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Analog Communication Unit No. 1

Amplitude (Linear) Modulation

Dr. Suvarna S. Patil BVCOEW, Pune-43 Mail id: [email protected] 2/12/2013

Amplitude (Linear) Modulation

1

Unit 1: Contents • • • •

Base Band & Carrier Communication Generation of AM (DSBFC) and its spectrum. Power relations applied to sinusoidal signals. DSBSC – Multiplier modulator, Non linear generation, switching modulator, Ring modulator and its spectrum. • Modulation index • SSBSC, ISB, VSB & their generation methods and comparison • AM Broadcast technical standards • Books : • T1: Modern Digital & Analog Communication Systems, by B. P. Lathi 3rd Ed. • T2: Electronic Communication, by D. Roddy & J. Coolen, 4th Ed. 2/12/2013

Amplitude (Linear) Modulation

2

Examination Scheme • 204189: Analog Communication • • • • •

2/12/2013

Lectures: 4 Hrs/Week Theory Online: 50 Marks Practical: 2 Hr/Week Theory Paper: 50 Marks Practical: 50 Marks

Amplitude (Linear) Modulation

3

Unit 1: Objectives • To understand base band signals and pass band communication • To study generation methods of various Amplitude Modulation (AM) schemes (such as DSBSC, SSB, ISB and VSB). • Plot frequency spectrum of different AM and compute the required bandwidth for AM channel • Derive power relation in AM and compute power required for AM transmission. • Define and calculate Modulation index • To understand the concept of suppressing the carrier in DSBSC and SSB techniques. • To study various AM broadcast standards

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Amplitude (Linear) Modulation

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Pre--requisites Pre • Electromagnetic spectrum • Non linear characteristics of switching devices • Basic block diagram of communication system • Need of modulation 1. 2. 3. 4. 5. 6. 2/12/2013

Reduces height of antenna Avoids mixing of signals Increases range of communication Allows multiplexing of signals Allows adjustments in BW Improves quality of reception Amplitude (Linear) Modulation

5

Basic block diagram of Analog Communication System Analog information source

Analog Modulation

Communication Channel

Analog carrier source

Noise

Analog Demodulation

Destination

What is Communication? What are its examples? What is Communication System? What are various information sources? What are available communication channels? 2/12/2013

Amplitude (Linear) Modulation

6

Base band and carrier communication • Communication systems are classified according to the range of frequencies they use to transmit information • Base band and Pass band / carrier systems • Base band communication is without any modulation (without frequency shifting)

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•

Carrier communication is with modulation

•

Shifting the baseband signal to higher frequency Amplitude (Linear) Modulation

7

Base Band Signal and Communication • Base Band Signal is band of frequencies of the signal delivered by the source • They have sizable powers at lower frequencies • Telephony – band of voice signals (0 to 3.5kHz) • Television – band of video (0 to 4.3kHz) • Digital data or PCM ( 0 to Rate Hz) •

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Eg. Communication between two telephones Video Communication Digital transmission of analog signals Communication between two PCs Amplitude (Linear) Modulation

8

Baseband and Carrier Wave (CW) Systems Linear modulation (AM...)

f w Baseband spectra

fc bw 2w

Exponential modulation (FM...)

bw 2w

• Figures show baseband message transfer by linear (AM) and exponential modulation (FM) • In linear modulation, transmission bandwidth is always below or equal to 2W (W: message bandwidth) • Non-linear (angle modulation) spreads message on much larger transmission bandwidth that 2W

2/12/2013

f

Amplitude (Linear) Modulation

f

9

Modulation • The process of shifting the base band signal to pass band is known as Modulation • Basic parameter such as amplitude, frequency or phase of a sinusoidal signal known as carrier of high frequency is varied in proportion to the baseband signal What is Modulation? Define AM. Define requirement of AM signal 2/12/2013

Amplitude (Linear) Modulation

10

AM waveform representation

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Amplitude (Linear) Modulation

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AM mathematical representation • Modulating signal

vm Vm sin mt vc Vc sin c t

• Carrier signal • Modulation index

Vm m Vc

• Amplitude of AM wave

A Vc (1 m sin mt )

• Instantaneous voltage of AM wave

v AM A sin c t Vc (1 m sin mt ) sin c t 2/12/2013

Amplitude (Linear) Modulation

12

AM mathematical representation v AM

mVc mVc Vc sin c t cos(c m )t cos(c m )t 2 2

Un-modulated carrier

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Lower sideband (LSB)

Amplitude (Linear) Modulation

Upper sideband (USB)

13

AM mathematical representation using frequency shifting property • Base band signal

m(t ) M ( )

• AM containing only two sidebands

1 m(t ) cos c t [ M ( c ) M ( c )] 2 Upper sideband (USB)

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Amplitude (Linear) Modulation

Lower sideband (LSB)

14

AM frequency domain representation

Amplitude Modulating band

0

m

Carrier band

mVc 2

Vc

c m c c m LSB

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m Vc 2

Amplitude (Linear) Modulation

(rad sec)

USB

15

AM Generation Methods • AM (DSBFC) Modulators • BJT Collector Modulator • Analog multiplier

