

-0.5

0

0.5

1

1.5

2

0 1000 2000 3000 4000 5000 60000

0.1

0.2

0.3

0.4

0.5

Frequency (Hz)

-2

-1.5

-1

Time (sec)-2

-1.5

-1

Time (sec)

0 1000 2000 3000 4000 5000 60000

0.1

0.2

0.3

0.4

0.5

Frequency (Hz)

Frequency domain Frequency domain

SSBFC WAVEPEAK CHANGE IN THE ENVELOPE IS HALF THAT OF THE DSB WAVE (ONLY ONE SIDEBAND)

SINGLE FREQUENCY MODULATING WAVE100% MODULATED SSBFC WAVE WITH A

( )( ) cos 2 2SSB LSB m c mv t V f f tπ π−

= −⎡ ⎤⎣ ⎦

SSBSC WAVE

( )( ) cos 2 2SSB USB m c mv t V f f tπ π−

= +⎡ ⎤⎣ ⎦

Advantages of SSBPower conservation

Bandwidth conservation

Selective fading

Noise reduction

Notes:

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17

Disadvantages of SSBComplex receivers

Tuning difficulties

Not compatible with existing p greceivers

Single sideband suppressed carrier AM modulation

There is wastage of transmitted power in DSB FCsystem in the form of carrier power and power in theredundant sideband.The upper and lower side bands are related to eachother.For transmission of information only one sideband isnecessary.If the carrier and one of the two sidebands issuppressed at the transmitter no information is lost.This type of modulation is called SSB SC.The SSB SC system reduces the BW by half.

Power Saving in SSB

If the carrier is suppressed

If the carrier and one of the sidebands is suppressed

2 2

4 4t c c cm mP P P P= + +

2

2DSBSC cmP P=

pp

Power saving can be expressed as a percentage

2

4SSBSC cmP P=

100%t DSBSC

t

P PP

−× 100%t SSBSC

t

P PP

−×

Power Saving in SSBAt 100% percent modulation

12t c cP P P= +

32 cP=

12DSBSC cP P= DSBSC

1.5 cP= DSBFC

0.5 cP=

P P

14SSBSC cP P= SSBSC

% 100%t DSBSC

t

P PSavingP

−= × 1.5 0.5 100%

1.5c c

c

P PP−

= × %66.7=

0.25 cP=

% 100%t SSBSC

t

P PSavingP

−= ×

1.5 0.25 100%1.5c c

c

P PP

−= × %83.3=

Power Saving in SSBAt 50% percent modulation

20.52c cP P= +

DSBSC

1.125 cP= DSBFC

0.125 cP=

P P

2

2t c cP mP P= +

2

2DSBSC cmP P=

20.52 cP=

20.54 cP= SSBSC

% 100%t DSBSC

t

P PSavingP

−= × 1.125 0.125 100%

1.125c c

c

P PP

−= × %88.9=

0.0625 cP=

% 100%t SSBSC

t

P PSavingP

−= × 1.125 0.0625 100%

1.125c c

c

P PP

−= × %94.4=

2

4SSBSC cmP P=

SSB SC with single tone modulating signal

Consider a single frequency sinusoidal signalx(t)=cosωmtThe frequency spectrum of this signal consists of twoimpulses located at ω=± ωm.When it is modulated on a carrier of frequency ω theWhen it is modulated on a carrier of frequency ωc theoriginal spectrum is translated to ± ωc.By removing the upper or lower sidebands we can obtainSSB SC signal.

Notes:

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18


mωmω−

ωω +−ωω −− ωω +ωω −cω− cω

)0(X

)]0([2/1 X

mc ωω +mc ωω mc ωω +mc ωω

mc ωω +− mc ωω −

mc ωω −− mc ωω +

)]0([2/1 X

)]0([2/1 X

1 cos( )2 c m tω ω−

1 cos( )2 c m tω ω+


The spectrum of the SSB SC signal with only lowersidebands can be expressed in time domain as1/2cos(ωc-ωm)t as the spectrum of cosine function containstwo impulses in its frequency domain.An SSB signal with only lower sidebands may beexpressed asp

SSB signal with only upper sidebands may be expressedas

[ ]1 1cos( ) cos cos sin sin2 2c m m c m ct t t t tω ω ω ω ω ω− = +

[ ]1 1cos( ) cos cos sin sin2 2c m m c m ct t t t tω ω ω ω ω ω+ = −


In general we can write the expression for the SSB signalas

The terms in the second part may be obtained by

[ ]1( ) cos cos cos( / 2)cos( / 2)2SSB m c m cS t t t t tω ω ω π ω π= ± − −

p y yintroducing a phase shift of for the first terms.In general the signal x(t)=cosωmt will not be a singlefrequency but any mix of frequencies.Then the SSB signal can be represented in the generalcase as

2/π

[ ]1( ) ( )cos ( )cos( / 2)2SSB c h cS t x t t x t tω ω π= ± −


[ ]1( ) ( )cos ( )cos( / 2)2SSB c h cS t x t t x t tω ω π= ± −

[ ]1( ) ( )cos ( )sin( )2SSB c h cS t x t t x t tω ω= ±

Where xh(t) is the signal obtained by shifting the phase of every component present in x(t) by 90 degrees.

