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ANALOG MODULATION

AMPLITUDE MODULATIONBy

Engr. Jose Espino

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Definitions

Communication – the transmission of intelligence between 2 or more points. This refers to the sending, receiving & processing of intelligence by electronic means.

Intelligence – refers to data, information or message that is to be transmitted.

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Communication System Block Diagram

Transmitter – device that processes the input signal to produced a transmitted signal suited to the characteristics of the transmission channel.

Channel – electrical medium that bridges the distance from source to destination.

Receiver – operates on the output signal from the channel in preparation for delivery to the transducer at the destination.

transmitter channel Receiver

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WHAT IS ANALOG MODULATION? Modulation – process of varying one of the properties of the

wave in proportion to another wave. This is performed at the transmitter to achieve efficient and reliable information transmission.

Analog modulation consists of two parts Analog message- a signal that causes a variation of some

of the properties of the carrier. Carrier- unmodulated signal that suits a particular

application. Analog message is then impressed upon the amplitude,

frequency or phase of the carrier We then say carrier is “modulated” by the message

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Why need for modulation?

Antenna sizeexample: at f=100MHz,λ/4=0.75m

at f=15KHz,λ/4=5KmSignal separation

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Types of Modulation

Amplitude Modulation (AM) – when you vary the amplitude of the carrier in proportion to the information signal.

Frequency Modulation (FM) – when you vary the frequency of the carrier in proportion to the information signal.

Phase Modulation (PM) – when you vary the phase of the carrier in proportion to the information signal.

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+MODULATING SIGNAL Vmsinωmt

CARRIER VCsinωct

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AM Example

0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2-3

-2

-1

0

1

2

3

AM BASEBAND

carrier

message

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AM expression

eAM = Vc sin ωct

+ Vm/2 cos(ωc-ωm)t – Vm/2 cos(ωc+ωm)t

Volts

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Modulation index, m – or coefficient of modulation, the ratio of the magnitudes of the modulated signal & the unmodulated carrier m = Vm/Vc

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Modulation Index

The quantity m is called modulation index or modulation factor.

Modulation index must be less or equal to 1

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Finding Modulation Index

VmVV

VmVV

cpeak

cpeak

)min(

)max(

tftfmV cmc 2sin2sin1

minmax

minmax=index modulationVVVVm

0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2-3

-2

-1

0

1

2

3

AM BASEBAND

VmaxVmin

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What is mHere?

0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2-3

-2

-1

0

1

2

3

AM BASEBAND

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AM – Percentage Modulation

Under modulated (<100%) 100% modulated

Envelope Detector

Can be used

Envelope Detector

Gives Distorted signal

Over Modulated (>100%)

Overmodulation as seen in the Reciever

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A waveform is seen on an oscilloscope screen. The max span is 1V,while the min span is 0.1V. What is the modulation index?

Given: Vmax=1V & Vmin=0.1VRequired:mSolution:

%818.81%100*81818.0%

81818.01.011.01=

=index modulationminmax

minmax

m

m

VVVVm

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AM Envelope

Envelope is a baseband signal and contains all the information an AM signal carries.

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Requirements on the Envelope

To prevent “overmodulation”, the envelope must always be positive,

100m100m

overmodulation

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Trapezoidal Patterns

0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2-3

-2

-1

0

1

2

3

AM BASEBAND

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Showing AM Spectrum

Baseband

AM

fm

upper sidebandlower sideband

fc

fm

fc+fmfc-fmfc

AM bandwidth=2fm

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AM Power

The power of AM signal is split between carrier and message

message

mcc

mcc

carrier

cc

cmccc

cmc

tffmV

tffmV

tfV

tftfmVtfV

tftfmV

cos2

cos2

2sin

2sin2sin2sin

2sin2sin1

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Components of AM Power

Carrier power=Upper sideband power=Lower sideband=

Total sideband power=

RVc2

2

22

81

cVm

22

81

cVm

RVcm

4

22

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Total AM Power

Total AM power is the sum of carrier and message power

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221

2

22

mPcP

RVmPPP

T

csidebandcarrierT

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Distribution of AM Power

How much of the total power goes into carrier and how much into message?