• AM (DSBSC) Modulators • • • •

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Multiplier (Product) Modulators Nonlinear Modulators Switching Modulators Ring Modulators

Amplitude (Linear) Modulation

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BJT Collector Modulator • Transistor is operated in Class C mode • Transistor is biased well below cutoff. • Carrier input to the base drives the transistor into conduction during the RF cycle. • Collector current flows in the form of Pulses periodic at carrier frequency. • Tuned circuit is used at the collector resonates at this frequency. • By applying modulating voltage in series with collector dc supply changes this steady collector voltage to slowly varying voltage. • This produces current pulses of varying amplitude • Finally modulated output is obtained by mutual inductive coupling 2/12/2013

Amplitude (Linear) Modulation

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BJT Collector Modulator Practical Circuit

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Collector Modulator Waveform

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Amplitude (Linear) Modulation

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Analog Multiplier AM Modulator m(t ) Modulating Signal

Analog Multiplier

cos c t

∑ DSBFC Modulated Signal

Carrier Signal

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Amplitude (Linear) Modulation

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Modulation index using practical method

V max V min

Vmax Vmin m Vmax Vmin 2/12/2013

Amplitude (Linear) Modulation

21

Trapezoidal method of Modulation index Calculation Modulated signal

L2 = 2 Vmin

2/12/2013

L1 = 2Vmax

Amplitude (Linear) Modulation

L1 L2 m L1 L2

22

Modulation index range :

0 m 1 m 1, (Vm Vc ) 100% modulation Requires high transmission power

m 1, (V m V c ) Under modulation Most preferably used

m 1 , (V m V c ) Over modulation Must be avoided

2/12/2013

Amplitude (Linear) Modulation

23

Bandwidth of AM For single tone modulation

BW f H f L f USB f LSB ( fc fm ) ( fc fm ) 2 fm For multi tone modulation

BW ( f c f max ) ( f c f max ) 2 f max 2/12/2013

Amplitude (Linear) Modulation

24

Power relations in AM PT Pc PLSB PUSB

Vc2rms R

2 VLSB rms

R 2

2 VUSB rms

R 2

Vc mVc mVc 2 2 2 2 2 R R R Vc2 m 2Vc2 m 2Vc2 2R 8R 8R 2/12/2013

Amplitude (Linear) Modulation

2

25

Power relations in AM V c2 Pc 2R PLSB PUSB

PSB PLSB PUSB m 2 Pc m 2 Pc 4 4 m 2 Pc 2

m 2 Pc 4

PT Pc PSB m 2 Pc Pc 2 m2 Pc 1 2 2/12/2013

Amplitude (Linear) Modulation

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Voltage and Current relations in AM 2

m VT Vc 1 2

IT I c

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m2 1 2

Amplitude (Linear) Modulation

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Power relations in multi tone AM 2 2 2 m3 m1 m2 PT Pc 1 2 2 2

m eff

2

2

m1 m 2 m 3

VT V c 1 IT Ic 1

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2

m eff2 2 m eff2 2

Amplitude (Linear) Modulation

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Transmission Efficiency in AM Ratio of useful power to the total power PSB 100% PT where PSB is the total sidebands signal power that contains information m 2 Pc m2 m2 2 2 2 m m 2 m2 21 Pc 1 2 2 If m = 1 (100% modulation), the average power, PSB = 50% Pc= Pc/2 It shows that the PSB is dependent on m. 2/12/2013

Amplitude (Linear) Modulation

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Transmission Efficiency in AM m2 100% 2 2m •

The transmission efficiency with m = 1 is only 33.33% practically still less on the order of 25% or lower.

•

Only 1/3rd of the transmitted power is used for carrying message.

•

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Efficiency increases monotonically with m

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Examples • Numerical 1: A 400 watt carrier is modulated to a depth of 75 %. Calculate the total power in the modulated wave. • Solution: m2 0.752 4001 PT Pc 1 2

512.5 W 2

• Note: Refer page no. 38 to 42 for more solved numerical from G. Kennedy

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Examples • Numerical 2: Sketch AM wave for modulation indices of m=0.5 and m=1, when m(t ) B cos mt • Solution: carrier signal is A cos c t

Vm B m Vc A • when the modulating signal is pure sinusoidal, the modulation referred to as Tone Modulation

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Amplitude (Linear) Modulation

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Disadvantages of AM • Needs large bandwidth • Requires more power for transmission • Maximum power is wasted in carrier. • Highly affected due to noise.

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Features of DSBSC • Carrier signal does not contain any information. • Information lies in sidebands only • Eliminating carrier signal saves transmission power • DSBSC contains only sidebands carrying same information • Bandwidth remains same as that of DSBFC (AM) • Exhibits phase reversal at zero crossing 2/12/2013

Amplitude (Linear) Modulation

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Multiplier/Product DSBSC Modulator

1 m(t ) cosct [M ( c ) M ( c )] 2

m(t )

DSBSC signal

Modulating Signal

cos c t Carrier Signal

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Amplitude (Linear) Modulation

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DSBSC signal

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DSBSC spectrum

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Nonlinear Resistance Characteristics • AM signal is generated using nonlinear device such as a semiconductor diode or a transistor. • Input output characteristics of nonlinear device is approximated by power series i