SSB SC with base band modulating signal

mωmω−

mc ωω +−mc ωω −− mc ωω+mc ωω −cω− cω

)0(X

)]0([2/1 X

mcmc mcmcc c

mc ωω +− mc ωω −

mc ωω −−mc ωω +

)]0([2/1 X

)]0([2/1 X

cω− cω

cω− cω

Balanced modulator using diodes

D1

AF input

RF input Output

D2

AF input

Notes:

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19

Balanced modulator using FET

RF in

Out

1di

D

SG

1V

1 2V V+

1T

AF inOut

2di

D

SG

2V

1 2V V−

oV

2T

Balanced modulator using FETThe modulating voltage V2 is fed in push pull and carriervoltage V1 is fed in parallel to a pair of two identical FETamplifiers.The carrier voltage is applied to the two gates in phase,where as the modulating voltage appears 1800 out ofphase at the two gates.The modulated output currents of the two FETs arecombined in the centre tapped primary of the push pulloutput transformer.The currents subtract and the carrier frequency will becompletely cancelled if the system is completelysymmetrical.

Balanced modulator using FETAnyway the carrier is heavily suppressed if notcompletely removed.The output of the balanced modulator contains the twosidebands and some harmonics which are removed byproperly tuning the output transformer’s secondarywinding.

Mathematical AnalysisThe input voltage at the gate of T1 will be V1+V2 and that of T2 will be V1-V2.Let the constants of the non linear device be a,b,c .The two drain currents may be calculated as

21 1 2 1 2( ) ( )di a b v v c v v= + + + +1 1 2 1 2( ) ( )d

2 21 2 1 2 1 22a bv bv cv cv cv v= + + + + +

22 1 2 1 2( ) ( )di a b v v c v v= + − + −

2 21 2 1 2 1 22a bv bv cv cv cv v= + − + + −

Mathematical Analysis

1 1 2d di i i= − 2 1 22 4bv cv v= +

1 sinc cLet v V tω= 2 sinm mv V tω=

1 2 sin 4 sin sini bV t cV V t tω ω ω= +1 s s sm m m c c mi bV t cV V t tω ω ω

2 sinm mbV tω=

0 1Let v iα=

( ) ( )14 cos cos2m c c m c mcV V t tω ω ω ω+ − − +⎡ ⎤⎣ ⎦

Balanced modulator using FET

2 sino m mv bV tα ω=

( ) ( )14 cos cos2m c c m c mcV V t tα ω ω ω ω+ − − +⎡ ⎤⎣ ⎦

2 mbt PL Ve α = 2 m ccV Qa d Vn α =

( ) ( )si co osn s co c m c mmv Q tP Q tt ω ω ωω ω= + − − +

ModulationFrequency

LowerSideband

UpperSideband

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20

Balanced ring modulator

ModulatingSignalInput

OutputSignal

D1

D3

D4

T1 T2

D2

Carrier Input

+ _

The amplitude of the carrier must be much greater than the amplitude of the modulating signal, preferablysix to seven times greater such that the carrier and not the modulating signal controls the condition ofthe diode switches


ModulatingSignal

OutputSignal

T1 T2D1 ON

++ + +SignalInput

Signal

Carrier Input

D2 ON

_

+ _

_ _ _


ModulatingSignal

OutputSignal

D3ONT1 T2

++ _ _

SignalInput

Signal

Carrier Input

D4ON_

+_

_ + +

Modulating signal

Carrier signal

Output signal1 2D and D ON

3 4D and D ON

Output signal

Output signalAfter filtering

Balanced ring modulatorDiodes D1 to D4 are electronic switches that controlwhether the modulating signal is passed from inputtransformer T1 to output transformer T2 as it is or with a1800 phase shift.With the carrier polarity as in figure2 D1 and D2 areforward biased and ON while diodes D3 and D4 are3 4reversed biased and OFF.Now the modulating signal is transferred through theclosed switches to T2 without phase reversal.With the carrier polarity as in figure3 D1 and D2 arereverse biased and OFF while diodes D3 and D4 areforward biased and ON.

Balanced ring modulatorNow the modulating signal is transferred through theclosed switches to T2 with phase reversal.Carrier current flows from its source to the centre taps ofT1 and T2 , where it splits and goes in oppositedirections through the upper and lower halves of thetransformers.Their magnetic fields cancel in the secondary windingsof the transformer and the carrier is suppressed.For proper operation the diodes and the transformerwindings must be perfectly matched.Other wise carrier leak will occur.

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21

Filter Method of sideband suppression

CRYSTALOSCILLATOR

SSB Output

BUFFER BALANCEDMODULATOR

SIDEBANDSUPPRESSION

FILTER

BALANCEDMIXER

AUDIOAMPLIFIER

CRYSTALOSCILLASTOR

AF Input

Filter Method of sideband suppressionThe filter method is the simplest and most widely usedmethod of generating SSB signals.The modulating signal is applied to the audio amplifier.The amplifier’s output is fed to one input of a balancedmodulator.A crystal oscillator provides the carrier signal which is alsoA crystal oscillator provides the carrier signal which is alsoapplied to the balanced modulator.The output of the balanced modulator is a double-sideband(DSB) signal.An SSB signal is produced by passing the DSB signalthrough a highly selective band pass filter.With the filter method, it is necessary to select either theupper or the lower sideband.