Even under full modulation, only a third of the transmitted power goes into the message component

1,31

0,0

2 2

2

m

m

mm

PP

total

sideband

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Example of Power Allocation

FCC power rating is based on average carrier power. If a transmitted is rated at 5KW and is connected to a 50 ohm load, find the total power and how it is split between carrier and message

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Answers...

Carrier peak amplitude is given by

Peak sideband(message) power is

volts70750005021 2

ccc VVP

WVmP cs 2500707141

41 222

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AM Power Efficiency

Here is AM power distribution Carrier power=5000W Sideband power=2500W Total power=7500 Efficiency=(sideband power)/total power=

(2500)/(7500)=1/3.Out of 3 watts of transmitted power, only 1

watt is going into the message

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Examples:What is the total power when the carrier is

700W and modulation index is 0.5A transmitter radiates modulated signal power

measured at 1500W, while the unmodulated value is 1000W.what is the modulation index? What is the unmodulated value if the modulation index is 0.85

A 1250W carrier is modulated with a resulting modulation index of 0.65. what is the sideband power?

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AM Current

21

21

21

21

221

2

2

2

2

2

2

2

2

22

mIcI

mRIcRIt

mPcP

mPcP

RVmPPP

T

T

T

csidebandcarrierT

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Examples:When a broadcast AM transmitter is 50%

modulated, the antenna current is 12A. What will the current be when the modulation depth is increased to 0.8?

The antenna current of an AM broadcast transmitter, modulated to a depth of 40% by an audio signal is 15A. It increases to 20A as a result of modulation with another audio signal. What is the modulation index resulting from the second signal?

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AMPLITUDE MODULATION WITH MULTIPLE SIGNALS

Pt = Pc (1 + mt2/2)

where mt = total modulation index

m1 , m2, m3 = modulation index of signal having index 1,2,3 respectively

2 2 21 2 3 ......tm m m m

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Exercises A 100KHz carrier is simultaneously modulated with

300Hz, 800Hz, and 2KHz audio sine waves. What will be the frequencies present at the output?

A broadcast AM transmitter radiates 50KW of carrier power. What will be the radiated power at 85% modulation?

When the modulation percentage is 75,an AM transmitter produces 10KW. How much of this is carrier power?

A 360W carrier is simultaneously modulated by two audio waves with modulation %s of 65 & 55, respectively. What is the total sideband power radiated?

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AM Signal Generation

Switching modulator

vm

~

R

vc=Vcsinωct

V2(t)=AMV1(t)

0,00,

2sin2sin

12

1

tvtvtv

tv

tfVtfVtv

c

c

ccmc

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How signal is modulated?

To produce a usable AM signal, the carrier and intelligence must be combine through a nonlinear device

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Nonlinear mixing

carrier AM WAVE

da

ta

Nonlinear device

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How to generate AM using tune circuit ?

It is necessary to apply the series of current pulses to a tuned (resonant ) circuit

Each pulse would initiate a damped oscillation in the tuned circuit

Initial amplitude is equal to the applied current pulse

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Flywheel Effect

The ability of a resonant circuit to maintain oscillation at an essentially constant frequency when fed with short pulses of energy

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Simple AM transmitter block diagram

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COMPONENTS OF AN AM TRANSMITTER

PREAMPLIFIER- a sensitive, class A, linear voltage amplifier with high input impedance which raises the amplitude of the source signal to a usable level with minimum nonlinear distortion & little thermal noise as possible

RF CARRIER OSCILLATOR-commonly uses crystal-controlled units to generate the carrier signal for accuracy & stability

BUFFER AMPLIFIER- a low-gain, high-input-impedance linear amplifier which isolates the oscillator from the high-power amplifiers

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MODULATOR- the circuit which translates the information signal to frequencies appropriate for transmission

-nonlinear devices that combine 2 input signals to produce a modulated output

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AM RECEIVER

To build up a receiver, one would probably go through the

following processes1. The signal form the antenna is

usually very small – therefore, amplification is necessary.