Positive C

i a bv cv 2 Negative C

a : dc component b: conductance c: coefficient of nonlinearity v 2/12/2013

Amplitude (Linear) Modulation

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Nonlinear Modulator y (t ) bx(t ) cx 2 (t )

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Amplitude (Linear) Modulation

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Nonlinear Modulator.. Cont. 2

2

z (t ) y1 (t ) y2 (t ) [bx1 (t ) cx1 (t )] [bx2 (t ) cx2 (t )] By substituting the two inputs

x1 (t ) cos c t m(t ), x2 (t ) cos c t m(t ) z (t ) 2b m(t ) 4c m(t ) cos c t After passing through BPF the output contains only one term with two sidebands so called as single balanced modulator

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Amplitude (Linear) Modulation

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Switching Modulator • The signal m(t ) w(t ) consists not only component of m(t) but also infinite number of modulated signals with carrier frequency ωc, 3ωc, 5ωc, …… • Multiplication of a signal by a square pulse is switching operation. • Switching the signal m(t) periodically is achieved using simple switching element. • This switching element is controlled by carrier signal w(t) 2/12/2013

Amplitude (Linear) Modulation

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Switching Modulator

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Amplitude (Linear) Modulation

42

w(t)

S(t) = m(t).w(t)

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Amplitude (Linear) Modulation

43

Diode Bridge: series and shunt modulator

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Amplitude (Linear) Modulation

44

Ring Modulator

Diode bridge act as an electronic switch For the first half cycle of carrier all diodes conduct For next half cycle all diodes remain open 2/12/2013

Amplitude (Linear) Modulation

45

SSB Modulation

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The two sidebands of an AM signal are mirror images of each other.

One of the sidebands is redundant

Using single-sideband suppressed-carrier transmission results in reduced bandwidth and therefore twice as many signals may be transmitted in the same spectrum allotment

Typically, a 3dB improvement in signal-to-noise ratio is achieved as a result of SSBSC

Amplitude (Linear) Modulation

46

SSB Spectra Frequency spectrum:

fc-fm

fc

fc+fm

Bandwidth = fm(max) Total Power = +PUSB

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Amplitude (Linear) Modulation

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Methods of Generating SSB i) Selective Filtering method

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•

A filter removes the undesired sideband producing SSB.

•

Balanced modulators is used to suppress the unwanted carrier and filters to suppress the unwanted sidebands

•

Quartz crystal filters are the most widely used sideband filters since they are very selective and inexpensive.

Amplitude (Linear) Modulation

48

SSB Generation by Filter Method Antenna Carrier oscillator

DSB signal Balanced modulator

Microphone Audio amplifier

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SSB signal Sideband filter Linear amplifier

Filter response curve Upper Lower sidebands sidebands

Amplitude (Linear) Modulation

49

Methods of Generating SSB ii) Phase shift method • This method avoids filters and their disadvantages. • It makes use of two Balanced Modulators and two phase shifters. • One of the BM receives 90° phase shifted carrier and in phase message signal

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Amplitude (Linear) Modulation

50

Phase Shift method •

Other BM receives 90° phase shifted message and in phase carrier signal.

• Both BM produce sidebands only. •

USB of both BMs will be in phase and LSB will be out of phase.

•

Adding the two DSB signals together results in one sideband being cancelled out.

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Amplitude (Linear) Modulation

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Block Diagram of Phase Shift method Vm cos wmt

m (t)

Balanced Modulator 1

A1(t)

Vc cos (wct + 90)

Output Signal

Phase shifter

+

Carrier signal Phase shifter

Balanced Modulator 2

A2(t)

Vm cos (wmt + 90)

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Amplitude (Linear) Modulation

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Advantages/Benefits of SSB • • • •

Low power transmission Less band width conservation Selective fading Noise reduction

Disadvantages of SSB Complex receivers Tuning difficulties

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Comparison of time domain representation of three common AM transmissions

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Amplitude (Linear) Modulation

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Examples For an AM DSBFC wave with a peak unmodulated carrier voltage Vc = 10 Vp, frequency of 100kHz, a load resistor of RL = 10 , frequency of modulating signal of 10kHz and m = 1, determine the following i) Powers of the carrier and the upper and lower sidebands. ii) Total power of the modulated wave. iii) Bandwidth of the transmitted wave. iv) Draw the power and frequency spectrum.

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Amplitude (Linear) Modulation

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Examples..cont’d • For the same given values, determine questions (ii)-(iv) for a AM DSB-SC, AM SSB-FC and AM SSB-SC systems. Determine also the percentage of power saved in each of the system design.