Filter Method of sideband suppressionA filter used for this purpose must have a flat pass bandand extremely high attenuation outside the pass band.In a radio communication system let the frequencyrange used for voice is 300Hz to 2800Hz.If it is required to suppress the lower side band and if thecarrier frequency is f, then the lowest frequency that thisq y f q yfilter must pass without attenuation is f+300Hz, whereasthe highest frequency that must be fully attenuated is f -300HzThe filter’s response must change from zero attenuationto full attenuation over a range of only 600Hz.

Filter Method of sideband suppression

300Hz 2800 Hz-f f

Baseband

Modulated

f-300Hz f+300Hz f+2800Hzf-2800Hz

600Hz

FilterResponse

Filter Method of sideband suppressionIf the carrier frequency is very high this is very difficult toachieve because the filters must have unreasonably highquality factor.Almost all practical filters have their operatingfrequencies limited to much below transmittingfrequencies.In order to overcome this, the filtering operation iscarried out at a lower frequency, and then, using asecond stage of mixing operation the selected sidebandfrequency is raised to the desired transmitting frequency.

Assignment Question 1.1Explain different types of filters used forsideband suppression.

Notes:

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22

Phase shift Method of sideband suppression

AUDIOAMPLIFIER

BALANCEDMODULATOR

M1

CARRIER PHASE

SHIFTER ADDERAF I t

SSB Output

sin mtω

( )sin 90ctω +

[ ]cos 90c mt tω ω− +[ ]cos 90c mt tω ω− + +

90 degree

BALANCEDMODULATOR

M2

AF PHASESHIFTER90 degree

CARRIERSOURCE

AF Input

sin ctω

( )sin 90mtω +

[ ]cos 90c mt tω ω− −[ ]cos 90c mt tω ω− + +

Phase shift Method of sideband suppressionOne of the modulators M1 receives the carrier voltageshifted by 900 and the modulating voltage whereas theother modulator M2 receives the modulating voltageshifted by 900 and the carrier voltage as it is.Let the modulating voltage be and carriervoltage be sinc cV tω

sinm mV tω

For the purpose of analysis the amplitudes may beignored as it does not affect the result.Modulator M1 receivesModulator M2 receivesThe output of the modulators will contain sum anddifference frequencies.

( )sin sin 90m cnt a d tω ω +( )sin sin 90c mnt a d tω ω +

Phase shift Method of sideband suppressionOutput of M1

Similarly the output of M2 is

( ) ( )1 cos 90 cos 90c m c mv t t t tω ω ω ω= + − − + +⎡ ⎤ ⎡ ⎤⎣ ⎦ ⎣ ⎦

[ ] [ ]cos 90 cos 90c m c mt t t tω ω ω ω= − + − + +

The output of the adder is

( ) ( )1 cos 90 cos 90c m c mv t t t tω ω ω ω= − + − + +⎡ ⎤ ⎡ ⎤⎣ ⎦ ⎣ ⎦

[ ] [ ]cos 90 cos 90c m c mt t t tω ω ω ω= − − − + +

1 2ov v v= +

Phase shift Method of sideband suppression

[ ] [ ]cos 90 cos 90o c m c mv t t t tω ω ω ω= − + − + +

[ ] [ ]cos 90 cos 90c m c mt t t tω ω ω ω+ − − − + +

[ ]2 cos 90c mt tω ω= − + +

SinceOne of the sidebands is cancelled in the adder, and theother is reinforced.This system gives the upper sideband. If both thesignals are phase shifted, we get lower sidebands.

[ ] [ ]cos 90 cos 90 0c m c mt t t tω ω ω ω− + + − − =

Third method of SSB generation

BALANCEDMODULATOR

M1

90o

PHASESHIFTER

LOWPASSFILTER

BALANCEDMODULATOR

M2

RF CARRIER

GENERATORADDER

AFIn

mtω

90otω +

90o mt tω ω+ ± 90o mt tω ω+ −

ctω

( ) 90mc o tωω ω+ − +

( ) 90mc o tωω ω− + −

90o

PHASESHIFTER

BALANCEDMODULATOR

M3

LOWPASSFILTER

BALANCEDMODULATOR

M4

AF CARRIER

GENERATOR

ADDER

SSBOut

otω

( )mo tω ω± ( )mo tω ω−

90ctω +

( ) 90mc o tωω ω+ − +

( ) 90mc o tωω ω− + +

TRF Receivers

AntennaCoupling Network

RF Amp

RF Amp

RF Amp

Receiving Antenna

AudioDetector

Audio Amplifier

Speaker

Notes:

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23

TRF ReceiversEarliest and the simplest type of AM receivers.The signal received from the antenna is passed throughseveral stages of tuned amplifiers to filter the desiredfrequency and amplify it to sufficiently high levels to drivethe detector.The detector converts the RF signals directly tog yinformation.The audio stage amplifies the information signals to alevel that can drive a loud speaker.

