Analog Communication Unit No. 1
Amplitude (Linear) Modulation
Dr. Suvarna S. Patil BVCOEW, Pune-43 Mail id:
[email protected] 2/12/2013
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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
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Examination Scheme • 204189: Analog Communication • • • • •
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Lectures: 4 Hrs/Week Theory Online: 50 Marks Practical: 2 Hr/Week Theory Paper: 50 Marks Practical: 50 Marks
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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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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
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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
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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
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Shifting the baseband signal to higher frequency Amplitude (Linear) Modulation
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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
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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
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f
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f
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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
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AM waveform representation
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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
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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)
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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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Lower sideband (LSB)
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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
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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
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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
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BJT Collector Modulator Practical Circuit
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Collector Modulator Waveform
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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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Modulation index using practical method
V max V min
Vmax Vmin m Vmax Vmin 2/12/2013
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Trapezoidal method of Modulation index Calculation Modulated signal
L2 = 2 Vmin
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L1 = 2Vmax
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L1 L2 m L1 L2
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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
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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
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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
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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
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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
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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
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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
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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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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
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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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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
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Nonlinear Modulator y (t ) bx(t ) cx 2 (t )
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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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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
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Switching Modulator
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w(t)
S(t) = m(t).w(t)
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Diode Bridge: series and shunt modulator
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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
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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
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SSB Spectra Frequency spectrum:
fc-fm
fc
fc+fm
Bandwidth = fm(max) Total Power = +PUSB
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Methods of Generating SSB i) Selective Filtering method
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A filter removes the undesired sideband producing SSB.
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Balanced modulators is used to suppress the unwanted carrier and filters to suppress the unwanted sidebands
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Quartz crystal filters are the most widely used sideband filters since they are very selective and inexpensive.
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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
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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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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.
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Adding the two DSB signals together results in one sideband being cancelled out.
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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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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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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
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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
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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
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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
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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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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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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
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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
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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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