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Amplitude (Linear) Modulation

56

Examples..cont’d i)

Solution for DSBFC

(V / 2 ) V (10) P 5W R 2R 2 10 mP P P 1.25W 4 2

2

c

2

c

c

2

c

usb

lsb

ii) m2 m2 Pt Pc Pc Pc 4 4 12 12 5 (5) (5) 7.5W 4 4

iii) Bandwidth=2xfmmax=2(10kHz)=20kHz 2/12/2013

Amplitude (Linear) Modulation

57

Examples..cont’d Solution : For DSB-SC

ii)

Power Saving with DSBSC

m m P P P 4 4 1 1 (5) (5) 2.5W 4 4 2

t

2

c

2

c

2

iii)Bandwidth=2xfmmax

Powersaved 7.5W 2.5W 5W 5W 100% 7.5W 66.67%

% Powersaved

=2(10kHz)=20kHz

iv) 2/12/2013

90kHz

110kHz

Amplitude (Linear) Modulation

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Examples..cont’d Solution :For SSB-FC

ii)

m P P P 4 1 5 (5) 6.25W 4 2

t

c

c

2

Powersaved 7.5W 6.25W 1.25W 1.25W % Powersaved x100% 7.5W 16.67%

iii)Bandwidth=fmmax=10kHz

iv) fc-fm 100kHz 2/12/2013

110kHz

Amplitude (Linear) Modulation

59

Example 1..cont’d • Solution : For SSB-SC

ii)

m P P 4 1 (5) 1 .25W 4

Powersaved 7.5W 1.25W

2

t

c

6.25W

2

iii)Bandwidth=fmmax=10kHz

6.25W % Powersaved 100% 7.5W 83.33%

iv) fc-fm 2/12/2013

110kHz

fc Amplitude (Linear) Modulation

60

Independent Side Band (ISB) Modulation • Satisfies the need of multiplex techniques for high density point- to- point communication • Two sidebands quite independent of each other with reduced carrier. • It can simultaneously convey a totally different transmission

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Amplitude (Linear) Modulation

61

ISB drive unit Input 1 Channel A Balanced Modulator 100 kHz crystal oscillator Balanced Modulator

USB filter 26-dB Carrier attenuator LSB filter

Channel B 2/12/2013

Input 2

3-MHz Crystal oscillator

Adder

Balanced Mixer 3.1-MHz Amplifier and filter

To the ISB transmitter Amplitude (Linear) Modulation

62

ISB spectrum Reduced Carrier 26-dB

LSB

USB

Independent information sidebands

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Amplitude (Linear) Modulation

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Vestigial Side Band (VSB) In the video signal, very low frequency modulating components exist along with rest of the signal These components give rise to sidebands very close to the carrier frequency – difficult to remove by physically realizable filters The low video frequencies contain the most important information of the picture Complete suppression of the lower sideband would result in phase distortion at these frequencies one complete sideband can not be fully suppressed As a compromise only a part of the LSB is suppressed Radiated signal consist of : Full USB + Carrier + Vestige of the partially suppressed LSB 2/12/2013

Amplitude (Linear) Modulation

64

VSB Generation x(t)

Product Modulator

DSBSC

Side Band Shaping Filter

VSB

Carrier

S(f) = Ac/2 [ M ( f – fc ) + M ( f + fc) ] H(f)

2/12/2013

Amplitude (Linear) Modulation

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Spectrum of the Modulating signal and corresponding DSB, SSB, and VSB M(w) DSBSC

SSB

VSB

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Amplitude (Linear) Modulation

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Bandwidth Calculation in VSB Frequencies up to 0.75 MHz of the LSB are fully radiated Attenuation slope of 0.5 MHz at either end FM sound signal occupies a frequency spectrum of about ±75 KHz around the sound carrier Guard band of 0.25 MHz – allowed on the sound carrier side – for interchannel separation

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Amplitude (Linear) Modulation

67

Advantages of VSB • Relatively easy to generate. • Less bandwidth as compared to DSBSC

Disadvantages of VSB • Small power is wasted in VSB filter. • Some phase and amplitude distortion still occurs • Needs careful design of VSB filter. • Critical tuning at the receiver 2/12/2013

Amplitude (Linear) Modulation

68

Sample MCQs 1.1 If Vc , Vl and Vu are the peak amplitudes of carrier, LSB and USB, then the relation among them in AM is a)

b)

c)

d)

1.2 The amplitude modulator works on the principle of a) Multiplication c) Subtraction

b) d)

Addition Division

1.3 To achieve 50 percent modulation with carrier Vc = 20cosωct ,the modulating signal amplitude Vm should be a) 15V b) 10V c) 12V d) 13V 2/12/2013

Amplitude (Linear) Modulation

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Sample MCQs 1.4 ideal value of m for maximum amplitude of modulating signal is a)

b)

c)

d)

1.5 Choose the correct statement related AM a) BW in AM is dependent b) on m c) Total power lies in d) sidebands 1.6

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The maximum power of AM wave under distortionless condition is a) b) c) d) Amplitude (Linear) Modulation

70

Sample MCQs 1.7

The instantaneous voltage of the SSB=LSB wave is a) b) c) d)

1.8

The bandwidth of SSB wave is given by a) b) c)

d)

1.9

Tone modulation refers to a) Modulating signal of b) Modulating signal is pure irregular shape sinusoid c) Modulating signal is d) None of them square wave 1.10 AM (DSBFC) wave can be generated with the help of a) Balanced Modulator b) Ring Modulator c) Class C tuned d) Phase shift method amplifier 2/12/2013

Amplitude (Linear) Modulation

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Outcomes of the Unit • Upon completing this unit Student will be able to • • • • •