Disadvantages of TRF ReceiversAt radio frequencies current flow is limited to theoutermost area of a conductor.The higher the frequency, the smaller the effective areaand the greater the resistance.The quality factor of the tank circuits (Q=XL/R) remainsconstant over a wide range of frequencies.1 constant over a wide range of frequencies.So the bandwidth (f/Q) increases with frequency .Hence the selectivity of the input filter changes over therange of input frequencies.If the band width is correct for low frequencies, it will beexcessive at large frequencies.

TRF Receivers

It is very difficult to tune a number of RF amplifiers tothe same central frequency at the same time.

The gains of RF amplifiers are not uniform over a

2

3 The gains of RF amplifiers are not uniform over awide range of frequencies and this producesvariations in the output amplitude with tuning.

3

Superheterodyne receivers

Superheterodyne receiver Heterodyne: to mix two frequencies together in anonlinear device or to translate one frequency intoanother using nonlinear mixing

Superheterodyne receiver Five stages:

RF Section

Mixer/ converter section

IF Section

Audio detector section

Audio amplifier section

Notes:

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24

RF SectionRF section is the first stage of the receiver and is thereforeoften called the receiver front end.It generally consists of a preselector and an amplifierstage.The preselector is a broadtuned bandpass filter withadjustable center frequency that is tuned to the desiredcarrier frequency.The primary purpose of preselector is to provide enoughinitial bandlimiting to prevent a specific unwanted radiofrequency.The other functions of the RF section are detecting, bandlimiting and amplifying the received RF signals.The RF amplifier determines the sensitivity of the receiver.

Mixer/Converter SectionMixer/converter section is the stage that down-convertsthe received RF frequencies to intermediate frequencies(IF) which are simply frequencies that fall somewherebetween the RF and information frequencies, hence thename intermediate.This section also includes a local oscillator (LO).( )The mixer accepts two inputs, the output of the RFamplifier and a steady sine wave from the localoscillator.Its function is to mix the AM signal with a sine wave to

generate a new set of sum and difference frequencies.

Mixer/Converter SectionIt can be shown that the mixer output is an AM signalwith a constant carrier frequency regardless of thetransmitter frequency.Although the carrier and upper and lower side

frequencies change frequency, the envelope remains thesame and the information contained in the sidebandsremains the same.The bandwidth is unchanged by the heterodyningprocess.

IF SectionIt consists of a series of IF amplifiers and bandpassfiltersMost of the receiver gain and selectivity is achieved inthe IF sectionThe IF centre frequency and band width are constant forall stations .all stations .The IF is always lesser in frequency than RF because itis easier and less expensive to construct high gain stableamplifiers for low frequency signals.Low frequency amplifiers are less likely to oscillate.The most common IF frequency for AM is 455KHz.

Detector sectionAM detector section is the stage that demodulates the AM wave and converts the modulated IF back to the original information signal.It can be as simple as a diode or as complex as a PLL or balanced modulator.

Audio amplifier sectionAudio section is the stage that amplifies the recoveredinformation.It comprises several cascaded audio amplifiers and oneor more speakers.The number of audio stages used depends on the audiosignal power required.g p q

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25

Operation of the receiverDuring the demodulation process in a super heterodynereceiver, the received signals undergo two frequencytranslations:First, the RF converted to IF; then the IF is converted tothe source information.Usually for commercial AM broadcast, the RF areUsually for commercial AM broadcast, the RF arefrequencies between 535kHz and 1605kHz , and IFsignals are frequencies between 450kHz and 460kHz .

Operation of the receiverThree main operations occurring in a super heterodyne receiver can be summarized as below.

(a) Frequency conversion

(b) Local oscillator trackingg

(c) Image frequency rejection

Frequency conversionIn the mixer/converter section, RF signals are combinedwith the local oscillator frequency in a nonlinear device.The output of the mixer contains the sum and differencefrequencies between the desired RF carrier and localoscillator frequencies.The local oscillator is designed such that its frequency ofThe local oscillator is designed such that its frequency ofoscillation is always above or below the desired RFcarrier by an amount equal to the IF center frequency.Therefore, the difference between the RF and the localoscillator frequency is always equal to the IF.

Frequency conversionThe adjustment for the center frequency of thepreselector (RF section) and the adjustment of the localoscillator are gang tuned.It means that the two adjustments are mechanically tied

together so that the single adjustment will change thecenter frequency of the RF section and, at the sameq ytime, change the local oscillator frequency.When the local oscillator frequency is tuned above theRF, it is called the high-side injection and when it istuned below the RF, it is called low-side injection.In AM broadcast-band receivers, high-side injection is

always used.

Frequency conversionMathematically, the local oscillator frequency is

High-side injection:

Low-side injection:

LO RF IFf f f= +

LO RF IFf f f= −

Assignment Question 1.2Explain the advantages of high side injection.