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2. This amplifier should be tuned to accept only the desired carrier and sideband frequencies to avoid interference from other stations and to minimize the receive noise

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3. A circuit to detect the intelligence from the radio frequency is required

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4. Further amplification is necessary to give it sufficient power to drive a loudspeaker

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Two major characteristics of any receiver

1.sensitivity- ability to drive the output transducer to an acceptable level

2.selectivity- ability to differentiate between the desired signal and unwanted signal

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CONVENTIONAL AM RECEIVERS

TUNED RADIO-FREQUENCY (TRF) RECEIVER

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ADVANTAGE OF TRF RECEIVER

Simple in construction with a relatively high sensitivity

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DISADVANTAGES OF TRF RECEIVER

1. inconsistent bandwidth when tuned over a wide range of input frequencies2. instability of the receiver due to the large number of RF amplifiers tuned to the same center frequency3. non uniform gain over a wide frequency range4. insufficient adjacent-frequency rejection

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Forms of Amplitude Modulation

1. A3E ( previously A3) DSBFC2. R3E (previously A3A) Single sideband Reduced carrier3. H3E (previously A3H) Single sideband , full

carrier4. J3E (previously A3J)Single sideband,

suppressed carrier5. B8E (previously A3B) two independent SB6. C3F (previously A5C) Vestigial sideband 7. Lincompex – Linked compressor and

expander

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Double Sideband

Baseband

AM

fm

upper sidebandlower sideband

fc

fm

fc+fmfc-fmfc

AM bandwidth=2fmcarrier

0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2-3

-2

-1

0

1

2

3

AM BASEBAND

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DSB-SC Spectrum

Spectrum is identical to before except for the removal of the carrier(two impulses)

upper sidebandlower sideband

fc fc+fmfc-fmfc

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Single sideband technique (SSB)

A technique in which the carrier is eliminated or highly suppressed, and one of the two sidebands must be filtered out.

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Single Sideband(SSB)

The two sidebands are identical. Only one is sufficient to carry all the message information. Block either the upper or lower sideband

Bandwidth=2fm Bandwidth=fm

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SSB Generation

One way to generate an SSB signal is through selective filtering

DSB-AMf

Ideal highpass filter

Leaks some of the lower sideband

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Issues in Sideband Suppresion

Consider a tone modulated signal with carrier frequency of 10MHz, and message frequency of 30Hz. Want to make a SSB signal out of it

60Hz

10MHz

carrier

10,000,030 Hz

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Required Q’s

We are trying to separate a frequency of 10.000030 MHz from 9.999970 MHz at 10 MHz.

The required Q is Q=10MHz/60Hz=166,667. Too high

Solution: Translate the signal down to 100KHz. There, Q=100KHz/60Hz=1666. Achievable

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SSB transmitter

Take a 60Hz tone message.

Audio amp Balancedmodulator

Highpassfilter

Balancedmodulator Power amp

Conversionoscillator RF carrier

100 KHz 10 MHz

60Hz

99,940 Hz 100,060 Hz9,899,940 Hz

10,100,060 Hz

100,060 Hz

200.120K

Passes the higherterm

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Power Distribution in SSB

If an SSB transmitter with total power of 10KW were to be replaced by a standard AM signal with the same total power, compare their respective carrier and sideband powers

WPPP

letm

PmPmPP

cc

c

sidebands

cccT

67.66662

000,10

1

000,1044

22

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Sideband Powers

Subtracting carrier power from total power leaves us with the sideband power

The power in one sideband is half at 1,666.67W

Psb PT Pc 10,000 6,666.67 3,333.33W

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Comparisons

A 10 KW AM transmitter ends up with 1,666 watts in each sideband

A 10 KW SSB transmitter ends up with 10 KW per sideband. There is nowhere else for the power to go

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Commercial AM

Here are some numbers Carrier frequencies: 540-1600 KHz Carrier spacing:10 KHz IF frequency:455 KHz IF bandwidth:6-10 KHz Audio bandwidth: 3-5KHz

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SUPERHETERODYNE RECEIVER

A type of receiver which mixes 2 signals in a nonlinear device & translates the desired signal to an intermediate frequency before extracting the original information

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COMPONENT SECTIONS OF AN AM SUPERHETERODYNE RECEIVER

1. RF Section2. Local Oscillator3. Mixer / Converter4. IF Section 5. Detector6. Audio Amp7. Speaker8. AGC

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1. RF SECTION

a wideband RF amplifier which is the first stage of the receiver. Often called the receiver front end; it is mechanically tied to the local oscillator to ensure precise tuning characteristics. It performs the detection, bandlimiting, & amplification of the received RF signals.