Describe theory of amplitude modulation Compare baseband and carrier communication Draw AM, DSBSC, SSB signals Compute total transmission power in each technique of AM Solve problems involving frequency components, power, current, bandwidth calculations. • Understand differences between AM and other techniques. • Draw and explain various AM modulators. • Explain the concept of non-linear resistance applied to generation of DSBSC signal

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Amplitude (Linear) Modulation

72

Thank You

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73

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Amplitude (Linear) Modulation

Dr. Suvarna S. Patil BVCOEW, Pune-43 Mail id: [email protected] 2/12/2013

Amplitude (Linear) Modulation

1

Unit 1: Contents • • • •

Base Band & Carrier Communication Generation of AM (DSBFC) and its spectrum. Power relations applied to sinusoidal signals. DSBSC – Multiplier modulator, Non linear generation, switching modulator, Ring modulator and its spectrum. • Modulation index • SSBSC, ISB, VSB & their generation methods and comparison • AM Broadcast technical standards • Books : • T1: Modern Digital & Analog Communication Systems, by B. P. Lathi 3rd Ed. • T2: Electronic Communication, by D. Roddy & J. Coolen, 4th Ed. 2/12/2013

Amplitude (Linear) Modulation

2

Examination Scheme • 204189: Analog Communication • • • • •

2/12/2013

Lectures: 4 Hrs/Week Theory Online: 50 Marks Practical: 2 Hr/Week Theory Paper: 50 Marks Practical: 50 Marks

Amplitude (Linear) Modulation

3

Unit 1: Objectives • To understand base band signals and pass band communication • To study generation methods of various Amplitude Modulation (AM) schemes (such as DSBSC, SSB, ISB and VSB). • Plot frequency spectrum of different AM and compute the required bandwidth for AM channel • Derive power relation in AM and compute power required for AM transmission. • Define and calculate Modulation index • To understand the concept of suppressing the carrier in DSBSC and SSB techniques. • To study various AM broadcast standards

2/12/2013

Amplitude (Linear) Modulation

4

Pre--requisites Pre • Electromagnetic spectrum • Non linear characteristics of switching devices • Basic block diagram of communication system • Need of modulation 1. 2. 3. 4. 5. 6. 2/12/2013

Reduces height of antenna Avoids mixing of signals Increases range of communication Allows multiplexing of signals Allows adjustments in BW Improves quality of reception Amplitude (Linear) Modulation

5

Basic block diagram of Analog Communication System Analog information source

Analog Modulation

Communication Channel

Analog carrier source

Noise

Analog Demodulation

Destination

What is Communication? What are its examples? What is Communication System? What are various information sources? What are available communication channels? 2/12/2013

Amplitude (Linear) Modulation

6

Base band and carrier communication • Communication systems are classified according to the range of frequencies they use to transmit information • Base band and Pass band / carrier systems • Base band communication is without any modulation (without frequency shifting)

2/12/2013

•

Carrier communication is with modulation

•

Shifting the baseband signal to higher frequency Amplitude (Linear) Modulation

7

Base Band Signal and Communication • Base Band Signal is band of frequencies of the signal delivered by the source • They have sizable powers at lower frequencies • Telephony – band of voice signals (0 to 3.5kHz) • Television – band of video (0 to 4.3kHz) • Digital data or PCM ( 0 to Rate Hz) •

2/12/2013

Eg. Communication between two telephones Video Communication Digital transmission of analog signals Communication between two PCs Amplitude (Linear) Modulation

8

Baseband and Carrier Wave (CW) Systems Linear modulation (AM...)

f w Baseband spectra

fc bw 2w

Exponential modulation (FM...)

bw 2w

• Figures show baseband message transfer by linear (AM) and exponential modulation (FM) • In linear modulation, transmission bandwidth is always below or equal to 2W (W: message bandwidth) • Non-linear (angle modulation) spreads message on much larger transmission bandwidth that 2W

2/12/2013

f

Amplitude (Linear) Modulation

f

9

Modulation • The process of shifting the base band signal to pass band is known as Modulation • Basic parameter such as amplitude, frequency or phase of a sinusoidal signal known as carrier of high frequency is varied in proportion to the baseband signal What is Modulation? Define AM. Define requirement of AM signal 2/12/2013

Amplitude (Linear) Modulation

10

AM waveform representation

2/12/2013

Amplitude (Linear) Modulation

11

AM mathematical representation • Modulating signal

vm Vm sin mt vc Vc sin c t

• Carrier signal • Modulation index

Vm m Vc

• Amplitude of AM wave

A Vc (1 m sin mt )

• Instantaneous voltage of AM wave

v AM A sin c t Vc (1 m sin mt ) sin c t 2/12/2013

Amplitude (Linear) Modulation

12

AM mathematical representation v AM

mVc mVc Vc sin c t cos(c m )t cos(c m )t 2 2

Un-modulated carrier

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Lower sideband (LSB)

Amplitude (Linear) Modulation

Upper sideband (USB)

13

AM mathematical representation using frequency shifting property • Base band signal

m(t ) M ( )

• AM containing only two sidebands

1 m(t ) cos c t [ M ( c ) M ( c )] 2 Upper sideband (USB)

2/12/2013

Amplitude (Linear) Modulation

Lower sideband (LSB)