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26

Preselector535 kHz-565 kHz

Mixer/Local

Oscillator

535 540 545 550 555 560 565

CH 1 CH 2 CH 3

RF Input 535 kHz to 1605 kHz Tuned to centre

Frequency of channel 2With BW 30 kHz

CH 3 CH 2 CH 1

Converter

IF Amp450 kHz-460 kHz

Oscillator1005 kHz

1005 kHz - 535 kHz = 470 kHz1005 kHz - 540 kHz = 465 kHz1005 kHz - 545 kHz = 460 kHz1005 kHz - 550 kHz = 455 kHz1005 kHz - 555 kHz = 450 kHz1005 kHz - 560 kHz = 445 kHz1005 kHz - 565 kHz = 440 kHz

⎫⎪⎬⎪⎭ ⎫⎪⎬⎪⎭⎫

⎪⎬⎪⎭

CH 1

CH 2

CH 3

440 445 450 455 460 465 470

450 kHz- 460 kHzChannel 2 only

Local oscillator trackingTracking is the ability of the local oscillator in a receiverto oscillate either below or above the selected radiofrequency carrier by an amount equal to the intermediatefrequency throughout the entire radio frequency band.With high side injection, the local oscillator should trackabove the incoming RF carrier by a fixed frequencyg y q yequal to fRF+fIF.For low-side injection, local oscillator tracks below theincoming RF carrier by the amount fRF - fIF.

RF A lifi Bandpass

Pre-selector Mixer/Converter

895 900 905

895LSFf kH z=905USFf kHz=

900Cf kHz=450LSFif kH z=460USFf i kHz=

455Cif kHz=

2250LSFif kH z=2260USFf i kHz=

2 2 5 5C if k H z=

RF Amplifier a dpassfilter

Ganged Tuning Local Oscillator

1355LOf kHz=450 455 460 kHz

Notes:

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27

Local oscillator trackingThe tuned circuit in the pre-selector is tunable from acentre frequency of 540 kHz to 1600 kHz.The local oscillator is tunable from 995 kHz to2055 kHz.The local oscillator should oscillate 455 kHz above thepre-selector centre frequency over the entire AMfrequency band.frequency band.

SelectivitySelectivity is a receiver parameter that is used to measurethe ability of the receiver to accept a given band offrequencies and reject all others.For example, with the commercial AM broadcast band, eachstation’s transmitter allocated a 10kHz bandwidth.Therefore, for a receiver to select a particular band only, it

t li it it b d idth t 10kHmust limit its bandwidth to 10kHz .If the passband is greater than 10kHz , more than onechannel may be received and demodulated simultaneously.If the passband of a receiver is less than 10kHz , a portion

of the modulating signal information for that channel isrejected or blocked from entering the demodulator and,consequently, lost.

SensitivitySensitivity is the minimum RF (radio-frequency) signal levelthat can be detected at the input of the receiver and stillproduce a usable demodulated information signal.Generally, the signal-to noise ratio and the power of thesignal at the output of the audio section are used todetermine the quality of a received signal.Therefore the sensitivity of an AM receiver depends on theTherefore the sensitivity of an AM receiver depends on thenoise power present at the input of the receiver and thereceiver’s noise figure.The best way to improve the sensitivity of a receiver is toreduce the noise level.Sensitivity is usually stated in microvolts of received signal(i.e. commercial broadcast am receiver is 50 uv, two-waymobile radio is between 0.1 uv and 10 uv)

FidelityIt is a measure of the ability of a communicationssystem to produce at the output of the receiver, anexact replica of the original source informationAny frequency, phase, or amplitude variations that arepresent in the demodulated waveform that were not inthe original information signal are consideredg gdistortion. the three distortions are:

amplitudefrequencyphase

Image frequency problem

A receiver tuned to receive a 20MHz station that uses a1MHz IF.The LO would, in this case, be at 21MHz to generate a1MHz frequency component at the mixer output. Thetuned circuit is designed to be at 20MHz .fIf an undesired station at 22MHz were also on the air, it

is possible for it to get into the mixer. Even though thetuned circuit at the mixer's front end is "selecting" acenter frequency of 20MHz .As soon as the 22MHz signal is fed into the mixer, wehave a problem.

Image frequency problemIt mixes with the 21MHz LO signal and one of thecomponents produced is 22MHz − 21MHz = 1MHz whichis the IF frequency .Thus, we now have a desired 20MHz station and anundesired 22MHz station. Both look correct to the IF

lifiamplifier.Depending on the strength of the undesired station, sucha situation can interfere with (or even completelyoverride) the desired station.The undesired 22MHz station is called the imagefrequency station.