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2. LOCAL OSCILLATOR

a variable oscillator which provides a signal at a certain frequency which, when combined with the received RF signal at the mixer stage, produces a group of frequencies which includes the intermediate frequency

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LOCAL OSCILLATOR

1. High-side injection – tuning the local oscillator frequency above the RF signal by an amount equal to the intermediate frequency

2. Low-side injection – tuning the local oscillator frequency below the RF signal by an amount equal to the intermediate frequency

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3. MIXER/CONVERTER SECTION

a class C nonlinear device which comes after the RF stage. It processes the received RF signals, producing the sum, difference, and original RF frequencies. The difference frequency is equal to the intermediate frequency & will serve as the input to the succeeding stage

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4. IF SECTION

a very narrow bandwidth class A device which receives the down-converted RF signal & filters the unwanted frequency components to prevent them from being demodulated with the original information signal; this section produces an amplified signal at the output & determines the sensitivity & selectivity of the receiver

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5. AM detection

Diode detectorone of the simplest and most effective types of detectors, and one with nearly an ideal nonlinear resistance characteristics

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Important motivation for using AM is the possibility to use the envelope detector that has a simple structure (also

cheap) needs no synchronization

(e.g. no auxiliary, unmodulated carrier input in receiver)

no threshold effect (SNR can be very small and receiver still works)

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8. AUTOMATIC GAIN CONTROL

a system in which the overall gain of a radio receiver is varied automatically with the changing strength of the received signal to keep the output substantially constant. It enables tuning to stations of varying signal strengths without an appreciable change in the volume of the output signal, except when the change in signal strength is enormous

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fc=900KHz

fl.o.=1355KHz

fc=900KHzCross products & harmonics

IF=455KHz

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Test Your Knowledge

1 A TRF receiver includes three stages: the RF, ________, and audio amplifier.a.IF b. detectorc.Trimmer d. mixer

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2 The extent to which a receiver can differentiate between the desired signal and others is termed its ________.a. Selectivity b. sensitivityc. figure-of-merit d. gain

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3 The main problem with the TRF design isa. Lack of selectivity in receiving all AM stationsb. Poor demodulation of an AM stationc. Frustration in tuning to receive more than one stationd. Lack of sensitivity in receiving all AM stations

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4 The sensitivity of a receiver has to do with its ability toa. Withstand shockb. Receive one station versus anotherc. Receive weak stationsd. All the above

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5 An AM signal having a carrier frequency of 940 kHz is to be mixed with a local oscillator output signal in order to produce an intermediate frequency of 455 kHz. At what should the L.O. frequency be set?a. 455 kHz b. 1395 kHzc. 910 kHz d. 1850 kHz

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6 A 7.0-Mhz carrier is modulated by a voice signal that has three frequency components of 100 Hz, 200 Hz, and 300 Hz. What three frequencies comprise the lower sideband?a. 6.9997 Mhz, 6.9998 Mhz, and 6.9999 Mhzb. 100 Hz, 200 Hz, and 300 Hzc. 6.9999 Mhz, 7.0000 Mhz, and 7.0001 Mhzd. 7.0001 Mhz, 7.0002 Mhz, and 7.0003 Mhz

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7 The total output power of an AM transmitter is measured to be 850 W. What is the total output sideband power if it has a percent modulation of 100%?a. 425 W b. 850 Wc. 283.3 W d. 141.65 W

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8 An AM waveform at maximum is 100 V p-p and at minimum is 40 V p-p. The modulation percentage isa. 250% b. 40%c. 25% d. 37.5%

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9 A transmitter having a 900-W carrier transmits 1188 W when modulated with a single sinewave. If the carrier is simultaneously modulated with another sinewave at 60% modulation, calculate the total transmitted power.a. 1084 W b. 1170 Wc. 1350 W d. 1224 W

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10 A carrier signal has _____.

A. constant peak amplitude B. the informationC. frequency range of 20-20,000 HzD. a varying amplitude