14

AM frequency domain representation

Amplitude Modulating band

0

m

Carrier band

mVc 2

Vc

c m c c m LSB

2/12/2013

m Vc 2

Amplitude (Linear) Modulation

(rad sec)

USB

15

AM Generation Methods • AM (DSBFC) Modulators • BJT Collector Modulator • Analog multiplier

• AM (DSBSC) Modulators • • • •

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Multiplier (Product) Modulators Nonlinear Modulators Switching Modulators Ring Modulators

Amplitude (Linear) Modulation

16

BJT Collector Modulator • Transistor is operated in Class C mode • Transistor is biased well below cutoff. • Carrier input to the base drives the transistor into conduction during the RF cycle. • Collector current flows in the form of Pulses periodic at carrier frequency. • Tuned circuit is used at the collector resonates at this frequency. • By applying modulating voltage in series with collector dc supply changes this steady collector voltage to slowly varying voltage. • This produces current pulses of varying amplitude • Finally modulated output is obtained by mutual inductive coupling 2/12/2013

Amplitude (Linear) Modulation

17

BJT Collector Modulator Practical Circuit

2/12/2013

Amplitude (Linear) Modulation

18

Collector Modulator Waveform

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Amplitude (Linear) Modulation

19

Analog Multiplier AM Modulator m(t ) Modulating Signal

Analog Multiplier

cos c t

∑ DSBFC Modulated Signal

Carrier Signal

2/12/2013

Amplitude (Linear) Modulation

20

Modulation index using practical method

V max V min

Vmax Vmin m Vmax Vmin 2/12/2013

Amplitude (Linear) Modulation

21

Trapezoidal method of Modulation index Calculation Modulated signal

L2 = 2 Vmin

2/12/2013

L1 = 2Vmax

Amplitude (Linear) Modulation

L1 L2 m L1 L2

22

Modulation index range :

0 m 1 m 1, (Vm Vc ) 100% modulation Requires high transmission power

m 1, (V m V c ) Under modulation Most preferably used

m 1 , (V m V c ) Over modulation Must be avoided

2/12/2013

Amplitude (Linear) Modulation

23

Bandwidth of AM For single tone modulation

BW f H f L f USB f LSB ( fc fm ) ( fc fm ) 2 fm For multi tone modulation

BW ( f c f max ) ( f c f max ) 2 f max 2/12/2013

Amplitude (Linear) Modulation

24

Power relations in AM PT Pc PLSB PUSB

Vc2rms R

2 VLSB rms

R 2

2 VUSB rms

R 2

Vc mVc mVc 2 2 2 2 2 R R R Vc2 m 2Vc2 m 2Vc2 2R 8R 8R 2/12/2013

Amplitude (Linear) Modulation

2

25

Power relations in AM V c2 Pc 2R PLSB PUSB

PSB PLSB PUSB m 2 Pc m 2 Pc 4 4 m 2 Pc 2

m 2 Pc 4

PT Pc PSB m 2 Pc Pc 2 m2 Pc 1 2 2/12/2013

Amplitude (Linear) Modulation

26

Voltage and Current relations in AM 2

m VT Vc 1 2

IT I c

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m2 1 2

Amplitude (Linear) Modulation

27

Power relations in multi tone AM 2 2 2 m3 m1 m2 PT Pc 1 2 2 2

m eff

2

2

m1 m 2 m 3

VT V c 1 IT Ic 1

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2

m eff2 2 m eff2 2

Amplitude (Linear) Modulation

28

Transmission Efficiency in AM Ratio of useful power to the total power PSB 100% PT where PSB is the total sidebands signal power that contains information m 2 Pc m2 m2 2 2 2 m m 2 m2 21 Pc 1 2 2 If m = 1 (100% modulation), the average power, PSB = 50% Pc= Pc/2 It shows that the PSB is dependent on m. 2/12/2013

Amplitude (Linear) Modulation

29

Transmission Efficiency in AM m2 100% 2 2m •

The transmission efficiency with m = 1 is only 33.33% practically still less on the order of 25% or lower.

•

Only 1/3rd of the transmitted power is used for carrying message.

•

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Efficiency increases monotonically with m

Amplitude (Linear) Modulation

30

Examples • Numerical 1: A 400 watt carrier is modulated to a depth of 75 %. Calculate the total power in the modulated wave. • Solution: m2 0.752 4001 PT Pc 1 2

512.5 W 2

• Note: Refer page no. 38 to 42 for more solved numerical from G. Kennedy

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Examples • Numerical 2: Sketch AM wave for modulation indices of m=0.5 and m=1, when m(t ) B cos mt • Solution: carrier signal is A cos c t

Vm B m Vc A • when the modulating signal is pure sinusoidal, the modulation referred to as Tone Modulation

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32

Disadvantages of AM • Needs large bandwidth • Requires more power for transmission • Maximum power is wasted in carrier. • Highly affected due to noise.