Notes:

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28

Image frequency problemAn image frequency is any frequency other than theselected radio frequency carrier that, if allowed to enter areceiver and mix with the local oscillator, will produce across-product frequency that is equal to the intermediatefrequency.It also said to be equivalent to a second radio frequencythat will produce an IF that will interfere with the IF from thethat will produce an IF that will interfere with the IF from thedesired radio frequency.Once an image frequency has been mixed down to IF, itcannot be filtered out or suppressed.If the selected RF carrier and its image frequency enter areceiver at the same time, they both mix with the localoscillator frequency and produce different frequencies thatare equal to the IF

Image frequency problemConsequently, two different stations are received anddemodulated simultaneously, producing two sets ofinformation frequencies.With high-side injection, the selected RF is below the local oscillator by an amount equal to the IF

LO RF IFf f f= +Therefore, the image frequency is the radio frequency that is located in the IF frequency above the local oscillator. For high side injection, the image frequency,

LO RF IFf f f

im LO IFf f f= +2im RF IFf f f= +

Image frequency problemThe relative frequency spectrum for the RF, IF, localoscillator and image frequencies for a super heterodynereceiver using high-side injection is given below.

2f

IF RF LO Image

fIF fIF

2fIF

f

Image frequency problemThe higher the IF, the farther away in the frequencyspectrum the image frequency is from the desired RF.The closer the RF is to the IF, the closer the RF is to theimage frequency.Therefore, for better image frequency rejection, a highintermediate frequency is preferredintermediate frequency is preferred.But, the higher the IF, the more difficult it is to buildstable amplifiers with high gain.So there is a trade off when selecting IF.Once an image frequency has been down converted toIF, it cannot be removed.It has to be blocked prior to the mixer/converter.

Image frequency problemIf the BW of the pre selector is sufficiently narrow , the image frequency is prevented from entering the receiver.Image frequency rejection is the primary purpose for the RF pre-selector.

Double conversion Receiver

RF AmplifierAnd

Preselector

FirstMixer/

Converter

First

BandpassFilter

SecondMixer/

Converter

Second

Antenna 2ndIF 455 kHz

1st IF 10.625 MHz

FirstLocal

OscillatorLocal

Oscillator

BandpassFilter

Second IFAmplifier

AudioDetector

LO = 11.08 MHzLO = fRF+10.625 MHz

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29

Double conversion ReceiverFor good image frequency rejection, a relatively highintermediate frequency is desired.For stable, high gain amplifiers, a low intermediatefrequency is preferred.The solution to this problem is to use two intermediatefrequencies.qThe first IF is a relatively high frequency for good imagefrequency rejection.The second IF is a relatively low frequency for easyamplification.

Double conversion ReceiverThe first IF is 10.625 MHz which pushes the imagefrequency 21.25 MHz away from the desired RF.The first IF is immediately down converted to 455 kHz and fed to a series of high-gain IF amplifiers.

Assignment question 1.3What is AGC. Why is it needed? How is it implemented?

Coherent ReceiversCoherent receivers are synchronous receivers wherethe frequencies generated in the receiver and used fordemodulation are synchronized to oscillator frequenciesgenerated in the transmitter.Coherent demodulation is where a locally generatedcarrier is synchronized to the transmitter’s carrieryfrequency.The receiver must have some means of recovering thereceived carrier and synchronizing to it.

Non-coherent receiversNoncoherent receivers are asynchronous receiverswhere either no frequencies are generated in thereceiver or frequencies used in the demodulation arecompletely independent from the transmitter’s carrierfrequencyThree noncoherent methods of am demodulation are:Three noncoherent methods of am demodulation are:

rectifier demodulation,envelope detection,square-law detection

Envelope detection

Notes:

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30

Envelope detection Envelope detection

Envelope detection Envelope detection

Assignment Question 1.4Explain how a diode can be used as an AM demodulator in three different ways.

1. rectifier demodulation,2. envelope detection,3. square-law detection

ISB Transmitter

BalancedModulator

100-kHzLF OSC

BPF A(USB)

HybridNetwork

Linear Summer Balanced Balanced

BPF 3

USB

BPF 4

Ch AInput

0kHz-5kHz

1

25 6

7

9

BalancedModulator

LF OSC

BPF B(LSB)

CarrierReinsertSwitch

Modulator

2.7-MHzMF OSC

Balanced Modulator

25-MHzHF OSC

USB USB

Ch BInput

0kHz-5kHz

3 4

8

Notes:

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31

A0k 5k

100kHz

ISB TransmitterModulate

A-5k

AA AA

1

100k 105k95k

A A100k 105k

-100k-105k -95k

-100k-105k

USB Filter

2

ISB Transmitter

B0k 5k

100kHzModulate

B-5k

BB BB

3

100k 105k95k-100k-105k -95k

LSB Filter

B B100k95k-100k -95k

4

ISB Transmitter

A A100k 105k-100k-105k

B B100k95k-100k -95k

100k-100k

A A100k 105k-100k-105k

B B95k-95k

Add

5

ISB TransmitterA A

100k 105k-100k-105kB B

95k-95k

2700kHzModulate

A AB B

6

AB2800k 2805k2795k2600k2595k 2605k

USB Filter

AB2800k 2805k2795k

7

ISB Transmitter

AB2800k 2805k2795k

25MHzModulate

ABAB

8

27.8MHz 27.805MHz27.795MHz22.2MHz22.195MHz 22.205MHz

USB Filter

AB27.8MHz 27.805MHz27.795MHz

Transmit

9

ISB TransmitterThe transmitter uses filter method to produce twoindependent single side band channels – Channel A andChannel B.The two channels are combined, then a pilot carrier is re-inserted.The composite ISB reduced-carrier waveform is upp pconverted to to RF with two additional stages offrequency translation.There are two 5-kHz wide information signals thatoriginate from two independent sources.Channel A information signals modulates a 100-kHz LFcarrier in balanced modulator A.

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32

ISB TransmitterThe output from balanced modulator A passes throughBPF A, which is tuned to the upper side band.Channel B information signals modulate the same 100-kHz LF carrier in balanced modulator B.The output from balanced modulator B passes throughBPF B which is tuned to the lower side bandBPF B, which is tuned to the lower side band.These two SSB frequency spectrums are combined in ahybrid network to form a composite ISB suppressedcarrier spectrum.The LF carrier is re-inserted in the linear summer to forman ISB reduced carrier waveform.

ISB TransmitterThe ISB spectrum is mixed with a 2.7-MHz MF carrier inbalanced modulator 3.The output passes through BPF 3 to produce an ISBreduced carrier spectrum that extends from 2.795 MHzto 2.805 MHz with a reduced 2.8 MHz pilot carrier.In the next stage, the signal is translated to 27.795 MHzg g

to 27.805 MHz in a similar way.

Non coherent BFO SSB Receiver

RF Amplifier

and RFMixer

IFAmplifier

And IFMixer

RFInput Signal

ReceivingAntenna

RFSSBSC

IFSSBSC

Pre-selector

Mixer BandpassFilter

Mixer

RFLocal

OscillatorBFO

Demodulated Information

Non coherent BFO SSB ReceiverThe selected radio frequency spectrum, which is asuppressed or reduced carrier SSB signal, is amplifiedand then mixed down to intermediate frequencies forfurther amplification and band reduction.The output from the IF amplifier stage is heterodynedwith the output from a beat frequency oscillator.The BFO frequency is equal to the IF carrier frequency.The difference between the IF and BFO frequencies isthe information signal.The receiver is non-coherent because the RF localoscillator and BFO signals are not synchronized to eachother or to the oscillators in the transmitter.

Noncoherent BFO SSB ReceiverAs a result, any difference between the transmit andreceive local oscillator frequencies produces afrequency offset error in the demodulated informationsignal.

Non coherent BFO SSB Receiver- Example

Let the information signal band is 0kHz to 5kHz.Let the received RF frequency band is 30MHz to30.005MHz.Let the RF local oscillator frequency is 20 MHz.Let the IF frequency band is 10MHz to 10.005 MHz..Let the BFO frequency is 10 MHz.The IF output from the RF mixer is

( )30 30.005 20IFf MHz MHz MHz= − −

10 10.005MHz MHz= −

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33

Noncoherent BFO SSB Receiver- Example

The demodulated information is

( )10 10.005 10mf MHz MHz MHz= − −

0 5kHz kHz= −

Coherent BFO SSB Receiver

RF Amplifier

and RFMixer

IFAmplifier

And IFMixer

RFInput Signal

ReceivingAntenna

RFSSBRC

IFSSBRC

Pre-selector


Mixer

Carrier recoveryAnd

Frequency synthesiser


BFO

RF Local

Coherent BFO SSB ReceiverIn coherent BFO receiver, the LO and BFO frequenciesare synchronized to the carrier oscillators in thetransmitter.The carrier recovery circuit is a narrow band PLL thattracks the pilot carrier in the composite SSBRC receiversignalsignal .It uses the recovered carrier to regenerate RF localoscillator and BFO frequency.The carrier recovery circuit tracks the received pilotcarrier.Minor changes in the carrier frequency in the transmitterare compensated in the receiver.

Envelope detection SSB Receiver

RF Amplifier

and RFMixer

IFAmplifier

And LinearSummer

RFInput Signal

ReceivingAntenna

RFSSBRC

IFSSBRC


EnvelopeDetectorPre-

selector


Summer

Carrier recoveryAnd

Frequency synthesiser

IF carrier LORF Local

Detector

RF Carrier

Envelope detection SSB ReceiverThe reduced pilot carrier is detected and regenerated inthe carrier recovery circuit.The regenerated carrier is used to synthesize acoherent local oscillator frequency and IF frequency.The received RF is mixed down to IF in the first mixer.Th t d IF i i dd d t th IF t iThe regenerated IF carrier is added to the IF spectrum inthe linear summer.The output of the summer is a full carrier SSB signalwhose envelope is the modulated signal.The envelope is demodulated in a diode detector toproduce the original information.