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33

Features of DSBSC • Carrier signal does not contain any information. • Information lies in sidebands only • Eliminating carrier signal saves transmission power • DSBSC contains only sidebands carrying same information • Bandwidth remains same as that of DSBFC (AM) • Exhibits phase reversal at zero crossing 2/12/2013

Amplitude (Linear) Modulation

34

Multiplier/Product DSBSC Modulator

1 m(t ) cosct [M ( c ) M ( c )] 2

m(t )

DSBSC signal

Modulating Signal

cos c t Carrier Signal

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Amplitude (Linear) Modulation

35

DSBSC signal

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36

DSBSC spectrum

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Nonlinear Resistance Characteristics • AM signal is generated using nonlinear device such as a semiconductor diode or a transistor. • Input output characteristics of nonlinear device is approximated by power series i

Positive C

i a bv cv 2 Negative C

a : dc component b: conductance c: coefficient of nonlinearity v 2/12/2013

Amplitude (Linear) Modulation

38

Nonlinear Modulator y (t ) bx(t ) cx 2 (t )

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Amplitude (Linear) Modulation

39

Nonlinear Modulator.. Cont. 2

2

z (t ) y1 (t ) y2 (t ) [bx1 (t ) cx1 (t )] [bx2 (t ) cx2 (t )] By substituting the two inputs

x1 (t ) cos c t m(t ), x2 (t ) cos c t m(t ) z (t ) 2b m(t ) 4c m(t ) cos c t After passing through BPF the output contains only one term with two sidebands so called as single balanced modulator

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40

Switching Modulator • The signal m(t ) w(t ) consists not only component of m(t) but also infinite number of modulated signals with carrier frequency ωc, 3ωc, 5ωc, …… • Multiplication of a signal by a square pulse is switching operation. • Switching the signal m(t) periodically is achieved using simple switching element. • This switching element is controlled by carrier signal w(t) 2/12/2013

Amplitude (Linear) Modulation

41

Switching Modulator

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42

w(t)

S(t) = m(t).w(t)

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43

Diode Bridge: series and shunt modulator

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Amplitude (Linear) Modulation

44

Ring Modulator

Diode bridge act as an electronic switch For the first half cycle of carrier all diodes conduct For next half cycle all diodes remain open 2/12/2013

Amplitude (Linear) Modulation

45

SSB Modulation

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The two sidebands of an AM signal are mirror images of each other.

One of the sidebands is redundant

Using single-sideband suppressed-carrier transmission results in reduced bandwidth and therefore twice as many signals may be transmitted in the same spectrum allotment

Typically, a 3dB improvement in signal-to-noise ratio is achieved as a result of SSBSC

Amplitude (Linear) Modulation

46

SSB Spectra Frequency spectrum:

fc-fm

fc

fc+fm

Bandwidth = fm(max) Total Power = +PUSB

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Amplitude (Linear) Modulation

47

Methods of Generating SSB i) Selective Filtering method

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•

A filter removes the undesired sideband producing SSB.

•

Balanced modulators is used to suppress the unwanted carrier and filters to suppress the unwanted sidebands

•

Quartz crystal filters are the most widely used sideband filters since they are very selective and inexpensive.

Amplitude (Linear) Modulation

48

SSB Generation by Filter Method Antenna Carrier oscillator

DSB signal Balanced modulator

Microphone Audio amplifier

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SSB signal Sideband filter Linear amplifier

Filter response curve Upper Lower sidebands sidebands

Amplitude (Linear) Modulation

49

Methods of Generating SSB ii) Phase shift method • This method avoids filters and their disadvantages. • It makes use of two Balanced Modulators and two phase shifters. • One of the BM receives 90° phase shifted carrier and in phase message signal

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Amplitude (Linear) Modulation

50

Phase Shift method •

Other BM receives 90° phase shifted message and in phase carrier signal.

• Both BM produce sidebands only. •

USB of both BMs will be in phase and LSB will be out of phase.

•

Adding the two DSB signals together results in one sideband being cancelled out.

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Amplitude (Linear) Modulation

51

Block Diagram of Phase Shift method Vm cos wmt

m (t)

Balanced Modulator 1

A1(t)

Vc cos (wct + 90)

Output Signal

Phase shifter

+

Carrier signal Phase shifter

Balanced Modulator 2

A2(t)

Vm cos (wmt + 90)

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Advantages/Benefits of SSB • • • •

Low power transmission Less band width conservation Selective fading Noise reduction

Disadvantages of SSB Complex receivers Tuning difficulties

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53

Comparison of time domain representation of three common AM transmissions

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54

Examples For an AM DSBFC wave with a peak unmodulated carrier voltage Vc = 10 Vp, frequency of 100kHz, a load resistor of RL = 10 , frequency of modulating signal of 10kHz and m = 1, determine the following i) Powers of the carrier and the upper and lower sidebands. ii) Total power of the modulated wave. iii) Bandwidth of the transmitted wave. iv) Draw the power and frequency spectrum.

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Examples..cont’d • For the same given values, determine questions (ii)-(iv) for a AM DSB-SC, AM SSB-FC and AM SSB-SC systems. Determine also the percentage of power saved in each of the system design.