Pilot carrier SSB - Transmitter

BalancedModulator

Carrier

USB Filter100-

104kHz

C i

BalancedModulator

Carrier

BandpassFilter3000-

3004kHz

Audio Signal

2900kHz

Carrier Oscillator100kHz

CarrierInsertion

Attenuator

CarrierOscillator2900 kHz

3MHz Linear

Power Amp100kHz

Notes:

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34


0k 4k 100k 104k96k

Modulate100kHz USB Filter

Add pilot Up convert

104k100k

BB USBUSBLSB

USB USBLSB100k 104k 3000k 3004k2796k 2800k

USB Filter

3000k

Transmit

3004k

USB

Pilot carrier SSBIn pilot carrier SSB system, a low level carrier signal istransmitted in its proper place in the spectrum.This carrier is at a low amplitude level when compared toFC transmission.This pilot carrier is used at the receiver to synchronizethe local oscillator used for demodulation.The audio signal in the range 0-4kHz is fed to thebalanced modulator to create upper and lower sidebandsaround the 100kHz carrier position.An upper sideband filter passes the upper sidebandbetween 100-104kHz.A portion of the carrier is added to it through anattenuator .

Pilot carrier SSBThis is now modulated on a 1900 kHz carrier by asecond balanced modulator to produce an uppersideband with signals between 3000-3004kHz and alower side band between 2800-2796kHz.The bandpass filter passes the upper sideband which isthen amplified and transmitted.

Pilot carrier SSB - Receiver

3MHzReceiver

USB Filter100-

104kHz

BalancedModulator

Low passFilter

0-4kHz AudioOut

Pilot CarrierFilter

100±0.1kHz

Phase Locked

Carrier Osc

IF Signal100±5kHz

100kHz


3000k

Down convert To IF, IF filter

3004k 100k 104kIF pass

USB pass

95k 105k

100k 104k

Band pass filter

USB USB

USBBand pass filter100k 104k

Carrier filterPhase locked Oscillator

Balanced Modulator 0k 4k

BB

Pilot carrier SSBAt the receiver, the 3MHz signal is picked up, amplifiedand down converted to the IF at 100±5kHz producing thepilot carrier at 100kHz and the USB signal in the range100-104kHz.This signal is passed through a USB filter to thebalanced modulator, and through a very narrow bandfilt t 100kH t t t th il t ifilter at 100kHz to extract the pilot carrier.The pilot carrier is applied to the input of the phasedlocked oscillator to produce the synchronized 100 kHzcarrier for the demodulator.The final lowpass filter removes sum components topass the 0-4kHz audio signal.

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35

Vestigial sideband transmissionThe use of single sideband techniques reduce bandwidthand power when compared to DSB.If we filter out one of the sidebands of a DSB signal wewill get SSB signal.But it requires a filter that has near-ideal characteristics.It is almost impossible to obtain such idealIt is almost impossible to obtain such idealcharacteristics from practical filters.When signals contain frequency components atextremely low frequencies, the USB and LSB of thetransmitted signal tend to meet at the carrier frequency.Under such conditions it becomes still more difficult toisolate one sideband from the other.

Vestigial sideband transmissionIn VSB, instead of rejecting one sideband completely, avestige of one sideband is transmitted along with theother one.The carrier may or may not be suppressed.VSB is mainly used for commercial TV broadcasting.

Vestigial sideband transmission

Video UpperVideo Lower

SoundCarrier

SoundSpectrum

PictureCarrier

Am

plitu

de

0.5

1

0 0.5 1.25 5.25 5.75 6

4.5MHz4MHz0.75

MHz

SidebandSideband 50kHz

Rel

ativ

e

Relative Channel Frequency

0

Spectrum of Transmitted Signals

Vestigial sideband transmissionIn TV transmission the BW of the baseband signal is4 MHz.Since sound carrier is also sent along the pictureinformation the base band BW is increased to 4.5MHz.When this base band signal is modulated using DSBsystem the BW of the resultant signal is 9 MHz.y gIf we are using such a high BW, the number of stationsthat can be accommodated in a given range of thespectrum is limited.So it is better to use SSB for TV transmission since theBW can be reduced by half.

Vestigial sideband transmissionSince TV transmission contains very low frequencyinformation content, it is very difficult to remove one ofthe side bands.If we are using practical filters, a portion of the requiredsideband get attenuated due to the imperfect filtercharacteristics.Also, near to the edges of the pass band, a practical filterproduces phase distortionIf low frequency content is lost, or phase distortion isintroduced, it produces noticeable degradation in thepicture quality, unlike in the case of audio signal.

Vestigial sideband transmissionSo in TV transmission, the first 1.25 MHz of the lowersideband is sent along with the upper side band.The first 0.75MHz is sent undiminished, where as thenext 0.5 MHz is attenuated.The total BW requirement of a station is now 6MHz.Approximately 3 MHz is still saved per TV channel.Approximately 3 MHz is still saved per TV channel.

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36

Vestigial sideband transmissionPictureCarrierFrequency

Res

pons

e

0.5

1

SoundCarrierFrequency

0 0.5 1.25 5.25 5.75 6

4.5MHz4MHz0.75

MHz

Rel

ativ

e

Relative Video Frequency

0

Receiver Video Amplifier Frequency Response

Vestigial sideband transmissionThe response of the input circuit of the TV receiver isshown in figure.Attenuation is purposely provided for video frequenciesfrom 0-1.25 MHz .Extra power is transmitted in these frequencies, sincethey are sent in both side bands.yThese frequencies would be unduly emphasized in thevideo output of the receiver, if they were not attenuated.

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