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Examples..cont’d i)

Solution for DSBFC

(V / 2 ) V (10) P 5W R 2R 2 10 mP P P 1.25W 4 2

2

c

2

c

c

2

c

usb

lsb

ii) m2 m2 Pt Pc Pc Pc 4 4 12 12 5 (5) (5) 7.5W 4 4

iii) Bandwidth=2xfmmax=2(10kHz)=20kHz 2/12/2013

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Examples..cont’d Solution : For DSB-SC

ii)

Power Saving with DSBSC

m m P P P 4 4 1 1 (5) (5) 2.5W 4 4 2

t

2

c

2

c

2

iii)Bandwidth=2xfmmax

Powersaved 7.5W 2.5W 5W 5W 100% 7.5W 66.67%

% Powersaved

=2(10kHz)=20kHz

iv) 2/12/2013

90kHz

110kHz

Amplitude (Linear) Modulation

58

Examples..cont’d Solution :For SSB-FC

ii)

m P P P 4 1 5 (5) 6.25W 4 2

t

c

c

2

Powersaved 7.5W 6.25W 1.25W 1.25W % Powersaved x100% 7.5W 16.67%

iii)Bandwidth=fmmax=10kHz

iv) fc-fm 100kHz 2/12/2013

110kHz

Amplitude (Linear) Modulation

59

Example 1..cont’d • Solution : For SSB-SC

ii)

m P P 4 1 (5) 1 .25W 4

Powersaved 7.5W 1.25W

2

t

c

6.25W

2

iii)Bandwidth=fmmax=10kHz

6.25W % Powersaved 100% 7.5W 83.33%

iv) fc-fm 2/12/2013

110kHz

fc Amplitude (Linear) Modulation

60

Independent Side Band (ISB) Modulation • Satisfies the need of multiplex techniques for high density point- to- point communication • Two sidebands quite independent of each other with reduced carrier. • It can simultaneously convey a totally different transmission

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ISB drive unit Input 1 Channel A Balanced Modulator 100 kHz crystal oscillator Balanced Modulator

USB filter 26-dB Carrier attenuator LSB filter

Channel B 2/12/2013

Input 2

3-MHz Crystal oscillator

Adder

Balanced Mixer 3.1-MHz Amplifier and filter

To the ISB transmitter Amplitude (Linear) Modulation

62

ISB spectrum Reduced Carrier 26-dB

LSB

USB

Independent information sidebands

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Vestigial Side Band (VSB) In the video signal, very low frequency modulating components exist along with rest of the signal These components give rise to sidebands very close to the carrier frequency – difficult to remove by physically realizable filters The low video frequencies contain the most important information of the picture Complete suppression of the lower sideband would result in phase distortion at these frequencies one complete sideband can not be fully suppressed As a compromise only a part of the LSB is suppressed Radiated signal consist of : Full USB + Carrier + Vestige of the partially suppressed LSB 2/12/2013

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64

VSB Generation x(t)

Product Modulator

DSBSC

Side Band Shaping Filter

VSB

Carrier

S(f) = Ac/2 [ M ( f – fc ) + M ( f + fc) ] H(f)

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Spectrum of the Modulating signal and corresponding DSB, SSB, and VSB M(w) DSBSC

SSB

VSB

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Bandwidth Calculation in VSB Frequencies up to 0.75 MHz of the LSB are fully radiated Attenuation slope of 0.5 MHz at either end FM sound signal occupies a frequency spectrum of about ±75 KHz around the sound carrier Guard band of 0.25 MHz – allowed on the sound carrier side – for interchannel separation

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Advantages of VSB • Relatively easy to generate. • Less bandwidth as compared to DSBSC

Disadvantages of VSB • Small power is wasted in VSB filter. • Some phase and amplitude distortion still occurs • Needs careful design of VSB filter. • Critical tuning at the receiver 2/12/2013

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68

Sample MCQs 1.1 If Vc , Vl and Vu are the peak amplitudes of carrier, LSB and USB, then the relation among them in AM is a)

b)

c)

d)

1.2 The amplitude modulator works on the principle of a) Multiplication c) Subtraction

b) d)

Addition Division

1.3 To achieve 50 percent modulation with carrier Vc = 20cosωct ,the modulating signal amplitude Vm should be a) 15V b) 10V c) 12V d) 13V 2/12/2013

Amplitude (Linear) Modulation

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Sample MCQs 1.4 ideal value of m for maximum amplitude of modulating signal is a)

b)

c)

d)

1.5 Choose the correct statement related AM a) BW in AM is dependent b) on m c) Total power lies in d) sidebands 1.6

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The maximum power of AM wave under distortionless condition is a) b) c) d) Amplitude (Linear) Modulation

70

Sample MCQs 1.7

The instantaneous voltage of the SSB=LSB wave is a) b) c) d)

1.8

The bandwidth of SSB wave is given by a) b) c)

d)

1.9

Tone modulation refers to a) Modulating signal of b) Modulating signal is pure irregular shape sinusoid c) Modulating signal is d) None of them square wave 1.10 AM (DSBFC) wave can be generated with the help of a) Balanced Modulator b) Ring Modulator c) Class C tuned d) Phase shift method amplifier 2/12/2013

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Outcomes of the Unit • Upon completing this unit Student will be able to • • • • •

Describe theory of amplitude modulation Compare baseband and carrier communication Draw AM, DSBSC, SSB signals Compute total transmission power in each technique of AM Solve problems involving frequency components, power, current, bandwidth calculations. • Understand differences between AM and other techniques. • Draw and explain various AM modulators. • Explain the concept of non-linear resistance applied to generation of DSBSC signal

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Thank You

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